Choosing the Right Subsurface Drain Pipes

Choosing the Right Subsurface Drain Pipes

Over the centuries sub surface drain pipes have been made from wood boards (box drains), circular clay or horseshoe shaped ceramic tiles, bricks, concrete tiles, corrugated plastic or perforated pipes, bituminised fibre or smooth plastic pipes, and (no doubt), various other materials. Modern drain pipe and accessory design criteria are however well established and clear, in respect of pipe size, perforation pattern, pipe material, and geometry. But the materials used can still be diverse. Modern drain and sewer pipes are still available in a variety of different types and materials including asbestos, iron, concrete, UPVC and vitrified clay. For much of the 20th Century most domestic drains in the UK were made from clay piping and later on from plastic. UPVC drain piping has become more popular since the 1980s. Less commonly however drains are sometimes still made from pitch fibre, asbestos, concrete, HDPE (High Density Polyethylene) GRP (glass fibre reinforced plastics), and cast (CI) or ductile iron. The main materials now however are Clayware, plastic and UPVC. Usually domestic clayware drain pipes have plain ends. Both ends of such plain ended drain pipes are identical in appearance and rely on couplings to join them together. Socketed drain pipes have ‘male’ and ‘female’ ends, which join together. The ‘female’ end of the drain pipe points upstream. A large number of older drain constructions still have socketed clay drain pipes measured in old imperial dimensions. Builders merchants now stock adapters that allow metric plain ended clayware drains to be connected on to the old socketed clayware ones.

Which type to Choose

Clay drain pipes with flexible plastic coupler joints

Clay drain pipes with flexible plastic coupler joints

Both plastic and clayware drain pipes have satisfactory hydraulic properties, so price and the relative strengths of each type of drain pipe are usually the main consideration when it comes to choosing the right drain pipes for your project.. Having said that it’s also wise, (particularly for the amateur), to consider ease of laying.. Vitrified clayware drain pipes are much more robust than plastic and are much less liable to become deformed when buried. So clayware drains rely much less on the performance of the granular bedding to bear external loads. Clay pipe drains can usually be laid straight into a trimmed and formed trench (class D bedding) but plastic drains have to be surrounded by small gravel or pea shingle. The essential building materials required to complete such work can cost more than the fittings and drain pipes themselves.

PVC  Drain system

PVC Drain system

Some professionals maintain that the range of gullies, junctions, bends, and other fittings available for clayware drains is more extensive than for drains manufactured from plastic. Professionals also regard the ‘play’ which clayware drains offer as being as being better than rigid plastic drains. There are however adaptor couplings which will join clayware and plastic drains together. Plastic drain pipes are obviously much easier to handle than clayware drains which are heavy, and have to be cut with a power saw or special pipe cutters. Clayware however is fragile and accidental breakage during handling is common. Until about 25 years ago clayware drains held the dominant share of the market but plastic has largely eroded its position in recent years. Clayware drains however are staging a comeback on the back of claims that they are more environmentally friendly, have lower energy costs in manufacture, and use less aggregate for drain backfilling and bedding. The Clay Pipe Federation is also focussing its marketing on the advantage that clayware drains are entirely resistant to rat and other rodent damage and are more robust when subjected to high pressure jetting which is now the most popular means of cleaning sewers and drains. The best clayware drains are capable of coping with pressures of up to 7.500psi. It’s usual to replicate the same drain pipes when extending or servicing an existing drain installation, but in general clayware drains are slightly cheaper than plastic for new projects, and are usually more flexible in their application. Most domestic drains are made from 110mm plastic pipes but if the drains are shared with other properties 150mm drain pipes may have been laid. Sometimes 150mm, drain pipes are also used when the drain is likely to be subjected to higher than normal loads. But drainage pipes up to 600mm are readily available from some suppliers. In some circumstances where ground movement or differential settlement is anticipated is anticipated it may be worth considering a more specialist flexible drain pipe which requires fewer joints but in circumstances such as this research the requirements carefully and if necessary take professional advice. Once the drain is laid it can be costly to rectify any miss judgement.

Clay drainage with plastic sockets

Clay drainage with plastic sockets

Bedding and Filling

A number of different types (classes) of bedding are recommended for drains. Each is designed to be suitable for different ground conditions, loadings, and types of drain piping. For more details check Approved Doc H of the Building Regulations. Drain fill material should be free of any stones bigger than 40mm, pieces of clay more than 100mm, and any frozen material, organic matter, and bits of wood. When using granular material for sidefilling and pipe bedding, you must to ensure that it conforms with BS EN 1601 (Annex B Table B15).

We explain the laying methods for the 3 most common types of sub surface drain piping in our article ‘Laying Drains’. Methods of cutting, jointing and handling, vary between different types of pipe and the fittings that come with them but the general principles are broadly the same. Pipes need to be laid in straight lines and to a steep gradient. A laser line can be used to maintain drain alignment level and accuracy, but a sight rail or a string line will suffice. Pipes must be laid in a full granular bed and never propped up on bricks or stones. And drain pipes must be set into bedding or aligned with bedding packed under the pipes to bring it to the level shown by the guide line.  But before embarking on the process of laying the drains the first thing to consider is the to ensure that the minimum Gradient and cover is available. Minimum Gradient for surface water drainage is 1:100. The minimum for foul water drains is 1:80 when there’s a WC connected, or 1:80 if there’s no WC. Minimum cover varies depending on the use to which the land is put and the type of drain pipe used. Owing to their robustness clayware drains require less cover than plastic.

Minimum Cover over Clayware Drains

300mm for Gardens and Fields

400mm for drains laid below drives and light roads

Minimum Cover over Plastic Drains

600mm in Gardens and Fields

900mm for drains laid below drives and light roads

All Drains laid under a road carrying traffic require at least a full metre of cover

Minimum Cover compliance is essential. If the required cover cannot be achieved the drains will need to be encased in concrete.