The short answer
The figure most often quoted for domestic flat roofs is a finished fall of 1:80, which is about 12.5 mm of drop for every metre of run. To make sure the finished roof still achieves that after the deck settles, designers usually build in 1:40, or 25 mm per metre. Less than that and water stands in pools, which is why ponding is so common on roofs built level.
Why a flat roof needs any fall at all
The name misleads. A roof built truly level would hold every drop of rain where it landed, and the surface would only dry by evaporation. A slight slope gives gravity something to work with, so rain runs to an outlet or a gutter and leaves. The less slope there is, the less margin there is for error, and a roof with no fall at all relies on the covering being perfectly flat, which no deck is.
The slope also protects the covering. Water that sits for days carries dirt, grows moss and algae, and finds any weak seam. A roof that drains properly keeps its surface cleaner and its laps drier. Our guide to how flat roofs drain water shows where the water goes once it is moving.
The figures in plain terms
A fall is written as a ratio. A fall of 1:80 means the roof drops one unit for every 80 units of run. Over a three-metre span, that is roughly 37 mm from the high side to the low. A fall of 1:40 doubles it, to about 75 mm over the same three metres. These are the numbers generally cited in roofing guidance for design and finished falls, and the difference between them is deliberate.
Decks move. Joists deflect under load, boards swell and settle, and a roof designed to 1:80 can end up flatter than that in places once the building has sat for a few years. By designing to the steeper figure, the finished roof is more likely to keep a working slope after movement. Some manufacturers set their own minimums for their systems, and a warranty can depend on meeting them, so the product data matters as well.
How a fall is built
There are two common methods. The first is to tilt the structure: joists are fixed at a slope, or tapered timbers called firrings (wedge-shaped strips) are laid on top of level joists to make the slope. The deck is then fixed to those. The second is tapered insulation, where boards cut to a gradient are laid on a level deck, giving a slope and a thermal layer together. The second approach is common on warm roofs.
Both methods work if they are set out properly before the covering goes on. The problem comes when a new membrane is laid over a roof that has already lost its slope, because the covering follows the deck and ponds again. What that looks like is described in our standing water guide.
What can undermine a fall
A sagging deck is the most common. Weak boards, undersized joists and rot all produce dips. A blocked outlet or an edge trim set too high can make a roof with a good fall behave as if it had none. Overlaying old roofs repeatedly can raise the surface at the edges until water has nowhere to go. In each case, measuring the fall is the first step. A spirit level and a straight edge give a rough read, and a laser or water level on the survey gives a more accurate one.
Valleys and gutters between roof planes need their own fall, usually a little steeper because they carry more water. Internal gutters that are laid too flat fill up and overflow into the building, which is a leak of a different sort.
Falls on different roof shapes
Roofs seldom drain in a single direction. A lean-to extension usually falls away from the house wall towards the garden, so the high point is against the wall and the upstand there is the tallest. A roof with a parapet on three sides may fall towards one corner outlet, which means the slope is diagonal and the figures are worked out along the longest run. A hipped or L-shaped plan needs two or more falls meeting at a valley, and the valley itself must fall to an outlet. These junctions are where mistakes are made, because a tidy fall on each plane can still leave a flat spot where they meet.
Getting it right on your roof
When we survey a flat roof, we measure the slope and look for dips, then show you the readings alongside photographs. If the fall is poor, the fix may involve new firrings or tapered boards, and we quote for that as part of the fixed written price rather than leaving it out and hoping. The Building Regulations side, where it applies, is ours to handle. The rest of our flat roof work is set out under flat roofing in Basingstoke.
Every roof is different. A free survey with photos settles it for yours.
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