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© 2011 Dodson Publications, Inc. Page 1 of 21 March 16, 2012 8:33 AM

Roof Drainage Issues

Designing Commercial Roof Systems In Compliance with the IBC Part 2:

Roof Drainage Issues

by John D. Shepherd, RRC, Shepherd Consulting Services

(Editor’s Note: John Shepherd is an RCI, Inc. Registered Roof Consultant

and Registered Roof Observer, a Building Science Certified Thermographer, a

Haag Certified Commercial and Residential Roof Inspector, and holds roofing

licenses in both Calif. and Ariz. He is also actively involved in RCI, Inc. and

the SoCal Chapter of RCI, and currently is a member of three committees

including the RCI Inc Advocacy Committee, the SoCal Chapter of RCI Hawaii

Winter Workshop Committee, and the ASTM D08 committee. You can reach

Shepherd at [email protected])

After proper attachment and installation of the low-sloped roof assembly on

a commercial building, it is this consultant’s opinion that proper roof

drainage is the singular most important issue that impacts a commercial roof

system. Roof drainage issues appear to some degree or other on just about

every low-sloped commercial reroof project I have inspected in my 40 plus

years as a roofer, roofing contractor, and roofing consultant. Roof drainage

issues also crop up in new construction projects and those not resolved

during the original construction process will resurface during a reroof

project. That said, while the focus of this article is on drainage issues

related to reroofing projects, I have included a section on pre-construction

inspections for new construction roof projects that I hope will slightly reduce

the roofing contractor’s future liability on new construction projects.

With the exception of new construction projects, this article goes

forward assuming that the reroofing project does not include the

involvement of an architect, engineer, or register roof consultant. While I

highly recommend that all reroofing projects have specifications and details


prepared for the specific project by one of these design professionals, I also

understand that this is not the reality of the world we all live in. In the vast

majority of cases, the roofing contractor has taken on the role as the

designer of record (refer to Part I of this series in the November/December

2011 issue of Western Roofing for a more detailed discussion on the

designer of record discussion).

Design related decisions made by the roofing contractor, in his/her

capacity as the prime contractor on the project, often include, but are not

limited to: the project’s overall scope of work, the type of materials to be

installed, which roofing material manufacturer to use, how existing

mechanical equipment and accessories will be addressed, addressing roof

drainage problems (ponding water conditions, missing overflow drains,

undersized crickets, etc.), how to deal with condensation and ventilation

issues, whether skylights are to be re-used or replaced, determining what to

do about areas with low base flashings, replacement of water damaged roof

sheathing, and support members, etc.

In this article I will discuss the typical roof drainage issues that a

contractor will run into on a reroofing project including ponding water

conditions, roof crickets and their impact on roof drainage, internal primary

and overflow roof drains, through-wall scupper primary and overflow drains,

raising and/or moving mechanical equipment, and penetrations in

waterways. This article will also include requirements and definitions for

roof drainage issues from the 2009 International Building Code (IBC) and

provide some common sense ideas on resolving the problems referenced

above.

2% Minimum Roof Slope Required

The 2009 International Building Code (IBC) and most roofing material

manufacturers required low-sloped roofs, often mischaracterized as flat roofs


even by experienced roofing contractors, to have a minimum 2% slope.

Under IBC Section 1507 “Requirements for Roof Coverings”, the IBC

requires that the, “roofs shall have a design slope of a minimum of one-forth

unit vertical in 12 units horizontal (2%) for drainage.” In IBC Section 1507

2% is listed as the minimum roof slope requirement for seven different roof

systems including Standing Seam Metal Roof Panels (Section 1507.4), Built-

up Roofs (Section 1507.10), Modified Bitumen Roofing (Section 1507.11),

Thermoset Single-Ply Roofing (Section 1507.12), Thermoplastic Single-Ply

Roofing (Section 1507.13), Sprayed Polyurethane Foam Roofing (Section

1507.13), and Liquid-Applied Coatings/Roofs (Section 1507.14).

