Flat Roof Drainage: Types and How They Work

Every flat roof needs a drainage strategy. Without effective drainage, water sits on the roof surface, degrades the membrane, adds structural load, and eventually finds a way inside. Here's how flat roof drainage works.

Falls and Slopes

The foundation of flat roof drainage is adequate falls. Building regulations require a minimum fall of 1:80 — that's 12.5mm of height for every metre of length. Tapered insulation is now commonly used to create falls in the insulation layer itself, which is particularly useful on existing flat roofs with insufficient fall.

Perimeter Drainage (Gutters)

The simplest and most common approach: the flat roof falls toward one or more edges, where water discharges into a conventional gutter and downpipe. This works well for domestic flat roofs where the perimeter is accessible for maintenance.

Internal Drainage Outlets

Larger flat roofs often drain to internal outlets — essentially a drain set into the roof surface, connected to a downpipe that runs through the building. These must be properly waterproofed at the membrane junction and have strainer covers to prevent blockage.

Overflow Outlets

Building regulations require flat roofs to have secondary overflow provision. This is a secondary outlet set slightly higher than the primary, which allows water to escape safely if the primary drain blocks. Without this, a blocked drain can cause dangerous roof loading.

Maintenance

All drainage components must be cleared at least twice a year — more frequently if there are nearby trees. A blocked drain is the most common cause of flat roof flooding and the easiest problem to prevent.