The short answer
The recommended minimum finished fall for a flat roof is 1 in 80, which is about 0.72 degrees. Because decks sag and builders work to tolerances, roofs are designed to 1 in 40 so the completed slope still reaches 1 in 80. The slope is made with timber firrings or tapered insulation laid over the deck.
The figures: 1 in 80 finished, 1 in 40 designed
The guidance most roofers work to comes from BS 6229:2018, the British Standard for flat roofs with continuously supported coverings. Manufacturers quoting it give the recommended minimum fall as 1 in 80, which is 0.72 degrees. That means a drop of 12.5 mm for every metre of run.
That figure describes the finished roof, not the starting point. A deck is never perfectly flat. Joists deflect under load, boards sit slightly proud of each other, and the work itself carries small tolerances. If you set out a roof at exactly 1 in 80 on paper, real-world variation leaves parts of it flatter than that. The standard answer is to design to 1 in 40, which is 1.44 degrees, so that, once the allowances are taken off, the finished roof still achieves 1 in 80.
Best practice under the same standard is that every roof surface, gutter beds included, is designed with a fall of 1 in 40 to achieve finished drainage falls of 1 in 80. These are the figures the manufacturers publish, and they are typical guidance, not a promise about any one roof. Your own survey will say what your deck allows.
Why water needs somewhere to go
Ponding is standing water left on the surface once the rain has gone. A thin film that is gone by the next morning is normal on plenty of roofs and need not worry anyone. Deeper, longer-lasting pools are a different matter. Water that sits over a seam keeps pressing on it, dirt and algae build up in the same place, and in winter the water freezes and expands against the membrane.
A flat roof with enough fall gets rid of most of that. Rain runs to the edge or to an outlet, the surface dries faster, and the membrane spends less of its life wet. Our page on flat roof ponding looks at what causes pools and when they point to a bigger problem.
Fall is also a question of where the water ends up. A roof that slopes towards a wall rather than a gutter, or towards a gutter of poor size or one not set level, will pond at the low end whatever the surface slope. The fall and the drainage are designed together, which is why a roofer looking at your roof checks the outlet and gutter as well as the surface.
Creating a fall with firrings
On a timber roof there are two ways to build the slope. The first is to set the joists themselves at an angle, so the deck slopes from the start. This is the neatest answer on a new build, though it needs the wall heights to match.
The second, much more common on repairs and re-roofs, is to use firrings. A firring is a tapering strip of timber, thin at one end and thick at the other, fixed on top of the joists so that the deck laid over it slopes. Firrings run in the direction of the fall, and they are cut or bought to give the slope required across the width of the roof.
Roofers measure the fall with a long straight edge and a level, or with a laser. They work out how much height difference the roof needs from one end to the other and choose firrings to suit. A wide roof needs a bigger total difference than a narrow one, because the fall is a ratio and a long run drops further. That extra height also affects the edge upstand and the abutment against the house wall, so the details are drawn up before work begins.
Where the existing deck is sound but too flat, firrings can sometimes be added on top without stripping the structure. If the deck has dipped or the boards have rotted, replacing the deck and building the fall in is often the more reliable way.
Creating a fall with tapered insulation
Tapered insulation does two jobs at once. It is made as boards that vary in thickness, with a built-in slope. Laid over a flat deck, it forms the fall and insulates the roof in one layer. That is the usual method on a warm roof, with the boards resting on the deck and the waterproof layer laid over them.
Suppliers typically cut tapered boards to a layout plan for your roof, with each board numbered. The plan includes crickets, which are small raised ridges that direct water around obstacles such as rooflights and chimneys, so that water does not collect behind them. If the plan is followed, the fall is accurate and the membrane laid over it is supported evenly.
Tapered insulation also suits roofs where the deck itself cannot be changed, such as a concrete slab. The slope is then created without touching the structure. Our guide to torch-on felt roofs explains how a felt system is laid over this kind of base, and the same principle applies to other membranes.
The thickness is the trade-off. Tapering means the thick end of the roof is raised by the full amount of the taper plus the insulation. That can affect door thresholds, window cills and the height of the parapet. It is something to check early, and the survey picks it up.
When a roof has too little fall
A roof with no discernible fall is one of the clearest reasons to rebuild the deck rather than simply re-cover it. If a membrane is laid over a flat or dipped deck, the same pools return. Laying new material over the same shape often buys only a short time.
It is not always possible to get the full design fall on an older roof. A low parapet, a high door threshold or a wall that was built to a fixed height can limit how much slope the roof can carry. In those cases the aim is the best fall the building allows, with extra attention to outlets and gutter levels, so water cannot gather in the worst place.
Fall also matters if you plan to walk on the roof or use it as a terrace. A roof built to take foot traffic still needs to drain, and the finishes added for people can hold water if the underlying slope is shallow. Read our answer on whether you can walk on a flat roof for how access is built in without spoiling drainage.
On survey day we inspect the deck from above and from the room beneath where we can, note where water has stood, and explain the fall your roof can reach. Afterwards you receive one fixed price, in writing. For more on how flat roofs are designed and fitted across Bath, see our flat roofing service. If the work changes the height of the roof, our note on flat roof planning permission covers when to ask the council.
