0.04 Roof and Stormwater3

7/31/2019 0.04 Roof and Stormwater3

1/6

T: 1300 815 127

www.pic.vic.gov.au

PO Box 536

Melbourne

Goods Shed North

733 Bourke Street

Docklands, Victoria 3008

TECHNICAL SOLUTION 0.040: ROOF PLUMBING (Inclues above groun stormater rainage)

ROOF ANd STORMwATER

Page 1

Dpcat stn f 4.04 (Dranag)

AIM

The aim of this technical solution is to clarifythe requirements for sizing stormwater drains andeaves gutters.

Plumbing Regulations 2008

The Pbng Cd f Astraa (PCA) is adopted byand forms part of the Pbng Rgatns 2008.Part D1 of the PCA species the objectives andperformance requirements related to the installation ofroof drainage systems.AS/NZS 3500.3: Pbng anddranag Part 3: Stratr dranag is a deemed

to satisfy document listed in Part D1 of the PCA andcontains a section on Roof drainage systems - design.

Th Pbng Rgatns 2008 also specify thatRoong (stormwater) work must comply with SAA/SNZ HB114 Gdns fr th dsgn f as andb gttrs.

Part D2 of the PCA species the objectives andperformance requirements related to the installation ofsurface and subsurface drainage systems.

AS/NZS 3500.3 is a deemed to satisfy document listedin Part D2 of the PCA and contains sections on Surface

drainage systems - design and Surface and subsoildrainage systems - installation.

SURFACE dRAINAGE SySTEMS dESIGNAS/NZS 3500.3

Summar of methos to esign surface rainagesstems

Two methods of design are used depending on the areaof the proposed allotment:

1. Nominal method for single dwellings (rural) andsingle dwellings (urban) where the allotment size

is less than 1000m. This method does not involveany calculations and some rules are providedregarding diameter of pipes, depth of cover, gradientand layout.

2. General method for all buildings. This methodinvolves hydraulic design calculations to determinedesign ows, and procedures to determine thedesign of channels and drains.

Nominal metho - Minimum iameter of pipe

1. For single dwellings in rural areas, and single

dwellings on urban allotments with areas less than1000m, the minimum diameter of pipe shall beDN90; and

2. For other properties, downstream of a stormwater

or inlet pit, shall be the greater of:- the diameter of the largest pipe entering the pit, or

- DN150.

An exception to this is at footpath crossings wheremultiple pipes of DN100 or less may be used.

External connections

The external connection can be to the street gutter, astreet drainage pipe or inter-allotment drain (see Figure 1).

Laout of sstemsAS/NZS 3500.3 lists a range of requirements for thelayout of stormwater drains. It is important to consider:

1. The allowance for possible stormwater dischargesfrom adjacent properties,

2. Protection of buildings, and

3. The location of stormwater drains in relation tothe sanitary drainage system.

Example of laout an sizing

Figure 2 is from Appendix K ofAS/NZS 3500.3 whichdetails a solution to stormwater drainage on an urbanallotment not exceeding 1000m. The drainage systemis constructed of approved non metal materials.

Solution

1. The layout should comply with AS/NZS 3500.3 sothat the overland ow path is directed away fromthe building.

2. The stormwater drains are sized in accordance withAS/NZS 3500.3 as follows:

- between a downpipe outlet and a stormwater orinlet pit, DN90

- between the stormwater pits A & B, DN150; and

- between pit B and the street kerb, two drainsDN100 or less

3. Minimum cover for PVC-U fromAS/NZS 3500.3

- under the paved driveway within the property,75mm below the underside of brick or unreinforcedconcrete for light vehicle loadings. Elsewhere forsingle dwelling properties 100mm.

- under the paved footpath outside the property,75mm below the underside of the paving.


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T: 1300 815 127

www.pic.vic.gov.au

PO Box 536

Melbourne

Goods Shed North

733 Bourke Street



ROOF ANd STORMwATER

Page 2


FiGuRe 1 - TyPiCAl ARRANGemeNT oF iNleT PiT AND FooTPATH CRoSSiNG

ROOF dRAINAGE SySTEMS - dESIGNAS/NZS 3500.3

saa/snZ Hb114 guidelines foR tHe designof eaves and box gutteRs.

Elements of Roof drainage design

1. Catchment areas

> Vertical walls abutting a roof must be included inthe catchment area. For hipped roofs with eavesgutters a simplied formula can be used to calculate

catchment area, but the slope of the roof must beknown. Refer toAS/NZS 3500.3.

> SAA/SNZ HB114 simplies the roof catchmentcalculations. See Appendix A, SAA/SNZ HB114for example calculations for typical roof styles. Seepage 14 step 2 for the method using the slope factorfor a pitched roof to an eaves gutter (hip & valleywith no abutting vertical walls or at roofs).

> The catchment area of a roof and any vertical wallsis greatly inuenced by the direction of wind driven

rain, therefore the combined catchment area mustbe used for sizing purposes.

2. Rainfall intensity

> Rainfall intensities are given in Appendix E,AS/NZS 3500.3. The appendix gives ve minuteduration rainfall intensities. There is also referenceto an Average Recurrence Interval (ARI) of 20 and100 years.

> The ARI is used in accordance with Table3.1, AS/NZS 3500.3 where the risk of property

damage, inconvenience or injury to people istaken into account.

