Drainage - NEW

Peter Eade suggests that while drainage may not be top of your list when planning a build, if it is designed and installed correctly it will function effectively for years.

Drains are an essential component of a house build. Domestic drainage not only includes foul drains from the house but also rainwater disposal from the roof gutters. All foul drains and waste pipes must conform to Part H of the Building Regulations and the design is best if it’s kept simple.

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The positioning of the proposed drains within the curtilage of the property will be shown on the approved Building Regulation plans. Before starting the task of setting out the build it is vital to know where the drains are planned. Is it to be a mains drain connection or possibly a septic tank or sewage treatment plant? Levels need to be taken to ensure it will be possible for the drains to naturally fall from the house to the outfall. This information will determine the height of the finished ground floor of the house.

Drains should be laid in straight lines and to an even gradient with an inspection chamber at each change of direction.

The drains should be laid in straight lines and to an even (minimum 1-in-40) gradient and should include an inspection chamber at each change of direction. In addition, further cleaning eyes may be required as it’s important that every part of the underground drains can be cleaned by using drain rods. The basic drain layout consists of a 100mm diameter pipe laid to a fall that flows from the head of the drain in the house to the external connection to the public sewer or private tank. If there is a problem with achieving a natural fall to the drain outfall a domestic pumping station may be required.

Below-ground Drains

When the foundation walls are being built ducts must be provided to allow the drains to exit the house. These ducts must be large enough to ensure the pipe passes freely through each wall. PS lintels need to be built in over each of the openings.

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At ground level, close to where the drain passes through the external wall of the house, there should be the first inspection chamber.

At ground level, close to where the drain passes through the external wall of the house, there should be the first inspection chamber, the depth of which (invert) needs to be at least 600mm below finished ground level.

The minimum depth of any drain is 600mm. 

The minimum depth of any drain is 600mm. Trenches for the drains can be excavated using a mini-digger but the bottom of the trench needs to be checked by hand to ensure there is a constant gradient. Before any pipes are laid a minimum of 100mm of granular material (pea shingle or similar) needs to be placed in the trench.

Once all the underground drains are laid, Building Control will need to see them and may ask for an air test to be carried out. Once the drain installation is approved the trenches can be backfilled, but first more granular fill is required at least up to the top of the pipes. Immediately above the granular fill, there should be a 100mm layer of stone-free soil followed with further soil, free of any stones over 40mm.

If it is intended to make a connection to public mains, permission must be sought from the relevant authority, which is likely to nominate an approved contractor to make the connection. Ideally, the main drain connection needs to be in place before any other drain work begins. If a septic tank is being planned a soil percolation test will be required to ensure that the soakaway drainage field is able to deal with the effluent. If the percolation test is not successful a mini-treatment plant or cesspool may have to be considered.

Off-mains Drainage

If mains drainage isn’t an option an alternative means of disposing of the sewage is required. This is likely to be a septic tank or cesspool. The tank can be no nearer than seven metres from the dwelling and no further than 30m from the access point for the sewage disposal truck.

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Septic tank installation.

Septic Tanks

The choice of tank will be largely down to a percolation test. If there is acceptable drainage then a septic tank is by far the better option. The capacity of the tank is dependent on the number of occupants; typically a house with four people would require a capacity of 2,700 litres, this being increased by 180 litres for each additional user. The tank can either be made of glass fibre and delivered by a truck or can be built on site using concrete blocks with a cast concrete top. If the tank is to be constructed on site the building inspector will need to approve the design before work starts.

The most important aspect of a septic tank installation is the drainage field which disposes of the effluent.

The soakaway system is made up of a series of 100mm perforated pipes laid to a uniform pattern and to a gradient of 1-in-200. The pipes are laid on 300mm of clean shingle or possibly broken stone which is graded between 20mm and 50mm. The perforated pipes must be laid in trenches up to 900mm wide to a minimum depth of 500mm below ground level. Once the pipes are in position further shingle or stones need to be laid over them before covering with a layer of geotextile to prevent silting up. The trenches can then be backfilled with earth. 

Cesspools

If a cesspool is planned the capacity will be somewhat greater than a septic tank, with 18,000 litres required for two persons, increasing by 6,800 litres for each additional occupant. Cesspools fill fairly rapidly and are likely to need emptying at least once a month.

Sewage Treament Plants

A better alternative to a cesspool might be a sewage treatment plant, the process of which is similar to the way that a septic tank works. The treatment plant breaks down solids to produce a cleaner and more environmentally friendly potable effluent which, subject to Environment Agency approval, can be discharged into local water courses or ditches.

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A mini-treatment plant converts raw sewage into potable water.

Where the position of the house makes it impossible to achieve a natural fall to the drain outfall a domestic pumping station may be required. The supply and installation of the plant are best carried out by a specialist. 

Above-ground Drains and Soil Pipes 

Above-ground drains consist of a 100mm vertical soil pipe which is connected directly to the toilets; all other waste pipe connections are in either 32mm for hand basins or 40mm for baths, showers and sinks. The soil stack within the house that is furthest from the underground drains has to provide ventilation and the pipe passes up through the roof and is open to the atmosphere. Further vertical sub-stacks may be required which do not need ventilation but they must be fitted with air admittance valves.

Once the vertical pipes are in position, connections can be made for the 32mm and 40mm waste pipes. This is done using boss connections which are solvent welded and strapped to the 100mm vertical pipes. All waste pipes must run to a fall and be designed with cleaning eyes at each change of direction; each sanitary fitting needs to be fitted with a trap. Baths and showers require traps that have a seal depth of 50mm. A kitchen sink, dishwasher and washing machine require a seal of 75mm.

Waste fittings can be either solvent welded or push-fit and need to be secured with support brackets.

When all of the above-ground pipes are installed the Building Inspector will want to see them

When all of the above-ground pipes are installed the building inspector will want to see them and again may request an air test.

Storm Water Drains and Gutters

drainage-guttering.jpgThe rainwater from the roof is collected by the gutters and flows to ground level via downpipes. From each downpipe, there is an underground pipe that takes the water at least five metres from the house and discharges it to a soakaway.

A soakaway should be at least 5m from the house.

The soakaways are square or circular pits, either filled with crushed rubble or soakaway crates and covered with geotextile and backfilled with soil. Some local water authorities allow the storm water to discharge into the foul drainage system, but this must be checked and approved before any connections are made.

The recovery of rainwater and grey water is worth considering and there are systems on the market that provide the pipes, filters and tanks that are required. It makes sense to recycle grey water where a septic tank or cesspool is proposed.

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A rainwater harvesting tank buried underground. The grey water can be used on the garden.


This Beginner's Guide to Drainage is from the August 2021 issue of SelfBuild & Design magazine.

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