The four most common roof areas catered for are 'Single', ' Hipped', 'Valley' and 'Double Valley' as illustrated below.

Reciept says he bought 40mm pipe, label on the display says 40mm pipe, didn't even know 43mm waste pipe existed, BS 5627: 1984 – Specification for plastics connectors for use with horizontal outlet vitreous china WC pans

He then proceeds to cut it to the size he needs, pokes it through the hole in the wall, and tries to connect it up to the trap supplied with the bath, pipe is too big, BS EN 1455-1 – Plastic piping systems for soil and waste DISCHARGE (low and high temperature) within the building structure – ABS. The grey area denotes the area of roof that is calculated. For a standard semi or detached house, with a 'Single' roof design, two calculations, one for each side, are required. Other roof areas can be accommodated, by using a combination of these four modules. BS EN 1451-1 – Plastic piping systems for soil and waste DISCHARGE (low and high temperature) within the building structure – Polypropylene (PP). Specifications for pipes, fittings and the system.

BS EN 1453 – Plastics piping systems for soil and waste discharge (low and high temperature) within the building structure. BS EN 5254 – Specification for polypropylene waste pipe and fittings (external diameter 34.6 mm, 41.0 mm and 54.1 mm) BS EN 681 – Elastomeric seals. Material requirements for pipe joint seals used in water and drainage applications. Part 1 vulcanised rubber BS 4514: 2001 – Unplasticized PVC soil and ventilating pipes of 82.4 mm minimum mean outside diameter, and fittings and accessories of 82.4 mm and of other sizes. SpecificationBS EN 1329 – Unplasticized poly vinyl chloride (PVC-U). Specifications for pipes, fittings and the system. So he tries the other end, fitting it into the boss in the soil pipe, too big, then he notices printed on the side of the pipe is says it's 43mm.