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INTRODUCTION There are two types of drainage water: (1) Foul water - Foul water consists of anything that comes from bathrooms, kitchens, utility rooms, car washing areas etc. This must always go to your foul water drain system. (2) Surface water - Surface Water is rainwater only. In older properties, the existing rainwater pipes are often discharged to the foul drain. This system is known as combined drainage. In a combined system, the rainwater pipes are discharged via gully traps, to stop foul air escaping from the drains. Modern systems keep the foul water and the surface water apart in separate drains. This is known as a separate drainage system. With this method, the rainwater is discharged either to a soakaway, watercourse or surface water sewer. Draining rainwater to these points does not create foul air. It is extremely important to ensure that you do not connect foul water to a surface water drainage system.

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INTRODUCTION

There are two types of drainage water:

(1) Foul water - Foul water consists of anything that comes from bathrooms, kitchens, utility rooms, car washing areas etc. This must always go to your foul water drain system.

(2) Surface water - Surface Water is rainwater only. In older properties, the existing rainwater pipes are often discharged to the

foul drain. This system is known as combined drainage. In a combined system, the

rainwater pipes are discharged via gully traps, to stop foul air escaping from the drains.

Modern systems keep the foul water and the surface water apart in separate drains.

This is known as a separate drainage system. With this method, the rainwater is discharged either to a soakaway, watercourse or surface water sewer.

Draining rainwater to these points does not create foul air. It is extremely important to ensure that you do not connect foul water to a surface water drainage system.

Underground drainage

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Underground drainage is water drainage which is routed underground, rather than along the surface. There are a number of reasons to choose underground drainage, and such systems need to be installed with care. People need to make sure that they drain to the correct location, and that they are installed with materials which are sturdy, and will be able to withstand incursions such as roots. Irrigation and drainage companies can design and install such systems for people who would prefer to leave things in the hands of experts.

Drainage is an ever present issue in any area where there is water. In some cases, the natural design of a property provides enough surface drainage that drainage is not a major problem. In other instances, people may notice pooling, soggy soil, and other signs of a drainage problem. Drainage problems need to be addressed because poor drainage can compromise the foundations of a home, damage plants in the garden, look unsightly, and make it difficult to walk around the garden or yard. Poor drainage can also contribute to erosion and other problems.

One option is to install a surface drainage system which collects water and routes it to storm drains, the street, or another suitable area. This can be unsightly, however, leading some people to prefer an underground drainage system. With underground drainage, a series of collection sites lead to underground pipes which dump out into a downhill location. Collection sites include downspouts from the gutters and grates in strategic low points around the yard or garden.

The pipes used for underground drainage need to be able to repel roots which could attempt to grow through them. Some people use a french drain system, with perforated pipes surrounded by gravel. This system allows water to quickly percolate through the gravel and into the pipes, allowing drainage to occur everywhere the pipes run, rather than collecting water at central points. This wicks subsurface water away quickly and efficiently.

Placing the outlet for a subsurface drainage system in the proper location is important. For people in urban areas, pipes should drain to storm drains or into the street, depending on local ordinances. It is important to avoid routing water into the yards of neighbors, and to avoid creating an outlet on the property itself. Residents of rural areas should try to place the outlet in a downhill location, taking some measures to prevent erosion such as putting a screen on the outlet to break up the water as it exits the drainage system.

Types of Drainage System

French drainage

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A French drain is one common type of drainage system that most often is found on flat land. It is made up of pipes that surround the structure and direct underground water away from both the foundation and any nearby landscaping. In most cases, it is necessary to dig several large, somewhat shallow holes to install this type of drainage system. Each drainage trench then needs to be filled with gravel. Some people can install this kind of drainage system on their own, mostly by renting the right equipment, but many landscaping companies offer this service.

Water damage easily can destroy any building, which is why a drainage system usually is needed in almost every kind of structure. Whether the excess water originates in the building or comes from the surrounding landscape, it generally should be drained as soon as possible. Fortunately, there are several types of drainage systems to choose from depending on the circumstances, though they all mostly perform the same task. The most common kinds are the French drain, the downspout drain, and the slope type.