Based on these code requirements I will ask you each to consider two

questions: 1) Does the 2% requirement also apply to the roof deck? 2) Is

there any flexibility in the 2% requirement? The answer to #1 is yes and

no, the answer to #2 is yes. Let me explain.

Roof Slope Deck Requirement: When Section 1507 references steep-

sloped type roof system, it specifically references what the minimum deck

slope shall be. However for low-sloped roof systems the code reference is

specific that the roof shall have a design slope of a minimum of 2%. So how

this is achieved on a reroof project is up to the contractor. If you have

verified that the existing roof deck slope is a minimum of 2%, then the

contractor can hypothetically install the new roofing system directly to the

roof deck (this discussion excludes discussions on energy code requirements

and specific materials that may be required to address fire-ratings of a roof

system). However, if the roof deck does not have the required 2% slope

then this slope must be achieved via either modifications of the actual roof

deck or via addition of materials over the deck (tapered insulation, light-

weight insulating concrete, etc.). Alternatively, slope can be achieved by

using sprayed polyurethane foam, which as a roof system can address both

the slope deficiencies and provide a waterproof roof assembly.


The methodology for addressing roof slope issues on projects without

the involvement of a design professional is really left up to the contractor,

who hopefully was prudent enough to address the issues in the pre-bid stage

and not after the roofing project is in progress. In the bid stage the

contractor will have the time to adequately evaluate the options and decide

whether they have the expertise to address the slope issues or recommend a

design professional to the building owner (see more on the above referenced

options later in this article). If decisions about slope modifications must be

made after the roofing project has started, the options available become

time dependent as the longer a roof is exposed the greater the risk of a

weather related catastrophe occurring.

When is a Slope of Less Than 2% Acceptable: Section 1510

“Reroofing” provides the one exception to the Section 1507 2% roof slope

rule. That exception, as listed under Section 1510.1 General, reads as

follows: “Reroofing shall not be required to meet the minimum design slope

requirement of one-quarter unit vertical in 12 units horizontal (2% slope) in

Section 1507 for roofs that provide positive roof drainage.” (It is important

to verify that the state or local jurisdiction the roof project is in has left this

code section intact when the code was adopted.)

So based on this code section, the contractor is not required to modify

slopes of less than 2%, as long as the roof provides positive drainage. This

code section now raises another question: What is “Positive Roof Drainage?”

Thankfully the IBC anticipates this question and provides us with a definition

for “positive roof drainage”.

Before moving on I want to recommend that you always consider

looking at either satellite images of the roof (via Google Earth or Bing) or

roof images taken via fixed wing aircraft (Pictometry, Eagleview, etc.) of any

building you are bidding on. Looking at such photographs prior to visiting

the site will provide a general understanding of the building layout, roof top


objects, and whether there is any possible ponding water conditions (refer to

photographs #1 and #2).

Positive Roof Drainage & Ponding or Standing Water Conditions

One of the most commonly mischaracterized conditions I have seen in

low-sloped related defect litigation cases are those related to ponding or

standing water conditions. For the last 20 plus years the so-called “48 hours

rule” was most often referenced with regards to defining what is ponding or

standing water conditions on low-sloped roofs. However, for many of those

years we (roofing contractors and consultants) depended on what the NRCA

1 and roofing material manufacturer’s 2 had to say about ponding or

standing water conditions. The building code had references to positive

drainage however in the past it did not articulate what is positive drainage,

and what is not.

With the adoption of the IBC nationwide we in the Western states now

have a clear legal definition for what is Positive Roof Drainage, and therefore

a legal definition for what is ponding water on a low-sloped roof. IBC

Section 1502 definitions defines Positive Roof Drainage as follows: “The

drainage condition in which consideration has been made for all loading

deflections of the roof deck 3, and additional slope has been provided to

ensure drainage of the roof within 48 hours of precipitation.”

This is a pretty direct statement: if all water from a rainfall is

dissipated from the roof within 48 hours of the last rainfall, the 2% slope

rule is not applicable on a reroofing project. This same statement is

applicable to any so called ponding water areas on a roof. Let’s discuss the

ponding issue more in depth. First though some clarifications on my

experience in this area. As a construction consultant, the majority of my

business is working as an expert of construction defect litigation cases.

