In this exclusive extract from NBS Shortcuts externallink, written by noted industry figure Austin Williams, we introduce the correct connection of domestic gravity-fed wastewater drainage.

'Is your home polluting the Environment?' Or even more accusatorially, 'Are YOU polluting London's rivers and streams?' Thus runs the Environment Agency's Lord Kitchener-esque campaign against foul water contamination of surface water systems.

You have to wonder what some of the European Standards are actually for. For example, BS EN 12056 Gravity drainage systems inside buildings is a suite of five parts. While Part 2 deals usefully with the actual layout and calculations of sanitary pipework, Parts 1 and 3 contain too little information to merit separate documents (nothing that couldn't have been added to Part 2 ). Part 4 on 'wastewater lifting plants' is limited. Part 5 on testing, admittedly, is reasonably handy. However, all of them replicate the glossary and descriptions; and most of them are so generalised as to be verging on trite ('thermal movement shall be considered'; 'noise shall be taken into consideration', for instance). The reason for the vagueness of this European Standard is that national and local regulations are expected to fill in the substantial issues - Building Regulations (England and Wales) and Scottish Technical Standards apply as outlined in the National Appendices. Where national and local regulations aren't referenced or don't exist (after all, the national standards from only 11 out of a total of 25 EU countries are cited) BS EN 12056 hints at a target level of basic provision.

That basic provision should ensure that every wastewater drainage system serving a building is 'designed and constructed in such a way as to ensure the removal of wastewater from that building without threatening the health and safety of the people in and around the building.'

According to Thames Water, plumbing mistakes have resulted in waste appliances being connected to the surface water system in around 1 in 10 houses in London, (and as many as 1 in 3 houses in some districts). This is eerily reminiscent of the 19th century when Dr John Snow discovered that the cause of cholera - a disease killing more than 10,000 Londoners between 1853 and 1854 - was cross contamination of the ground water supply with wastewater. Mercifully, we don't draw water from wells any more, but if the figures are to be believed, there is an accident waiting to happen. This article looks at above-ground wastewater connections for standard domestic arrangements, although Approved Document H (AD H) does stipulate slightly different requirements for domestic buildings over 3 storeys.

The National House Building Council (NHBC) requires that the installation of internal soil and waste systems be in accordance with AD H. Once again, the Scottish Technical Handbook (Section 3: Domestic: Wastewater Drainage), amended in 2007, contains the most contemporary information written in a straightforward format. Ironically it relates design and maintenance issues back to BS EN 12056-2. However, there is less core 'technical' information in the Scottish standards and so here we outline some of the essential requirements of AD H.

Primary ventilated stack system

This was previously known as the single stack system. The discharge stack and branch pipes are sized to avoid the need for separate ventilating pipes (see Box). In dwellings especially, this system saves space and costs but must be designed within the limitations of unventilated pipework, summarised in BRE Digests 248 and 249.

The distances of sanitary fittings from the discharge stack can be exceeded if using resealing (or antisiphon) traps and branch pipe air admittance valves although these will require access for maintenance.

BS EN 12056-2 (National Annex ND 3.1) allows external pipework on buildings up to three storeys, although internal discharge stacks and branches are preferred. To minimise noise transmission, NHBC Standards (8.1 S8[c]) recommend 25 mm of insulation and a duct casing of at least 15 kg/m2 around soil pipes passing through a bedroom or living room. However, locating air admittance valves in sealed boxing will inevitably affect the performance of the valve and BS 8313 clause 12 states that ducts for discharge branches and stacks should be ventilated (except for ducts with a cross sectional area of less than 0.05 m2 in houses, offices and shops, other than food shops).

Coming back to the Environment Agency's concerns, foul drainage must not be connected into rainwater drainage, but where combined drainage exists, it is sometimes a simple and economic solution to connect rainwater from small areas of roof into a foul drainage stack. Before this course of action is considered, the drainage and Building Control authorities must be consulted to ensure that the sanitary pipework and below-ground drainage will not be overloaded during rainstorms.

About this article

The full text of this NBS Shortcut is exclusively available as part of NBS Building Regulations externallink.

NBS Shortcuts is a new series of illustrated "how-to" articles and guides, covering a wide range of practice, regulatory and design guidance. The easy to follow text and detailed hand-drawn graphics will aid any building designer. They are available online as part of your subscription to NBS Building Regulations.