4. The minimum gradient for stormwater drains (fromAS/NZS 3500.3) for DN90, DN100 and DN150 is1:100.

5. Stormwater pits, A & B are sized based onAS/NZS3500.3. Each pit would be 450mm x 450mm with aminimum fall of 20mm between invert of inlet andinvert of outlet.


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T: 1300 815 127

www.pic.vic.gov.au

PO Box 536

Melbourne

Goods Shed North

733 Bourke Street



ROOF ANd STORMwATER

Page 3


FiGuRe 2 - SoluTioN To SToRmwATeR

DRAiNAGe oN uRBAN AlloTmeNT

For example:

- external gutters eaves gutters 20 years ARI

- Internal gutters, box gutters 100 years ARI

n: for a 500 year ARI, multiply 100 year ARI x 1.5

> Table E1 lists latitude and longitude of selectedplaces, some of which are marked on the maps. Ifthere is any doubt regarding the rainfall intensity fora particular area, refer to the relevant council / shirefor accurate rainfall intensity gures.

3. Overow measures for eaves gutters

> Always consider what will happen if the gutteroverows. If water cannot ow back into the building,e.g. gutter xed to rafters without eaves linings, nooverow measures are required. If water can owback into the building; e.g. through eaves linings,then overow methods must be provided.

> Blockages can occur in eaves gutters anywherealong the gutter; therefore an overow device at theoutlet may not prevent such overows.

> Examples of acceptable overow measures foreaves gutters can be found in Appendix G of

AS/NZS 3500.3.

> Simplied ways of providing for overow ofexternal eaves gutters can be found on page 12 ofSAA/SNZ HB114. A gure of 20mm is adopted forconvenience in the examples shown in Figure 3 .

design Example

The following example is shown to illustrate theprocedure adopted by SAA/SNZ HB114 to designeaves gutters and downpipes.

A house is to be constructed in Melbourne. The planis shown in Figure 4. There roof pitch is 23. Gutteroverow could cause signicant damage, thereforeoverow measures are required.

sp 1: dr h 5/20 yr aRi rh cy

From AS/NZS 3500.3 Figure E6, theARI for Melbourne is 130mm/hour.

sp 2: sc r r p

The eaves gutter selected is quad spoutingwith an effective cross sectional area of

6125mm2 installed with a slope of 1:500.


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T: 1300 815 127

www.pic.vic.gov.au

PO Box 536

Melbourne

Goods Shed North

733 Bourke Street



ROOF ANd STORMwATER

Page 4


step 7: divie the roof into approximately

equal catchment area an etermineownpipe poition

Divide the roof into ve catchments

and nominate the high points betweendownpipes (see Figure 5).

Step 8: Select an overow method if required

The example building requires overowmeasures.

SSA/SNZ HB114 and AS/NZS 3500.3both provide various methods dependingon the type of gutter used. For thisjob select Example 1 from Figure 1 inSSA/SNZ HB114 where the gutter isinstalled with the top of the front bead aminimum of 20mm below the top of thefascia (see Figure 6).

SSA/SNZ HB114 also provides informationto size box-gutters, rain-heads, sumps andoverow devices plumbers must beaware of design requirements, and applythem on the job.

n:

> There are many possibilities for downpipe positions.In some cases, depending on roof shape or buildinglayout, extra downpipes may be needed as it is notalways possible to achieve approximately equalcatchment areas.

> Valley gutters should be at high points to allowdrainage away from internal angles.

> The sub catchment areas may not add exactly dueto rounding off during calculations.

> In this particular example, as per step 5, nocatchment area should exceed the allowable 47m2.

sp 3: dr wpp z

From Table 3.3 in AS/NZS 3500.3,OR Table 3.2 in SAA/SNZ HB114, theminimum size downpipes compatiblewith spouting of 6125mm2 cross sectionalarea (installed at a 1:500 gradient)are 90mm diameter round or 100 x 50rectangular.

Note that a minimum gradient of 1:500is preferred.

sp 4: sc 100 50 rcr wpp.

From Figure 3.5 (A) inAS/NZS 3500.3, ORFigure 4 in SAA/SNZ HB114 the maximumcatchment per downpipe is 47m2.

sp 5: dr r wpp

To calculate the minimum number ofdownpipes, divide the roof catchmentarea by the allowable maximumcatchment per downpipe.

Min number of downpipes =

roof catchment area

allowable maximum catchment perdownpipe

= 223.8

47

= 4.76m2

= Five downpipes are required(round up to the next whole number)

sp 6: dr h r cchr pr wpp

To calculate the average catchment perdownpipe, divide the roof catchmentarea by the number of downpipes.

Average catchment per downpipe =

Roof catchment area

No. of downpipes

= 223.8

5

= 44.76m2


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T: 1300 815 127

www.pic.vic.gov.au

PO Box 536

Melbourne

Goods Shed North

733 Bourke Street



ROOF ANd STORMwATER

Page 5


FiGuRe 3 - PRoviDiNG FoR oveRFlow oF exTeRNAl eAveS AND GuTTeRS (20mm)

20mm 20mm

20mm 20mm

20mm

10mm

FiGuRe 4 - DeSiGN exAmPle HouSe PlAN


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T: 1300 815 127

www.pic.vic.gov.au

PO Box 536

Melbourne

Goods Shed North

733 Bourke Street



ROOF ANd STORMwATER

Page 6


FiGuRe 5 - RooF DiviDeD iNTo Five CATCHmeNTS

FiGuRe 6 - exAmPle oF oveRFlow meTHoD

20mm

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0.04 Roof and Stormwater3