Generally I am an expert on roofing, waterproofing, and deck issues, though


I also handle exterior wall issues as well (stucco, EFIS, siding, elastomeric

paint, windows and doors, etc). Though I occasionally work as a plaintiff

expert, I primarily work as a defense expert for either the developer or the

subcontractor (roofing, sheet metal, waterproofing, decks). It is my

experience as a roofing expert that has provided the basis for what I am

about to say.

In virtually every litigation case that I have worked on involving

commercial roofing (low-sloped roof systems) the allegation of ponding

water conditions is always on the defect list. Why, well it’s an easy target.

When the plaintiff’s expert sees staining and/or an accumulation of dirt and

debris in what appear to be low spots on a roof, typically in front of drains

and scuppers, or adjacent to cricket waterways and large mechanical

equipment, they automatically assume it’s a ponding water condition. In

most cases how I deal with this issue is I disagree with the allegation that

ponding water is actually present, until someone actually performs a 48-hour

ponding water test. At times when I am pretty certain that the water testing

will confirm there are no ponding conditions, I will actually push for the

water testing as part of any defense related testing.

Now sometimes this condition is actually present on a roof and it is

important to recognize it early in the litigation, or in the case of a reroofing

project, prior to starting the roof construction. From a contractor’s

perspective you want to know if there are issues you need to address related

to ponding that could void the manufacturer’s warranty. So it is important

to include some type of language in your proposal that includes the

possibility of additional work being conducted to correct ponding conditions.

It should include a reference to verify the adequacy of the roof slope and

roof drainage via a 48-hour water test prior to removing the roofing,

especially if your preliminary inspection leads you to suspect that ponding

water conditions do exist. One thing that doesn’t work is to put language in


your contract that you are not responsible for ponding water conditions.

Regardless of what language you insert into your roofing contract, you are

the roofing professional and the party that should be familiar with the

building code requirements and the exclusions in the roofing material

manufactures warranty. The average building owner or property manager is

typically not aware of these issues until you inform them of these issues. So

if you suspect ponding water conditions then push for a water test. If your

client insists that the work be done and the ponding be ignored, you have to

make a decision to either walk away from the project (before signing the

contract) or risk ignoring the building codes and hope that nothing goes

wrong later that brings attention to the warranty exclusion.

The testing for ponding water is pretty simple. First check the weather

and determine whether rainfall is eminent. Is so let the rain water the roof

for you and then follow the below procedures. If no rain is predicted then

you will need to run water on the roof.

Start the testing by first sweeping up all loose debris from the

waterways and in front and around internal roof drains and through-wall

scuppers. Run the water over the roof until all little dips, birdbaths, and any

suspected ponding area is covered with water. Note the time the water is

turned off and then use spray paint (my preference) or chalk to mark the

outline of each area holding water (refer to photograph #3). Return to the

site 48-hours after you last turned off the water. If the roof is 100% dry,

there are no ponding water areas, then the roof has positive slope as defined

by the building code (refer to photograph #4). If there are small puddles of

water on site-specific areas of the roof then, using a different color of

marking paint, mark the perimeter of each remaining pond and then

photograph the condition. Minor ponding water conditions can often be

corrected with minor alterations to the roof slope. These include adding


Henry’s Pond Patch, Siplast’s Paraslope, or small amounts of tapered

insulation to the location in question.

If the water test identifies significant ponding conditions you still need

to identify, mark and photograph the ponding conditions but now you need

to have a serious discussion with the building owner about the options for

addressing the ponding conditions.

