Flat roof falls and ponding calculator
Work out the fall a flat roof should have over its length, and how much standing water weighs on the deck when it ponds.
The fall figures follow the British Standard for flat roofs, and the water load is simple arithmetic. Neither tells you whether the structure can carry it; that takes a structural engineer. A roof condition survey records the drainage, and where the deck is concrete, a RAAC survey shows whether it is RAAC.
Fall over a length
How much the roof should drop between the high point and the outlet.
- Drop over the run
- Gradient
- At 1:80, the least the finished roof should have
Falls from BS 6229:2025. The finished roof should fall at 1:80 or more once deflection and construction tolerances are allowed for. Where the deck has had no structural analysis, falls are designed at 1:40 or more, and a small roof, up to 50 m², should aim for 1:60.
Standing water
What a pond on the roof weighs, and the load it puts on the deck.
- Weight of water
- Volume
- Load on the deck under the pond
Worth a survey. Ponding that is still there after the rest of the roof has dried points to a low spot, a blocked outlet or a deflecting deck. The deeper the pond, the more the deck bends, and the more water it holds next time.
1 mm of water over 1 m² weighs 1 kg. A 25 mm pond is 25 kg on every square metre it covers.
These results are a guide only and don’t replace a survey or professional advice.
How to use it
Pick the fall
1:40 is the design fall where the deck has had no structural analysis, and a small roof should aim for 1:60. 1:80 is the least a finished roof should have. Then enter how far the water has to travel to the outlet.
Read the drop
The tool gives the drop in millimetres over that run. Compare it with what the roof actually has, from the drawings or a level survey.
Measure the pond
Pace out the pond and estimate its depth. The tool gives the weight of the water and the load it puts on the deck underneath.
What it tells you, and what it cannot
The arithmetic is simple. What it means for the roof takes a survey.
It tells you
- The drop a roof needs over a given run at the standard falls
- The fall as a gradient, in degrees and as a percentage
- How much a pond weighs, and the load per square metre it adds
- Whether ponding is worth taking seriously, and what usually causes it
It cannot tell you
- What fall the roof actually has, which needs levels taken on the roof
- Whether the deck can carry the load, which is a structural question
- Whether the membrane under the pond is sound, which needs a leak detection or moisture survey
- Why the pond is there: a blocked outlet, a sagging deck or a roof laid without falls
Questions about falls and ponding
What fall should a flat roof have?
The British Standard for flat roofs, BS 6229, calls for a finished fall of at least 1:80. Its 2025 edition removed the advice to design to 1:40 to achieve 1:80. Where the deck has had no structural analysis, falls are designed at 1:40 or more, so that deflection and construction tolerances still leave the roof draining, and a small roof, up to 50 m², should aim for 1:60. The tool gives all three.
Does ponding damage a roof?
Water that stays on a roof after the rest has dried keeps the membrane and the laps wet, holds dirt and growth, freezes in winter, and adds weight that bends the deck further, which deepens the pond. Most membranes tolerate some ponding, but it shortens their life and it marks where the roof is lowest.
How heavy is standing water?
One millimetre of water over one square metre weighs one kilogram. So a pond 25 mm deep adds 25 kg to every square metre it covers, and a pond across 60 m² weighs one and a half tonnes.
Can the roof carry it?
That depends on the deck and the structure, and only a structural engineer can say. The tool gives the load so the question can be asked properly. Where the deck is RAAC, standing water is one of the risk factors the IStructE lists, and a RAAC survey is the right next step.
How do I find out what fall my roof actually has?
From the drawings, if they exist and the roof was built to them, or from levels taken on the roof as part of a measured survey. The tool then shows how far short it falls.
Get the right survey for your roof
Whether you’re investigating a problem or planning ahead, we’ll help you choose the right survey for your roof. Get in touch for a free quote.
Roof condition surveys
Every defect photographed and located on a plan of the roof, with ponding and drainage recorded.
About condition surveysRoof measurement
Outline the roof on the map for its area and perimeter, to size the pond or the whole roof.
Measure a roofAll measurements, calculations and estimates provided by Roofcheck’s online tools are for guidance purposes only and should not be relied upon as definitive or technically accurate. Results should be independently verified through a professional roof survey or assessment before undertaking any works or making decisions. Roofcheck accepts no liability for any loss, damage or costs arising from reliance on these results, to the fullest extent permitted by UK law.