Another kind of drainage system is a downspout drain which connects to the gutter system of a building and carries water away from the roof toward the ground. Downspout pipes can be rectangular or round, and mostly are made of aluminum, copper, or steel. The most common downspout pipes on houses are rectangular and white, though they often can be replaced to match the exterior décor of a home. No matter what color or shape they are, they should divert rain water not only toward the ground, but away from the foundation of a structure, as well.

Similar to the other types of drainage systems, the main job of a slope drain is to direct water away from a building and its landscaping. The primary method is to allow the water to follow a natural route down a slope. Typically, a pipe is installed and anchored to the small incline on which it sits. The pipe can be made out of metal, concrete, or plastic, and often is covered with a grate to prevent small animals or children from entering the area.

It usually is quite clear when there is a drainage issue. If there is no obvious flooding, the mildew smell that accompanies standing water usually gives it away, even if water is not visible. A slow leak also could cause an eventual issue, weakening the structure over time. The loss of structural integrity in a building, a strong mildew smell, mold, damage to the landscaping, and mosquitoes could be consequences of water damage when no proper drainage system is in place.

Foundation drainage

A sloped finish grade and properly placed perimeter drains will keep the basement dry

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Underground water and runoff from rain and snow pose a threat both to the structural integrity of the foundations I build and to below-grade interior living space. Wet basements and cracked foundations are difficult to fix after the fact, but good perimeter drainage, both at grade and down at the footings, is a cheap and easy way to prevent problems. If you follow these rules of thumb for perimeter grading and drain tile, you’ll sleep easy knowing that the water control systems you buried today won’t bubble up into a callback tomorrow.

Surface RunoffAlthough some wind-driven rain strikes the siding and drains onto the ground, most surface runoff comes from the roof, and the amount of runoff varies according to the size and style of the roof. A gable roof deposits all runoff onto the ground under the eaves, with little runoff at the gable ends; a hip roof distributes the runoff more evenly on all sides (see Figure 1).

Gable Roof RunoffFigure 1. Both of these

roofs cover approximately 2,500 square feet. The

gable roof deposits runoff along two sides of the

house; the hip roof spreads the runoff more or less evenly along all sides. Main roof valleys and

dormers concentrate the runoff into smaller areas on

the ground.

Hip roof Runoff

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Roof Runoff

(from 2500 sq. ft. roof)

Rainfall Rainfall Volume VolumeAmount Rate (cubic ft.) (cubic ft.)

1 in. per hr. 200 15001 in. per day 200 15002 in. per hr. 400 30002 in. per day 400 300

Note: Every inch of rain, whether it falls during a one-hour downpour or an all-day rain, deposits 1,500 gallons of water onto the ground around a typical 2,500-square-foot roof surface. During a winter rainstorm, every

foot of melting snow on the roof adds an additional 1,500 gallons.

In addition, valleys at main roof intersections and dormers can concentrate runoff into a relatively small area on the ground. In cold climates, runoff increases significantly during spring rainstorms when higher temperatures and rain combine to melt snow on both the roof and the ground, adding to the total amount of surface water that must be drained away from the foundation.

Sloped grade.

Most basement water problems can be solved by properly sloping the ground around the house. The finish grade should slope away from the foundation at the rate of 1/2 to 1 inch per foot for 6 to 10 feet. A 2- to 4-inch cap of silty-clay material will keep runoff from percolating down through the backfill.

A sloped grade will not work for long, however, if the perimeter fill is not mechanically compacted, which is rare in residential construction. Instead, compaction is left to chance and occurs slowly over a period of months or years,

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depending on climate and the type of backfill used. Gravels and sands percolate faster and may reconsolidate more quickly — typically, from three months to a year. Silts and clays, which have a much slower percolation rate, may not compact for several years.