Addressing Serious Ponding Conditions

There are a number of things that can be done to address serious

ponding water conditions, however the extent and location of the ponding

conditions is the key to the repair options. First, if the conditions are

extensive and widespread, it is likely that a complete resloping of the roof

may be required. Such resloping can be done with lumber and tapered 2x’s,

with light-weight insulating concrete, with tapered insulation and/or with

spray-polyurethane foam. With the exception of the wood framing option,

each of the choices can not only provide the positive slope needed, but also

provide additional R-value for the buildings roof via the insulating aspects of

the materials. The method and materials chosen for this task will depend on

the structural integrity of the roof, costs, the needs of the building owner,

and what the building is used for. If the building is primarily an office

building the long term benefits of energy savings should be calculated for

the building owner to assist in the methods used to address the inadequate

slope issues. If the building is in California, you may already need to add

insulation to the building to comply with the current energy codes so this

becomes a win-win situation.

With each of the choices above the roofing contractor should not try to

go-it alone. With tapered insulation you can get design assistance from the

tapered insulation manufacturers. While there is no direct cost for this

service the indirect cost could be high and the manufacturer’s design might


include more insulation than say a design done by a third part design

professional.

With regards to modifying the roof slope with tapered 2x’s and

plywood or light-weight insulating concrete, you will need to have a

structural engineer determine if the building can handle the load. An

architect or engineer should design the new wood deck. The light-weight

insulation industry will provide design assistance for their product though,

like the tapered insulation you need to have the design reviewed by a third

party.

Addressing Moderate Ponding Conditions

The most common scenario related to a positive finding of ponding

water conditions is that the condition is not minor, but not so extensive that

the entire roof slope must be redone. In my experience I often found that

the problems can be resolved using a combination of one or more remedies.

This includes creating recessed sumps around ponding at roof drains and

through-wall scuppers, adding additional roof drains and/or through-wall

scuppers, adding tapered crickets on the upslope side of skylights and

mechanical equipment, rebuilding the existing crickets to have a steeper

slope and therefore more positive drainage, breaking up large crickets into

smaller crickets, and adding additional roof drains and/or scuppers. On

some commercial buildings tenant improvements or use changes in the

building create roof drainage problems by locating mechanical equipment in

the water-ways (refer to photographs #5 and 6). In these cases the only

option is to relocate the equipment that is blocking the direct flow of water

to roof drains. Again the choice of options used will depend on the specific

needs and issues on the building. And each option brings about another set

of questions to determine whether the option is actually viable.


If the main area of ponding is along the waterways, and it is not

caused by mechanical equipment, it is likely that the slope of the cricket is

not double the roof slope. In this case the best solution to solve the

drainage problems may be increasing the slope of the large crickets, if there

is adequate room in the height of the parapet walls. However if the parapet

wall is too low for such an adjustment, then look at breaking up large

crickets to smaller crickets and adding roof drains or through-wall scupper

drains as an alternative option.

When it comes to roof penetrations and objects blocking the flow of

water to the drains, you need to consider all options. When evaluating the

relocation of mechanical equipment that has been set in the waterways first

look at relocating the unit and the platform. But also consider the option of

replacing a closed mechanical equipment platform with an opened frame

support system that allows water to flow to the roof drains. Individual roof

penetrations in waterways are often the source of future roof leaks and they

should be moved out of the waterway if at all possible.

Adding roof drains also provides its own complications. If you want to

add internal roof drains, can they be added with only minimal impact on the

interior of the building? For a warehouse type building adding the internal

drains is easily doable. But in an office building adding an internal drain

could cause a significant disruption to the building. Same goes for the

adding of through-wall scupper drains. Is the intended location directly over

a doorway or a window? Can downspouts be added? An important thing to

remember is that if you add roof drains or through-wall scuppers you will

also need to add overflow drains or scuppers.

It is important that your sales team, or at least one key person on

your staff, have a keen understanding of each option available, and how to

apply the various options to issues found at your specific project. Also keep

in mind that at some point you may cross a threshold where the cost of


addressing each ponding area becomes more expensive than a whole roof

solution such as tapered insulation or even switching the roof system to an

SPF system.

Let’s move on to talk about existing roof drains and scuppers.

