Getting rainwater off the roof, down to the ground and away from the building is not always as simple as it seems. All the rain falling on the building has to be led down to ground level without spilling, and into a below-ground drainage system. That system has to be able to cope with potentially large and sudden influxes of storm water. This Good Building Guide shows how to avoid some common pitfalls in roof drainage systems and soakaways for new housing. It covers design, sizing and installation of roof drainage systems. Designing and constructing soakaways is briefly described (this is covered in more detail in BRE Digest 365 - DG365). 6 pages.