In either case, however, the result is a negative grade that directs runoff back toward the foundation. Depending on the type of backfill, sooner or later the runoff will overwhelm the footing drainage system, and basement water problems will appear. Silt or clay fill, which hold water longer than gravel or sand, can make the foundation more susceptible to cracking from frost action; hydrostatic pressure may also develop with these types of fill, forcing water through the slab-footing joint. Rarely will any of these problems appear immediately, but down the road, you’ll be faced with a messy and expensive repair job.

Gutters. While gutters can dramatically reduce the total ground area onto which roof water drains, it is crucial to use a sloped leader to extend downspouts along the ground to carry water away from the foundation (Figure 2).

Downspout with Sloped Leader

Figure 2. Sloped downspout leaders should discharge at least 10 feet away from the

foundation wall.

Downspout with Catch Basin

Use solid drain pipe to carry runoff

from a concrete

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catch basin to daylight or a

drywell .

Otherwise, a gutter-and-downspout system compounds the drainage problem by concentrating the entire roof runoff load into a few small areas, usually at the house corners. Leaders should discharge onto sloping ground at least 10 feet from the foundation. If downspouts dump directly into a catch basin on the surface or underground, the collected runoff should be carried through a solid drain pipe to a drywell or to daylight.

Keep gutters clear of leaves, pine needles, and ice. Overflow from blocked gutters can follow the contour of the gutter and saturate the soffit and siding, often making its way into the wall and wetting the insulation, drywall, and floor. Similarly, gutters in cold climates can encourage ice damming, with the same damaging results.

Hardscape.

Concrete or paver block sidewalks can also control percolation of runoff into the backfill (Figure 3) — I’ve measured reductions in runoff percolation of between 300% and 500%.

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Concrete or Paver Sidewalk

Figure 3. A properly sloped

concrete or paver sidewalk will

reduce the amount of runoff that

percolates through the backfill (top). Where perimeter

plantings are used to landscape,

improve drainage by burying a sheet

of polyethylene below the plant

bed, with openings cut out for roots

(bottom). Tie shallow perforated drain tile to solid

pipe to carry water to daylight or a

drywell.

Plant Bed with Drain

Again, the hardscape should be wide enough to cover the entire backfilled area, and the surface should slope away from the foundation walls.

A less expensive technique is to bury a sheet of polyethylene in a plant bed. The poly should cover the backfilled foundation trench and slope to a perforated drain tile laid parallel to the foundation. Use solid pipe to carry runoff to daylight or to a drywell. In landscaped areas, cut openings in the poly to accommodate plant and tree roots.

Buried poly works well, so long as the backfill has been compacted. With a negative grade, however, the poly actually directs the water into the foundation wall. Plant and tree roots near the foundation can also compound problems with

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uncompacted fill, because their root systems absorb water and cause the soil to reconsolidate quickly. In a drought, tree roots can pull so much moisture out of the soil that the foundation may settle.

Perimeter Footing DrainsFoundation perimeter drains work in both directions. They not only carry rainwater percolating down through the backfill away from the foundation, they also relieve excessive hydrostatic pressure from rising groundwater. By helping the backfill dry out more quickly, properly installed perimeter drains reduce lateral soil pressure, which in turn means that foundation walls can be designed to use more porous materials and less steel.

There’s a right way and a wrong way to install perimeter drainage. Unfortunately, many foundation contractors and home builders labor under a false sense of security, reasoning that if complaints about leaky basements don’t surface within the first year or two after a project is completed, their construction techniques must be working. The fact of the matter is that basement water problems that occur within the first twelve months are usually related to waterproofing defects. Drain tile problems typically take many years to develop. Thus, many contractors have buried time bombs that will eventually blow up in their faces.

Holes DownAlthough porous cement-based tile is still in use today, most residential contractors would agree that perforated 4-inch-diameter plastic pipe produces tighter joints and is easier to work with. Not all would agree, however, on which direction to place the holes in the pipe when installing footing drains.