Verifying the Roof Has the Right Drain Size

As a consultant I always conduct what I call a preliminary evaluation of

the roof drain size for each specific roof area serviced by the roof drain to

determine if the roof drains are the correct size for the area of roof and

parapet walls being serviced by the roof drain. And I recommend that you

the contractor consider doing the same. I do these calculations primarily to

see if there are any red flags I need to caution my client about. However, I

caution you that unless you are an architect or an engineer licensed in the

state you are practicing, your calculations should used as an internal use

document only and should not be sent out to the client. If I find the drains

are undersized, I tell my client I am suspicious of the drain size and then

recommend that the drains be analyzed by a licensed engineer or architect.

The method of determining drain size for a given roof area is fairly

simple:

• Roof Drawing: Create a roof drawing that includes roof drains,

scuppers, crickets, ridge lines, etc. Identify the roof that sends water to

each drain and mark off the areas on the drawing. Also measure the

diameter of any internal roof drains and the area of any through-wall

scupper drains.

• Calculate the Total Roof Area: Calculate the total roof and wall area

service by each existing roof drain. Add 100% of the roof deck square

footage and 50% of the parapet wall square footage to determine the total

roof area serviced by each drain.


• Determine Rainfall Intensity: For the Western USA use figure 1106.1

on page 87 of the 2009 International Plumbing Code. Look on the figure and

identify the geographical location of the building to be reroofed and

determine the maximum hourly rainfall in inches for your geographical area.

• Select the Table Needed for the Next Step: For internal drains use

Table 1106.2(1) and for through-wall scupper drains use figure 1106.2(2).

Both are on page 90 of the 2009 IPC.

• Determine the Roof Area the Drain Can Handle: Find the rainfall

intensity for your area (as obtained from figure 1106.1) along the top of the

table and the diameter or the size along the left hand column and cross

reference to determine the square foot area the buildings existing drains or

scuppers can handle.

• Compare Existing to Design Requirements: If the existing roof area

(and 50% of walls) is less than the design load for the specific drain then the

existing drain or scupper is acceptable. If the roof area exceeds the drain

design load then either the existing drains or through-wall scuppers must be

enlarger or additional drains or scuppers must be installed.

Requirement for Overflow Drains/Scuppers

As far back as I can remember the various building codes have always

required the installation of overflow drains and/or overflow scuppers in

conjunction with the installation of primary drain system of internal roof

drains and/or through-wall scupper drains. So I am still surprised by the

number of buildings I have encountered over the last five years that do not

have overflow drains or overflow scuppers (refer to photographs #5 and 6).

In each case I made recommendations for adding overflow drains or

scuppers. If you, the contractor, should come across a building without

overflow drains or scuppers you also should make recommendations for

adding the overflow drains or scuppers to your client.


As with previous versions of the building code, 2009 IBC Section 1503

still required roof drainage systems to comply with the plumbing code.

Section 1503.4 read as follows: “Roof drainage. Design and installation of

roof drainage systems shall comply with Section 1503 and the International

Plumbing Code” (in California that would read “and the California Plumbing

Code). And though the wording had changed, the code made it relatively

simple for the contractor to determine if secondary roof drainage was

required on any building scheduled to have the roof replaced.

The 2009 IBC Section 1503.4.1 and IPC Section 1107.1 both read as

follows, “1503.4.1 Secondary drainage required. Secondary (emergency)

roof drains or scuppers shall be provided where the roof perimeter

construction extends above the roof in such a manner that water will be

entrapped if the primary drains allow buildup for any reason.” Translation, if

there are any parapet walls or any objects that would allow water to collect

on the roof if the roof drain should become blocked, the roof must have

overflow drains. This sounds straight forward enough. However the first

question I often hear from the roofing contractor, especially in construction

defect cases is, “This building was built X number of years ago under a

different code so why did I have to bring the roof and roof accessories into

compliance with the current code?”

The answer is, “because the building code says you have to.” Section

1510 Reroofing, includes the following statement: “1510.1 General.

Materials and methods of application used for recovering or replacing an

existing roof covering shall comply with the requirements of Chapter 15.”

