What Fall Does a Flat Roof Need?

The fall of a flat roof is how much it drops over a given run, written as a ratio. A fall of 1:80 means 1 unit of drop for every 80 units across, so 12.5mm for each metre. Over a 4 metre run the far end sits 50mm lower than the high side.

Which standard sets that figure?

BS 6229 is the British Standard code for flat roofs whose waterproof layer is a flexible membrane resting on a continuous deck. Its scope stops at roofs steeper than 10 degrees. BSI shows the 2018 edition as withdrawn on 16 December 2025 and the 2025 edition as current, published on that date. The 2025 edition holds on to 1:80 as the lowest finished fall, across general roof areas and internal gutters.

Notice the word finished. The minimum describes the roof as it ends up, after the deck, the insulation and the covering are all in place, and not the angle it is set out at. An old reference to the 2018 code may turn up in a specification or a quote, but it is no longer the current edition.

A designer cannot count on perfect workmanship, so a margin is added. IKO, a membrane manufacturer, advises designing for 1:40 (it was citing the 2018 edition). That works out at 25mm per metre, twice the finished minimum, which is the margin the designer is buying.

Why not just design to 1:80?

Because things move. Timber decks deflect under load, joists shrink and settle, insulation compresses at the points where it is walked on, and laps in boards or sheets can steal a few millimetres. A roof laid out at exactly 1:80 may end up flatter in places, and water stands in exactly those places. Starting with more fall leaves room for the unavoidable loss.

The 1:40 figure is therefore the design target, and 1:80 is the floor the finished roof must not go below. Mixing them up is a common error: describing 1:80 as the design fall leaves no margin at all.

Our post on flat roof ponding in Somerset shows what happens when the margin is used up.

BS 6229 deals with flexible waterproof coverings, which means felt, single-ply sheets such as EPDM, and similar membranes laid over a continuous deck. It does not cover lead. A lead roof is a different system, with its own sheet codes, rolls or welts, and its own guidance on falls and bay lengths.

Does the same fall apply to a lead or zinc roof?

Not on the strength of this code. If your flat roof is a lead-covered bay or a metal porch, the fall comes from the metal sheet's own guidance and the details of the build, and a roofer who works in lead will know where to look. Do not assume that 1:80 transfers across.

Gutters follow separate rules again. BS EN 12056-3:2000, on roof drainage layout and calculation, is listed by BSI as current and sizes the drainage, which is a separate question from the deck's slope. Brett Martin, a guttering maker, suggests 1mm for every 600mm of rainwater gutter, which is a much gentler slope than a roof deck needs.

If the roof is not yet built, a designer or builder should state in writing which standard and which fall they are working to.

You can learn a lot without tools. After rain, a flat roof should be wet but not puddled within a day or so. A pool that stays for days, a tide mark of dried dirt or a patch of moss points to a low spot. A long spirit level on the deck, or a string line across it, shows the same thing more precisely.

Can a roof with poor falls be corrected?

Often, yes, when the roof is being re-covered. Tapered insulation, which is insulation cut in wedges to build a slope on top of a level deck, can create falls without lifting the joists. A new deck laid to a slope is the other route. Raising the finished level has knock-on effects at doors, windows and upstands, so it is worth planning early.

See tapered insulation for flat roofs and, if an outlet seems to be the cause, why a flat roof outlet overflows.

For a roof of your own, we survey it first, measure the falls, and show you what we find on the deck. One written price follows. The flat roofing page lists the work we do.