The answer depends on the type of pipe. Flexible HDPE (high-density polyethylene) is slotted all the way around, and some rigid PVC has a pattern of holes around the entire circumference. With these types of drain tile, there is no “right” direction because there are openings on all sides. Plugged holes on the bottom are cleared by water entering through the sides and top.

The most popular drain tile, however, is rigid PVC that has just two parallel rows of holes close together along its length. The classic approach is to lay this type of drain tile with the holes facing down, in the five-o’clock and seven-o’clock positions. This allows a rising water table to enter the pipe at its lowest point.

Filter fabric. While hydrostatic pressure helps to flush silt from the pipe, all buried drain tile should be surrounded with coarse gravel or crushed stone, and wrapped with a filtering material. Without a filter, silt will contaminate the stone and eventually enter and plug the holes in the pipe (Figure 4).

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Figure 4. Without a filter to keep silt from contaminating the surrounding stone, drain tile can be rendered useless within just a few seasons (left). Pipe that is pre-wrapped or “socked” with filter material will prevent drain tile from becoming plugged (above).

Various geotextiles are available in rolls, and pre-wrapped or “socked” pipe — pipe that is manufactured with a filter sleeve already in place — is also available.

Drain Tile Location

Filter paper and properly oriented perforations, however, will not guarantee that drain tile will work. The pipe must also be installed carefully and in the right location with respect to the footing and any interior slab.

From a pure engineering point of view, the ideal place to lay exterior drain tile is alongside the footing, because water from a rising water table enters the pipe sooner (Figure 5).

Pipe Even with Top of FootingFigure 5. The best location for rigid drain tile is alongside the

footing. Minimum requirements for stone cover depend on

whether the tile is flush with the top of the footing (top) or the

bottom (middle). In either case, the top of the interior slab should be at least 6 inches

above the top of the drain tile. The pipe can be laid level or

pitched slightly.Where drain tile must be located lower than the bottom of the footing (bottom), Pipe at Bottom of Footing

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avoid undermining the footing by keeping the pipe outside of a 60-degree angle measured from the corner of the footing.

This location also requires more stone cover for the pipe.

Pipe Below Footing

The drain tile does not need to be sloped, although a slight pitch helps keep the pipe clear of silt and clay (particularly when the pipe has just two rows of holes on the bottom). Avoid trying to slope flexible drain tile, however, because you can inadvertently create dips and sags that will eventually collect silt and clog the pipe (Figure 6).

Figure 6. Regardless of the type of pipe used or its shape, unfiltered drain tile can easily be plugged with silt and clay (left). Water-seeking roots from trees growing too near the foundation can also completely clog perimeter drains (right).

In fact, undulating drain tile can result in premature failure of the drainage system. This problem is more pronounced when trees are growing close to the foundation, because wet silt and clay accumulating in low spots become targets

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for water-seeking tree roots in dry periods or in dry climates. In a relatively short period of time, tree roots can completely plug drain tile.

Some contractors create an even lower elevation for the tile by digging a small trench next to the footing. To avoid undermining the foundation, however, most codes require that the tile be placed outside a 60-degree angle from the footing.

Drain tile can also be placed on top of the footing. The advantage here is that the tile will be as level as the footing — a good strategy when using flexible pipe (Figure 7).

Figure 7. To keep flexible drain tile from developing low spots that will collect silt, place it on top of the footings, making sure that the top of the pipe is not higher than the top of the interior slab.

But this higher placement doesn’t control a rising ground water table as effectively, and may require raising the elevation of the interior slab.

Specialty drainage products.

Today there are several products on the market, such as Form-A-Drain (CertainTeed Corp., P.O. Box 860, Valley Forge, PA 19482; 800/233-8990; www.certainteed.com), that provide both the footing form and the drain tile (Figure 8).

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Figure 8. Form-A-Drain stay-in-place footing forms ensure a level perimeter drain and have a larger capacity than pipe systems (left). To control hydrostatic pressure, dimpled drainage panels fastened against the foundation wall carry water from the backfill into the perimeter drains (right).

These systems not only ensure that the drainage system is level, they often provide more flow capacity than traditional pipe systems.

