The short answer
The recommended minimum finished fall for a flat roof is 1:80, which is about 0.72 degrees. Because decks sag and builders work to tolerances, BS 6229 says to design for 1:40 so that 1:80 is still there once the roof is built. In plain terms, design for 25 mm of drop over every metre, and expect the finished roof to hold at least 12.5 mm.
The two figures: 1:40 on the drawing, 1:80 on the roof
People quote one number and get it wrong, so it is worth being exact. IKO, a membrane manufacturer, gives 1:80 (0.72 degrees) as the minimum to aim for on a flat roof. The code of practice, it says, wants the design set at 1:40, which leaves room for building tolerances, a deck that is not quite true and some deflection under load. Rockwool, citing BS 6229:2018, gives the same pairing: design at 1:40 to achieve finished drainage falls of 1:80.
It is not the other way round. A roof designed at 1:80 has no margin at all, so any small dip in the deck leaves water sitting on the membrane. The design figure is steeper precisely so that the finished figure survives real building.
Neither figure is a promise of a dry roof. They are the starting point for a roof that drains, and a specification for a particular membrane may ask for more.
What 1:40 and 1:80 look like in millimetres
A ratio of 1:40 means a drop of one unit for every forty units of run. Turned into figures you can check on site, it looks like this.
| Run of roof | Drop at 1:40 (design) | Drop at 1:80 (finished minimum) |
|---|---|---|
| 1 m | 25 mm | 12.5 mm |
| 2 m | 50 mm | 25 mm |
| 3 m | 75 mm | 37.5 mm |
| 4 m | 100 mm | 50 mm |
On a typical garden-room or kitchen extension, a 3 m run therefore wants roughly 75 mm of fall designed in. That is nearly the thickness of a hand, and it is why a flat roof that looks level from the garden may still be draining correctly.
The degrees are small, which makes them easy to misjudge by eye. IKO gives 1:80 as 0.72 degrees and 1:40 as 1.44 degrees. A level app on a phone will not measure that well on a rough deck, so we use a long straightedge and a level on the deck itself.
How a fall is actually built into the roof
There are three usual ways to get water moving, and the best choice follows the structure and the budget.
Sloping the joists
The joists themselves can be set or cut to a slope, or new timber firrings (tapered strips fixed on top of level joists) can be laid across them. This is the most direct route and is the one we prefer where the structure allows it.
Tapered insulation
Where the insulation sits above the deck, it can be supplied in tapered boards that create the fall within the insulation layer. Rockwool's guidance on compliance and tapered falls is aimed at exactly this method. The deck stays level, and the board schedule produces the slope.
Screeded falls
Sand and cement screed can be laid to a slope on a concrete deck, such as an older garage or a commercial roof. It adds weight, so the structure needs to be checked first. The Approved Document A guidance on roof loading notes that a structure unable to take extra load may show up as ponding, which is a good reminder that more weight on a tired deck can create the very problem the fall was meant to cure.
Why the finished fall can end up shallower than designed
A roof designed at 1:40 and finished at 1:80 has already used up half its allowance. Several things can use the rest.
Timber decks deflect under load, particularly across a long span or where the joists are slender. Boards can bow. Old roofs settle at the walls. A new membrane laid over an existing deck inherits every dip in it, which is one reason we survey the deck before talking about coverings.
Where the fall has gone, water lies in shallow ponds, usually in the middle of the roof and along the lowest edge. Standing water does not always cause an immediate leak, but it loads the deck, collects silt and algae, and ages the surface faster. If that sounds like your roof, our guide to flat roof falls and drainage deals with the ponding itself.
Getting the fall right when you replace a covering
When the covering is renewed, the fall is easiest to correct. The deck is bare, the levels can be checked, and adding tapered insulation or firrings costs far less than reworking the roof later. A new membrane also falls under the renovation rules of Approved Document L, so the insulation layer is likely to be part of the same decision. Our answer on whether you must insulate when replacing a flat roof sets that out.
The fall also affects how the roof can be used. A roof built to drain is still not a terrace, and one that will be walked on regularly needs different detailing again. We cover that in whether you can walk on a flat roof.
On a survey, we take levels across the deck, photograph any standing water and mark where the fall is lost. The written quote then includes tapered boards or firrings, and our 10-year workmanship guarantee, in writing, covers the finished roof. For the wider range of coverings, start from our flat roofing page.