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A Guide to Installing an Effective Underground Drainage System

Depending upon whether you are installing a new drainage system or intending to make alternations to an existing drainage network; you should always ensure that you liaise with the Building Control Department at your local Council. And present plans which outline the scope of works to be executed, making sure that the works comply with the latest Building Regulations, since fundamentally the works will need inspecting afterwards and passing off. Conversely if you are simply replacing sections that are damaged then this alleviates the necessary aforesaid notification.

Types of Drainage Systems

Generally speaking, drainage water can be classified as being one of two types – foul or surface. Foul water is classified as being any soiled water coming from your bathroom, kitchen or utility room; and is always conveyed to a foul water drain. Whereas with surface water this is simply rainwater.

Some properties – predominantly the older ones, have a combined drainage system whereupon the rainwater is discharged to the foul drain via a series of gully traps to prevent foul odour escaping from the drain.

Modern houses and systems keep the two drainage systems separate and rainwater is either discharged to a soakaway, watercourse or surface water sewer. Typically speaking a soakaway is no more than hole in the ground which has been backfilled with coarse stone; thereby facilitating the percolation of the surface water back into the ground.

Planning Beforehand

Before digging up your garden or wet surface retaining area, always make sure you have the appropriate drawings showing the new network layout. Consult an expert if unsure and always wear the appropriate safety equipment.

Underground Drainage Pipes & Fittings

To ensure compliance with the Building Regulations and prevent the works from having to be rejected by the Building Control Department; all drainage pipes and underground drainage fittings should be BROWN in colour.

Modern underground drainage systems rely upon a network of flexible plastic pipes, inspection chambers, rodding points to cleared ensuing blockages and access fittings. And since such materials are susceptible to deformation under load, then it’s imperative that there is adequate cover of at least 600mm. The backfill material used to provide the cover should allow percolation i.e. pea shingle or other granular type stone is primarily used.

The trenches themselves can either be hand dug or preferably made using a small excavator or micro digger. The depth should adhere to the Building Regulations and anything deeper than 1,200mm should always be shored up or support – by law. Preferably the sides should be dug at angle of repose which will safeguard a little against the excavated material collapsing in on itself, which could result in serious injury or fatality. The bottom of the trench should be filled with granular material to serve as a bedding material to receive the pipe and underground drainage fittings; and ideally to aid conveyance of the water in the pipe it should be laid at a slight gradient.

The drainage pipes should be accurately measured and cut to size using a hacksaw and mitre block. When cut, the edges of the newly cut pipe will be rough so need chamfering with a wood file at an angle of 45 degrees and the excess burrs removing. Wiping the pipe with a cloth should suffice.

Fittings, traps and junctions should sit on a paving slab to provide adequate support from lateral soil pressures, and prevents any cracking of the pipe.

Underground drainage pipes and their associated fittings can easily be connected but require both the male (the pipe) and female (the fitting) ends lubricating to receive one another. When lubricated they should slot together with relative ease.

Maintenance and Repair

The underground drainage network is liable to cracking and general deterioration over time, so the network should include adequate inspection chambers and rodding access points to remove any blockages that could build up over time.