Despite their name, flat roofs are not actually completely flat — and understanding why is important for anyone considering this type of roofing system. The term “flat roof” is a common misnomer that refers to a roof with a very shallow pitch, rather than a perfectly level surface. Here’s what you need to know.
Why Flat Roofs Are Not Truly Flat
A truly flat roof would be a disaster from a drainage perspective. Water that cannot drain off a roof surface will pond, and standing water is the primary enemy of any roofing membrane. Over time, even a small amount of persistent standing water will accelerate the deterioration of almost any flat roof covering, leading to leaks, premature failure, and costly repairs.
For this reason, building regulations and roofing industry standards require all “flat” roofs to be constructed with a minimum fall (slope) to ensure adequate drainage. In the UK, Building Regulations Approved Document H recommends a minimum fall of 1:80 for flat roofs, though most roofing manufacturers recommend a minimum design fall of 1:40 to account for construction tolerances and long-term deflection of the roof structure.
How Much Fall Does a Flat Roof Need?
A fall of 1:40 means the roof drops 1 unit in height for every 40 units of horizontal distance — so a 4-metre wide flat roof would have a height difference of 100mm (10cm) from the high point to the low point. This is enough to ensure water drains effectively towards the outlet, gutter, or drainage point without being visible or creating significant structural challenges.
What Happens If a Flat Roof Has Insufficient Fall?
If a flat roof is constructed without sufficient fall, or if the roof structure deflects over time causing low spots to develop, water will begin to pond. While modern flat roof membranes such as EPDM, GRP, and single-ply systems are designed to be waterproof even under standing water, prolonged ponding significantly shortens their service life. It also increases the risk of debris accumulation and biological growth, which can block drainage outlets and worsen the problem.
How Is the Fall Created in a Flat Roof?
The fall in a flat roof can be created in several ways. Tapered insulation boards — cut at an angle to create a consistent slope — are the most common method used in modern flat roof construction. The fall can also be built into the structural deck using timber firring pieces, or by designing the structural elements themselves at the required angle. Tapered insulation is particularly effective as it combines the required drainage fall with the necessary thermal insulation in a single layer.
Flat Roof Materials for Proper Drainage
Choosing the right flat roof system and ensuring it is installed with adequate fall are both critical to long-term performance. North East Roofing Supplies stocks a comprehensive range of flat roof materials designed for use in properly designed and drained flat roof systems. Browse our product range or visit one of our North East branches for expert advice. You can also contact our team with any technical questions.