On sites where an exceptionally high ground water table creates intermittent hydrostatic pressure on the foundation walls, dimpled sheets can be used in conjunction with standard drain tile. These membrane systems provide a waterproof barrier while also directing excess ground water from higher up on the foundation walls into the perimeter drains.

Discharging Collected WaterCapturing ground water in a perimeter drainage system is only half the battle — once you’ve collected water in the drain tile, you have to dispose of it somewhere. Discharging water into sanitary sewer systems is generally illegal, which leaves two basic ways to get rid of the water: On sloped sites, you can extend unperforated drain tile to daylight and discharge the water on the ground; on flat sites, you can collect the water in a sump basket and pump it to a discharge area away from the basement.

Gravity discharge. Two elements are critical to proper function of a gravity drainage system. First, although the perforated drain tile around the foundation itself may be level, solid pipe running from the foundation to daylight should slope at the rate of 1/16- to 1/8-inch per foot. Second, the open end of the discharge line should prevent entry by rodents, frogs, snakes, and reptiles. One method is to cover the exposed end of the pipe with 1/4-inch hardware cloth. Alternatively, you can bury the end of the pipe in crushed stone, which will allow the water to seep out below grade.

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Pumped discharge. While gravity discharge to daylight is cheap and easy, I recommend installing a sump basket as a backup. A submersible sump in the bottom of the sump basket connects to a hose or rigid pipe system that carries the collected water out of the basement. If you provide for the collection sump at the time the foundation and slab are placed, the pump and discharge piping can be installed later if needed.

The sump basket should be located inside the foundation, where it can pick up ground water that rises under the slab. On a flat site where all ground water must be pumped away, water from perimeter drains should also be directed into the sump through drainage sleeves in the footing (Figures 9a & 9b).

Figure 9a. An interior sump basket picks up excess water flowing through sleeves in the footing.

Figure 9b. A submersible pump connected to a hose or rigid pipe discharges the water on the ground away from the foundation.

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To avoid having to excavate later, be sure to place sleeves before the footings are poured. Use 4-inch-diameter pipe, and space sleeves 6 to 8 feet apart around the entire perimeter of the footing. In special cases where the slab is placed a foot or more above the top of the footings, you can locate sleeves in the foundation wall. Although water passing through the sleeves or under the footing will generally find the sump basket on its own, I recommend an interior drain pipe at the perimeter, terminating in the sump basket.

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Setting outSetting out is a fairly quick exercise in the scale of things, but obviously it has to be done with great care.  A few of the things that I will be mentioning below might sound like I am writing a how to for dummies.  Believe me mistakes made in the setting out can come back to haunt you.I have heard of houses being built on the wrong blocks.  I have a friend who when he was planning an extension to his house, found out that his side fence was 1.5M out of position at one corner, in his neighbours favour.  The neighbour had a nice brick barbecue built partly on the wrong block land.  the neighbours was understandably peeved when he was asked to remove it. (Plus the expense of new fence etc.).

Setting out - A concrete footing with a profile in the foreground.  In this case it uses star pickets for the pegs and the timber is fixed with tek screws through the holes in the picket.  Quick, solid, easy and reusable.

First checks, before you start.

I am assuming that the relevant permits to build have been obtained.

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You should have an accurate block plan, with the lengths and angles of all the boundaries marked on it.Use it to check every fence line.  You may have to buy or hire a long tape measure for this.  I have used steel ones in the past, but they are expensive and prone to damage.  I have had a 50M fibreglass one for years.  It is not as accurate as a steel one (they stretch a touch) but it is indestructible.Every time that I have built on a boundary line (not a fence, but a proper building) I have had a certified surveyor to do a check.   I just don't trust the existing fences.  The very last job that I did, our surveyor pitched up an 85 to 50 discrepancy in one of the boundary walls. (The builder who put it in was a bit like me, and on the cautious side, he gave us the odd 50mm just to make sure). I could quite easily have assumed that the wall was OK and built partly on his block.

Preliminary site works.