So as there are no exceptions, all of the roof drainage requirements listed in

Chapter 15 would apply to a reroofing project. For clarity I think the

requirement bears repeating. “If any portion of the building could

accumulate water if the primary drain becomes blocked, and the building


had no overflow drains or scuppers, then overflow drains or scuppers must

be added during the roof replacement project.”

The choice of using overflow drains or overflow scuppers is one of both

cost and access. Installing a through-wall overflow scupper could entail as

little as cutting a hole in the parapet wall and inserting a through-wall

scupper flashing. If concrete then little extra work is required. If wood-

framed and stucco covered then plaster and lath needs to be removed and

replaced, and the hole needs to be framed to allow attachment for new lath

around the area. Installing an internal overflow drain involves cutting

through the roof deck, adding extra structural support as needed around the

point where the drain penetrates the roof, and then running drainage pipe

down the interior of the building. This is an easy fix, though more costly

than a through-wall scupper in a concrete tilt-up wall, if the area below is a

warehouse. It is a more difficult task, though still doable, if the area below

was office space.

Placement & Size of Overflow Drains

Previous versions of the building code would reference internal drains

but would defer to the plumbing code. And though the wording has been

modified, IBC Section 1503.4 still directs the reader the IPC (or in the case

of California, the California Plumbing Code) for roof drain issues. A search of

the 2009 IPC finds two specific sections related to overflow drains including

IPC Sections 1107.2 and 1107.3.

Section 1107.2 is as follows: “Separate systems required. Secondary

roof drain systems shall have the endpoint of discharge separate from the

primary system. Discharge shall be above grade, in a location that would

normally be observed by the building occupants or maintenance personnel.”

Translation: the primary and overflow drain systems should not be using

the same drain pipe system to drain water from the roof, and the discharge


point need to be highly visible so that if water is coming out of the overflow

drain discharge point someone will know the primary drains are blocked.

This section of the IPC may be the most overlooked section by any

roofing contractor, which is not good as it could also be the most costly issue

down the road if some existing condition does not comply with the code. I

have had a half dozen roof collapse cases over the years and each was

related to either the lack of an overflow drainage system, or as a result of a

lack of code compliance with the overflow drainage system that was in place

at the time the contractor installed the roof. Therefore it is my opinion that

on each commercial building that has overflow drains, the contractor has two

fundamental additional tasks. The first is to verify that that the overflow

drain system is separate from the primary drain system by both a visual

inspection from below the roof and by pouring water down the primary and

secondary drains and document the discharge points of both (alternatively,

the drains could be scoped with a camera by a plumber to provide a copy of

the film to keep in the contractors file). The second must-do task for the

contractor is to verify the location of all overflow drain discharge points and

then verify that it is not hidden behind landscaping or some other object. If

the discharge point is through a curb at a paved parking lot, the contractor

should also determine if the discharge point opening has not been partially

reduced in size by asphalt from a past paving of a parking lot.

Section 1107.3 of the IPC is as follows: “Sizing of secondary drains.

Secondary (emergency) roof drain systems shall be sized in accordance with

Section 1106 based on the rainfall rate for which the primary system is sized

in Tables 1106.2, 1106.3 and 1106.6.” To determine the correct size of

internal overflow drains, you use the same basic principal of drainage design

steps used to determine if the existing roof drains were the correct size. The

size of the overflow drains will be the same as the internal roof drains, if the


existing internal drains size meets the requirements for the geographical

location and the roof area being services by the drain.

Placement & Size of Overflow Scuppers

In all previous versions of the building code it was easy to determine

the size and placement location of a through-wall overflow scupper. The

through-wall overflow scupper was to be three times the area of the primary

roof drain or scupper and was to be set 2” above the surface of the roof

drain or scupper. With the adoption of the 2009 IBC things related to

through-wall scuppers are no longer so simplistic. There are four sections of

the code to review for placement and sizing of scuppers including one

section in Chapter 15 of the 2009 IBC and three sections in Chapter 11 of

the IPC.

