Flat Roof Fall Ratio Calculator
Calculate the correct drainage falls for flat roofs per BS 6229:2018 and UK Building Regulations Part H
Required Fall Height
Fall in Millimetres
Fall in Centimetres
Slope Angle
Slope Percentage
What is a Flat Roof Fall Ratio?
A flat roof fall ratio determines the gradient needed for proper water drainage on flat roofs. Despite their name, flat roofs must never be completely level. They require a slight slope to prevent water pooling, which can cause leaks, structural damage, and premature roof failure.
The fall ratio is expressed as a proportion, such as 1:80, meaning the roof falls by 1 unit for every 80 units of horizontal distance. This translates to 12.5mm of fall per metre of roof length. Proper drainage falls are mandatory under UK Building Regulations Part H and British Standard BS 6229:2018.
UK Building Regulations for Flat Roof Falls
Minimum Fall Requirements
According to Building Regulations Part H (Drainage and Waste Disposal) and BS 6229:2018, all flat roofs in the UK must incorporate adequate drainage falls. The regulations specify a minimum finished fall of 1:80, with a design fall of 1:40 recommended to compensate for construction variations.
| Standard | Design Fall | Minimum Finished Fall | Equivalent |
|---|---|---|---|
| BS 6229:2018 | 1:40 | 1:80 | 25mm per metre |
| BS 8217 | 1:40 | 1:80 | 12.5mm per metre |
| Building Regs Part H | 1:40 | 1:80 | 0.72° minimum |
Why Design at 1:40 for a 1:80 Finish?
Construction never achieves perfect accuracy. Roof decks deflect under their own weight and load, timber can warp slightly, and installation tolerances mean the theoretical design may differ from the finished result. Designing at 1:40 provides a safety margin so that even with these imperfections, the roof still achieves the minimum 1:80 fall required for effective drainage.
A 1:40 fall equals 25mm per metre run. For a 10-metre roof span, this requires 250mm of fall from the highest to lowest point. When combined with insulation and roofing materials, this can significantly affect roof height at abutments and perimeters, requiring careful planning of upstands, kerbs, and damp-proof courses.
How to Create Falls on Flat Roofs
There are several methods to achieve the required drainage gradient on flat roofs. The optimal approach depends on roof size, complexity, whether the deck requires replacement, and budget constraints.
Firring Pieces Method
Tapered timber battens (firring pieces) are fixed to the structural deck to create the fall. This method works well for smaller roofs or when the deck needs replacement due to deterioration. Firring pieces are cost-effective for straightforward layouts but become complex with long drainage runs or roofs requiring multiple fall directions.
Tapered Insulation Method
Cut-to-falls insulation boards are manufactured with precise tapers to create the drainage gradient. This system offers flexibility for complex roof shapes, multiple outlet positions, and changes in fall direction. Tapered insulation eliminates the need for timber firring and can accommodate valleys and hips more readily than cut timber.
Screeding to Falls
Applying a tapered screed layer over the structural deck creates the necessary gradient. This method suits refurbishment projects where the existing deck remains in place. However, screed adds significant weight to the roof structure, which must be verified by structural calculations, and deflection under load can reduce the finished fall.
Common Problems with Inadequate Falls
Ponding Water
Water that remains on the roof for more than 48 hours after rainfall is considered ponding. This adds dead load to the roof structure, accelerates deterioration of waterproofing membranes through UV exposure and freeze-thaw cycles, and creates an environment for algae and moss growth.
Structural Stress
Ponded water significantly increases roof loading. A 25mm deep pond across a 10m² area adds approximately 250 litres (250kg) of weight to the structure. In winter, if this water freezes, expansion can damage the waterproofing layer and substrate.
Membrane Degradation
Constant water exposure reduces the lifespan of roofing membranes. Single-ply membranes, built-up felt systems, and reinforced bitumen all suffer accelerated ageing when subjected to permanent or frequent ponding. This can void manufacturer warranties and necessitate premature roof replacement.
Frequently Asked Questions
What is the minimum fall for a flat roof in the UK?
The minimum finished fall for a flat roof in the UK is 1:80 (12.5mm per metre), as specified in BS 6229:2018 and Building Regulations Part H. However, roofs should be designed with a fall of 1:40 (25mm per metre) to account for construction tolerances and ensure the minimum 1:80 is achieved in practice.
How do I calculate the fall needed for my flat roof?
Multiply the roof length by the fall ratio. For a 1:40 fall, divide the length in millimetres by 40. For example, a 6-metre roof equals 6,000mm ÷ 40 = 150mm of fall required. Alternatively, use 25mm per metre for 1:40 (6m × 25mm = 150mm) or 12.5mm per metre for 1:80.
Can I have a completely flat roof with no fall?
No. UK Building Regulations and British Standards prohibit flat roofs without drainage falls. Even roofs described as “flat” must incorporate a gradient to prevent water accumulation. Zero-fall roofs experience ponding, leaks, structural overload, and membrane failure.
Do internal gutters need the same fall as the main roof?
Yes. BS 6229:2018 specifies that internal gutters should be designed to the same criteria as the general roof area, with a design fall of 1:40 to achieve a minimum finished fall of 1:80. Internal gutters are particularly vulnerable to ponding and blockage, making adequate falls essential.
What happens if my flat roof has less than 1:80 fall?
Roofs with insufficient fall will experience water ponding, which reduces membrane lifespan, increases structural loading, creates leak risks, and typically voids manufacturer warranties. If an existing roof has inadequate falls, refurbishment should include measures to correct the drainage gradient through firring pieces, tapered insulation, or deck replacement.
How does roof deflection affect the fall?
All roof decks deflect under their own weight and imposed loads such as snow, maintenance foot traffic, and plant equipment. This deflection reduces the effective fall, particularly at mid-span between supports. Designing at 1:40 instead of 1:80 provides a safety margin to counteract deflection and maintain adequate drainage even after settlement.
References
British Standards Institution. (2018). BS 6229:2018 Flat roofs with continuously supported flexible waterproof coverings. Code of practice. London: BSI Standards Limited.
British Standards Institution. (2017). BS 8217:2005 Reinforced bitumen membranes for roofing. Code of practice. London: BSI Standards Limited.
HM Government. (2015). The Building Regulations 2010: Drainage and waste disposal (Approved Document H). London: NBS.
British Standards Institution. (2000). BS 12056-3:2000 Gravity drainage systems inside buildings. Roof drainage, layout and calculation. London: BSI Standards Limited.