First off tidy up the site, remove all trees that are in the way.  We all love trees on our blocks, but don't try to save ones that are just too close to the new building.  They are an absolute pain to work around with scaffold etc. and they usually have to go in the end anyway.

On the drawing there is always one point and one line, or two reference lines that cross one another, given to start the set out.Usually they are referenced to a couple of boundary lines, or an existing building.Go around with a few steel pegs and bang them in near enough at the corners of the proposed slab. From them mark out roughly the area of the job.

Somewhere on your drawings or in the building specifications there will be a clause that says something to the effect that you should strip the area of the work plus an extra 1m all around, of all topsoil and deleterious materials to a min. depth of 150. before starting to place the compacted approved fill.  This means get rid of all vegetable matter, grass tree roots etc.

Get a machine in to clean up and remove top soil.

first layout

1. Go around again and put in pegs for the corners more accurately this time. 2. At this stage your pegs can be short wooden things that you can tap a nail

in, steel rods, even screwdrivers or just besser blocks placed on the ground.

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3. Anything that will give you the positions of the corners. 4. As you do this do checks for square. (See section on squaring below). 5. At this stage, if the job is small and you getting machinery in for

excavation work, you may put string lines between the pegs, and mark out the lines of trenches, or pier holes with lime.

6. You could then get the excavation work done first, before going on to the next stage.

Profiles, batter boards or hurdles.

Setting out - A plan of a concrete slab showing the profile positions.

Profiles consist of pegs, stakes or pickets, driven into the ground, with cross piece of timber attached to them.

Like formwork they are only temporary and as such they don't always look too neat, made up of all sorts of odds and ends and yet they have to contain quite a lot of information, even on a simple house extension.

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Setting out - A 3D view showing the use of profiles on the above slab.

They are used to transform the original pegs in the ground to something that is a semi permanent but accurate reference of the important sizes, measurements and offsets etc for a particular stage of a job. In the case above, when the concrete slab is poured the profiles can then be removed, because further measurements can be made from the actual concrete.

For the slab drawn, you may have marked on the profiles, before the excavator starts work:-

1. The position of all the foundations, for external and internal walls. 2. The wall positions to let the plumber accurately to position his sewerage

pipes and floor wastes.  To let the concretor place wall starter bars in the slab or column HD bolts.

3. Possibly underground power supply and entry point.

The first trade to use the profiles will be the excavator, so a reasonable clearance between the work and the profile itself is needed, to allow the machine to do it's job without squashing a hurdle her of there. In the sketches shown here I have shown them marked out with the overall sizes of a concrete slab. They could just as easily have a set out for the width of excavation trenches etc.

Quite often a profile may consist of a board nailed to an existing boundary fence.  There is no absolute rule, just something that can be marked out, take a nail or a screw and is fairly robust so it can't get moved out of position easily.

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Setting out - A hurdle or profile used for setting out.

In most cases the guy on the job, say the plumber setting a floor waste or the carpenter fixing the perimeter formwork, will use a spirit level to plumb down from the string line to his job.

Many times, in sloping ground or when working in an excavation where it is hard to use a level, then the plumb bob can be very effective.  I have used it in basement type situations where I simply hang the plumb line off the profile lines. I use a tie wire hook through a slip knotted loop to adjust the length of the plumb line.

A lot more convenient than straight edges and spirit levels.

Getting it square.

In my opening paragraph I mentioned that mistakes in setting out can come back to haunt you.  One of the classic mistakes is getting the floor plan out of square.  It has repercussions for the roofer, but worse still is when you have the floor tiler in towards the end of the job and you can't escape the fact that the width of the

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tiles varies from one end of a wall to the other.  Far easier to get it right in the setting out, right in the concrete (check the formwork before the pour) and right when laying out the internal walls.

We mainly work with rectangles in building work.  A rectangle has the following attributes which help us in setting out building work.

1. Each of the four angles is 90 degrees, or square as we call it.Great, this means that if we have a base line side set up, and then get a second line set up off it at the right spot, at 90 degrees to the first we have two of our side done.

