Roof Drainage Basics

About this course.

Roof drainage works by using the slope of the roof and of the drainage components to collect the rain that lands on the roof and carry it, by gravity, off the roof and away from the building to a safe discharge point — and it matters because a roof sheds a large volume of water very quickly during rain, and if that water is not collected and carried away, it will find its way into the building or undermine its surrounds. The system is conceptually simple — catch the water at the roof edge or in valleys, funnel it into downpipes, and take it away — but it must be sized, laid and detailed correctly to cope with heavy rain, and it must include safeguards for when it is overwhelmed or blocked. When roof drainage is right, it is invisible; when it is wrong, it is one of the more damaging and common building problems.

The basic path of water. Rain lands on the roof and runs down its slope to the low edges (for a pitched roof) or to internal drainage points (for a flat or complex roof). At the eaves, gutters catch this run-off and carry it along their length, falling gently towards outlets. From the gutters, water drops into downpipes, which carry it down the building. The downpipes discharge to the site's stormwater drainage — an underground pipe system, a legal point of discharge, or (increasingly) to rainwater tanks, on-site detention or infiltration systems, depending on the site and local requirements. On larger or flat roofs, box gutters, sumps and rainheads collect and channel water to downpipes. At every stage, gravity and correct falls move the water along, and the components must be sized to handle the flow.