Minimum Pitch for a Flat Roof: What the Rules Say

Despite being called 'flat' roofs, all flat roofing systems require some degree of slope to ensure water drains off the surface effectively. Getting this wrong is one of the most common causes of premature roof failure.

The Minimum Requirement

British Standards (BS 6229) and NHBC guidance specify a minimum fall of 1:80 — that's 12.5mm of height difference per metre of length. In practice, many roofing contractors specify 1:60 or 1:40 to allow for inaccuracies in construction.

Why Insufficient Falls Cause Problems

A roof without adequate falls will pond water. Standing water degrades most roofing membranes over time, adds structural load, and encourages moss and algae growth. It also increases the risk of water finding a way through any minor weakness in the membrane or flashings.

Creating Falls

Falls can be created in several ways: by the structural deck itself (tapering the timber framework), by using tapered insulation above the deck (very common in re-roofing), or by using screed to create falls on concrete decks.

Tapered Insulation

Tapered PIR insulation is a popular solution for both new roofs and re-roofing projects where the existing structure doesn't have adequate falls. The insulation is cut in a wedge profile to create the required slope. It also helps to achieve the required U-value.