2. The opposite sides are parallel, that is, they are the same distance apart.That is it! Measure the correct length from one end of the first side and the same length from the other end and we have our third side fixed.  Do the same again and we have our fourth side.  That could be it - finished.

3. The diagonals are equal lengths.  That is the length from one set of opposite corners is the same or equal to the length from the other set.  This means that if we have got a touch out with either of the first two steps (as you do:-), we have an excellent way of checking our set out

Diagonal measures work well when they are at a reasonable angle.  When the rectangle is long and narrow then the angle is flatter this way is a less reliable way of checking.

Setting out - Using a 3,4,5 triangle

Look at the sketch here.  Let's say it is a plan of a house slab that you have to form up.

You should first have an idea of which will be the most important side, that the rest are made square and parallel to it.

Split it into rectangles and check the diagonals for equal. The dotted red lines.

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Also on this plan I have incorporated a right angle triangle known to builder's the world over as a three four five triangle.

In our setting out of the rectangles in the sketch you can clearly see that each rectangle could be also seen as two triangles.

Two thousand five hundred years ago the ancient Greek philosopher and mathematician Pythagoras discovered his truths about right angle triangles.

Anyone with a cheap student's calculator can use Pythagoras' theorem to get the length of the diagonal side if you know the lengths of the other two sides. (The sum of the squares of the two shorter sides is equal to the square of the other).

There is a unique triangle that is often used to demonstrate this principle, 3,4,5. ( (3x3=9)+(4x4=16) 9+15=25, so does 5x5=25)

So any triangle in this ratio, will have a 90 degree corner opposite the long side.

Ratio is the key word. The units don't matter as long as the ratio is the same. 3,4,5 miles, or 3,4,5 centimeters, the right angle is still there. So If we are looking at a house plan in the sketch above, I could use say

2M as my unit. I would bang a nail in the formwork 8M (that is 4units of 2M each) from the

bottom right hand corner. Another one would go in at 6M (3units of 2M) along the other edge. I would check the distance with my long tape between the two nails and if

it was within 5mm of the 10M (5units at 2m) that I was expecting, I would modestly say to my offsider, "crikey we were lucky there mate".

If it was a bit more out like 40mm or so then I would move the nail holding my string line on the profile to correct the error, and then work from the altered line, thinking myself lucky that I had checked the set out before we had done a lot more work.

Back to top

Offsets

Setting out - Offset line around obstacles.

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It is not good practice to use only a short section of an existing wall to get a line that is a continuation of it.  Far better to try to use as much of the existing wall as possible, even if the wall has obstruction in the way.  The way to do it is simple, offset the line around the obstacles.Even if the wall is clear, using an offset is the more accurate way, because there could be local bumps in the wall that could throw your line off.

← In the sketch above I show a method of using the profile to offset the line to miss obstacles and then reset to get the point that is a true extension of the main building line.

← I want to mark out, on my profile (to the left), a building line for a new extension, that is exactly a straight continuation of the main house wall.

← I just pick a distance from the house that gets me clear of the various obstructions. In this case it is 145mm.

← I make sure the string line is offset the same distance from the house,and then I put a nail or a screw into my profile at the continuation of that line. That is I place a line parallel to the wall 145 off it.

← I then measure back the offset distance to get a point exactly in line with the wall.

← I bang a nail into a brick joint or a peg touching the wall, and string a line, shown dashed, to fix the true building line.

One a similar vein, sometimes when you have to get machinery into a job, rather than just pull out the profiles that are in his way, first set up some offset or recovery profiles. Say you set the recovery profile 6M away from the real one, then it is a case of a simple 6M measurement offset to get back to the original set out, and reinstate the original profiles, rather than starting from scratch again, squaring etc.

REFERENCES

http://www.servicemagic.com/article.show

www.wikipedia.org/wiki/drainage system

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CONCLUSION

In the end ;

We know what the meaning of drainage system.

We learn types of underground drainage system.

Setting out at the site is important.