Starting out in the plumbing code the first reference to scupper

location is in Section 1106.5 which reads as follows: “Parapet wall scupper

location. Parapet wall roof drainage scupper and overflow scupper location

shall comply with the requirements of the International Building Code.” First

up in IBC Chapter 15 is Section 1503.4.1, which tells us when we need to

install secondary drainage on a roof (this was discussed in more detail

earlier in this article). Next up is Section 1503.4.2, which reads as follows:

“Scuppers. When scuppers are used for secondary (emergency overflow)

roof drainage, the quantity, size, location, and inlet elevation of the scuppers

shall be sized to prevent the depth of ponding water from exceeding that for

which the roof was designed as determined by Section 1503.4.1. Scuppers

shall not have an opening dimension of less than 4” (102mm). The flow

through the primary system shall not be considered when locating and sizing

scuppers.” Say what?

Let me start with the back half of this code section. The second and

third sentences are pretty clear: the scupper will have a minimum of a 4”


opening dimension and the location and the size of the overflow scuppers

are not based on the primary drain system. Obviously this is a major

change from the past where the area of the overflow scupper was based on

three times the area of the roof drain. However the first sentence raises

some problems as it dictates that the size, quantity, and the location of the

overflow drains is based on what will not exceed the design load of the roof,

when it has X amount of water on it. At this point, unless you are an

engineer, we (consultants, architects, and contractors) must turn to an

engineer and say, “What size, where, and how many.”

The engineer will need to calculate the load capacity of the existing

roof deck (likely without the availability of original drawings) and then

determine what size the overflow scuppers must be, how many are required,

and where they should be installed. This requirement is supported by the

2009 IPC in section 1107.3, which reads similar to 1503.4.1: “Sizing of

secondary drains. Scuppers shall be sized to prevent the depth of ponding

water from exceeding that for which the roof was designed as determined by

Section 1101.7. Scuppers shall not have an opening dimension of less than

4” (102mm). The flow through the primary system shall not be considered

when sizing the secondary roof drain system.” Enough said on this part,

things get a bit more complicated with the above reference to IPC section

1101.7.

2009 IPC Section 1101.7 reads as follows: “Roof design. Roofs shall

be designed for the maximum possible depth of water that will pond there

on as determined by the relative levels of roof deck and overflow weirs,

scuppers, edges, or serviceable drains in combination with the deflected

structural elements. In determining the maximum possible depth of water,

all primary roof drainage means shall be assumed to be blocked.” This

section tells me that it is not about the structural load capacity of one roof

section when one drain is plugged. The IPC wants the engineer to determine


the maximum possible depth of water on the roof when all drains are

plugged. Ouch.

Lets recap what we just covered. I now know that I, or you the

roofing contractor, can no longer just determine the size, location, or

number of through-wall overflow scuppers to install on a building without

overflow drains or scuppers, based on our years of experience and

knowledge of a past simplistic, yet easy to use, building and plumbing code.

Now an engineer needs to conduct a structural analysis of the roof in order

to allow for the installation of through-wall overflow scuppers. I think the

cost of installing internal overflow drains just might now be cheaper than

installing through-wall overflow scuppers.

Roof Crickets

Roof drainage crickets are located on every commercial building I can

ever remember being on. Crickets, made from either lumber, tapered

insulation, or concrete, are generally constructed between roof drains and

between a roof corner and a roof drain, with the overall number of crickets

being dependent on the number of roof drains. In order to push water

towards the roof drains, crickets typically have a slope that is twice or more

than the slope of the main roof area. And when there are ponding water

conditions on a roof I often find that the crickets are a contributing factor.

Either the slope on the cricket is less than double that of the roof field

and/or the crickets stop too far from the roof drains. Or in some cases the

crickets installed do not comply with the cricket drainage design that were

on the original construction drawings.

On a new construction project that does not have a consultant

involved or any third party inspections occurring, I want to encourage the

roofing contractor to review the construction drawing and determine if the

crickets built appear to be as designed. If not, I recommend you send


documentation of such to the general contractor informing them of the issue

and the possible drainage problems that may arise in the future for which

you might get blamed for. If the building ends up in litigation in the future

and a roof consultant opines to ponding water conditions on the roof, your

letter could save your firm money in the future litigation.

