What fall does a flat garage roof need?

A fall of 1 in 80 means the roof drops 1 unit for every 80 units it runs. Over one metre, that is 12.5 millimetres. Across a garage 5 metres deep, it comes to about 62 millimetres between the high edge and the low edge. A fall of 1 in 40 doubles that: 25 millimetres in a metre, and about 125 millimetres across the same 5 metres.

These are small numbers, and the eye will not spot them, which is why a flat roof can look fine and still pond. A spirit level held on the deck tells you more than a view from the drive does.

The scope of BS 6229:2025 is roofs, flat or curved, that are no steeper than 10 degrees and carry a continuously supported flexible waterproof covering. On a garage that means felt, EPDM and GRP, the usual candidates on our page about the best garage roof material.

The 2025 edition of BS 6229 retains 1:80 as the minimum finished fall, covering general areas as well as internal gutters, as the trade summary from December 2025 reports. The 2018 edition was withdrawn on 16 December 2025, so advice that quotes it is out of date.

The design figure is higher on purpose. IKO's guidance, written under the 2018 edition, gives 1:80 as the recommended minimum but adds that the design should target 1:40. GreenSpec also says at least 1 in 40. The gap exists because a deck rarely finishes exactly as drawn. Boards deflect and joists settle a little, and a design that aims at the minimum can end up below it once the work is done.

So when someone describes a garage flat roof as built to 1:80, ask what fall they are building in, and what the finished roof will actually measure. The two answers should differ, and the second matters more.

There are three common routes. The first is to lay the joists themselves to a slope, which works best when the whole roof is being rebuilt. The second is to add tapered timber strips, called firrings, on top of level joists. The third is to use tapered insulation boards over a level deck, which gives a fall and warmth in a single layer.

A garage whose joists already slope needs only a deck and a covering. One with level joists needs the extra layer. The choice depends on the height available at the door lintel and the wall, because extra depth at the high edge eats into the room left for the weather details.

A fall can run one way, usually towards the gutter, or be split with a gentle crossfall to two outlets. At the survey we measure what exists and photograph it, then explain which route fits your garage and what each one adds to the work.

Water leaves the roof into a gutter, and the gutter has its own slope. Brett Martin's guidance is 1 mm for every 600 mm, with supports no more than 900 mm apart, while FloPlast gives 1:350 for its MiniFlo system. A safe reading is roughly 1:600 to 1:350, depending on the system.

On a short garage run, one outlet at the low end usually does the job. A long run, or one that serves a double garage, may need a stop end and an outlet at each end, or one in the middle.

Gutters fed by a flat roof also catch grit from the membrane edge and leaves from nearby trees, so an autumn clear stops overflows. If your garage gutters are the old grey cement type, read our answer on asbestos garage gutters before anyone touches them.

Look for water still lying on the roof a day or two after rain. Look for tide marks, green algae in the same patch, a dark band along the edge, or a soft, spongy feel in the boards seen from inside. Each suggests water is sitting where it should be running off.

Ponding on its own is not proof of a leak, yet it wears out a covering sooner, and it is a fault to correct when the roof is next renewed. Our garage roof repair or replace page explains when a repair holds and when a new fall calls for a new roof.

The survey gives you photographs and a plain verdict, and a written quote follows with a single price. Where the fall can be corrected without rebuilding the deck, that is what we recommend. The wider job is described on the garage roof replacement page.