Flat Roof Fall
FALL · GRADIENTYour Results
FALL · PER METRE · ANGLEA 1:80 design fall is a common minimum; build to 1:40 to allow for deflection.
Fall · Gradient · Drainage
Flat Roof Fall Calculator
“Flat” roofs are never truly flat — they need a slight fall so water drains. Enter the roof length and the fall ratio (for example 1:80) to get the total fall, the fall in millimetres per metre, the gradient percentage and the angle in degrees.
- Fall ratios like 1:80, 1:60 or 1:40 supported
- Total drop, mm per metre, gradient % and angle at once
- Answers “fall over 4m” and similar questions
- Guidance on recommended minimum falls
How Flat Roof Fall Works
Flat roof fall is the gentle slope that moves rainwater to the outlets. It is usually written as a ratio — 1:80 means the roof drops 1 unit for every 80 units of length. The total fall over a length is simply:
total fall = length ÷ ratio
Over a 4 m roof at 1:80 the fall is 4 ÷ 80 = 0.05 m = 50 mm, or 12.5 mm per metre. The gradient is 100 ÷ 80 = 1.25%, an angle of about 0.72°. For a sloped (pitched) roof instead, use the main roof pitch calculator.
Recommended Minimum Falls
UK guidance (BS 6229) commonly targets a 1:80 finished fall, designed at 1:40 so that construction tolerances and deflection do not create ponding. Membrane manufacturers may state their own minimums — always follow the specification for your covering and local code.
Full Roof Pitch Calculator
Working with a pitched roof rather than a flat one? The complete rise-over-run calculator is below.
Roof Pitch Calculator
LIVE RESULTStandard run = 12 in
Your Results
PITCH · ANGLE · SLOPEConventional / standard slope
Drag the orange apex to change both rise and run, or the blue handles to adjust one at a time. The boxes above stay in sync.
Frequently Asked Questions
What is the minimum fall for a flat roof?
A common design target is 1:80, built to 1:40 to allow for deflection and construction tolerances so water does not pond.
What is 1:80 fall in mm per metre?
A 1:80 fall is 12.5 mm per metre, because 1000 mm divided by 80 is 12.5 mm.
How much fall is needed over 4 metres?
At 1:80, a 4 metre roof needs a 50 mm fall. At 1:40 it would be 100 mm.
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