On a reroofing project you are not usually going to have the luxury of

having the original plans to review so when you inspect the cricket areas of

the roof you need to look for the telltale signs of ponding water conditions.

As mentioned earlier in this article, the modification of roof crickets to

correct drainage problems is almost always doable, unless the parapet walls

are too low. However, my preference for addressing poor drainage issues at

wood framed crickets is to replace the large crickets with smaller crickets

and add more roof drains. With concrete crickets you generally have to find

a way to increase the slope of the crickets, which sometimes means adding

some height (if allowable) to the parapet walls. Regardless of the solution,

the one thing that will be required for cricket modifications is a structural

analysis of the roof area in question. So again we hit an area where even

when the roofing contractor is the prime contractor, there are things that

cannot be addressed without assistance.

Crickets on the upslope side of mechanical equipment and skylights

are an area that can be addressed by the roofing contractor. It is rare for

me to find a roof where these crickets were built large enough to drain off all

the water from the upslope side of the object. Even though the water is

often gone from the roof areas at each end of the cricket, I often find the

roof membrane slightly more deteriorated at the cricket ends than the roof

membrane in the field area of the roof. So on a reroof project, enlarge the

crickets when you can.

Closing


Identifying drainage issues and then presenting the appropriate

solutions to correct the drainage deficiencies to the building owner prior to

the start of a roofing project can save everyone time and money in the long

run. The owner will have a roof that will not degrade early due to

inadequate roof drainage and will have a less chance of leaking. The

contractor will make more money on the project by being involved in the

extra work needed to correct the roof drainage deficiencies, and will have a

lower risk of being sued in the future if the deficiencies were ignored and the

roof fails prematurely or if a consultant comes along a few years later and

identifies the problems that should have been addressed.

Also consider where level of expertise your firm has in dealing with

roof drainage problems. Know your limitations, not just of what your firm

can do, but also what your insurance will and will not cover. Carefully

consider the level of design responsibility you want to take on. Be aware of

the overall scope of the drainage issues to be addressed and when in doubt

do the responsible thing and get a design professional involved.

What do you do about the building owner who wants the roof installed,

but doesn’t want to pay to correct any of the roof drainage issues that are

required by the building and/or plumbing code(s)? First off, when you

present the options available to address the problem conditions, also include

the specific section of the building or plumbing code that requires corrective

action be taken. Get the support of a design professional, the local building

official, and or the manufacturer’s representative as needed to convince the

owner of the need for addressing drainage issues. In the end, the prudent

roofing contractor should walk away and not get involved in the project if the

owner wants the reroof done without the needed corrections.

However, I understand that in this economy many of you may choose

to not walk away. If you find yourself in a situation where you don’t want to

walk away and the owner just wants the new roof installed, I suggest you at


least list the drainage issues that need to be addressed, and get the owner

to sign a hold harmless agreement for these issues to limit your risk. You

may also want to choose a roof system that has no ponding water exclusion

and then try for a variance from the building department.

In the end the choice is yours. I only hope that I have provided you

with enough information to assist you in making the right choices.

If you want to learn more about roof drainage issues I suggest you

attend Roof Drainage – What You Don’t Know Can Hurt You! which is being

presented by Phil Dregger, FRCI, RRC, PE of DNG Group at the June 27th

RCI Region IV meeting in Las Vegas (as part of the 2012 WSRCA

Convention).

1 Though often mischaracterized as standards, the NRCA’s Roofing and

Waterproofing manual are not standards, though they are an excellent

reference source for learning about various roof decks, roofing systems, and

evaluating roofing-related issues. However, when it comes to specific roof

details, I find the lack of stucco related roof flashing details problematic and

therefore usually look to the WSRCA details, which do include stucco related

roof flashing details.

2 Today, most manufacturers have one or more products that are not

negatively impacted by ponding water conditions and therefore they allow

ponding water on some roofs, however the majority of each manufacturer’s

low-sloped roofing products still exclude damages from ponding water

conditions.

3 We will not get into the structural references in this statement in this

article.

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