Excerpts & references from BS EN 12056-2:2012.

[ It should be noted that these requirements also form part of The Building Regulations 2010 – Document H (2015 Edition)]

When designing pipe systems for use in the UK, it is imperative that the requirements of the above standard and Building Regulations are adhered to. To this end, some key guidance notes are included here for reference purposes. For full details, BS EN 12056-2 should be consulted before a soil system design is finalised.

Branch pipe bends and junctions

This section refers to the use of swept-radius branches

Bends in branch discharge pipes should be avoided, especially for single and ranges of wash basins, as they can cause blockages and increase self-syphonage effects.

When they are unavoidable, they should be of a large radius.

Junctions between branch discharge pipes of about the same diameter should be swept in the direction of flow using swept entry branches, with a 25mm minimum root radius, otherwise, 45˚ branches should be used. To minimise the risk of blockage, branches up to DN 40 size joining larger diameter horizontal branches of DN 100 or over should, if practicable, connect to the upper part of the pipe wall of the larger branch. For the same reason, opposed branch connection in the horizontal place to a main branch discharge pipe should be avoided.

Bends at the base of stacks

Bends at the base of a discharge stack should be of large radius (minimum centre line radius 200mm) or two 45˚ radius bends may be used. Increasing the diameter of the bend at the base of a stack is an alternative but this may oversize the drain and be uneconomic.

Branches at the base of stacks

Generally, for systems up to five storeys, the distance between the lowest branch connections and the invert of the drain should be at least 750mm, but 450mm is adequate for low rise single dwellings. For larger multi-storey systems it is better to connect the ground floor appliances to their own stack or the horizontal drain and not directly to the main stack. For buildings over 20 storeys high, it may be necessary to connect both the ground and first floor appliances in the same manner.


Offsets in the wet portion of a discharge stack should be avoided. When they have to be fitted, large radius bends should be used as described in ND.3.5.2. In a secondary ventilated stack system, connections to the discharge stack should be made above and below the offset. Offsets above the topmost appliance or branch connection do not require venting.