What is the minimum fall for a flat roof with a rooflight?
The minimum finished fall on a domestic flat roof is 1:80, and to achieve that reliably the roof is designed to 1:40. That is the whole answer, and the gap between those two numbers is where most of the trouble on real roofs comes from. Where a rooflight sits in that roof, the fall matters more than it does across open field, because the kerb is an obstruction and anything the roof fails to drain will find its way to the base of it.
This page works through where the figures come from, how a fall is actually built, which direction it should run past a rooflight, and how to check what you already have. The rest of the installation sequence is on the flat roof rooflight installation hub.
Where 1:80 and 1:40 come from
BS 6229, the British Standard for flat roofs with continuously supported flexible coverings, sets a minimum finished fall of 1:80. That is a drop of one unit for every eighty across, so twelve and a half millimetres per metre.
The same standard recommends a design fall of 1:40, twenty five millimetres per metre, precisely so that the finished result is not less than 1:80. The doubling is not belt and braces. It is a construction tolerance allowance, and it exists because timber is not perfectly straight, joists are not perfectly level, decking is laid by hand, and every one of those small errors eats into the theoretical gradient. Design at 1:80 and you will finish somewhere below it.
Deflection under load, and the allowance for it
The second reason for designing steeper is that the roof moves. A timber flat roof deflects under its own weight, under the covering, under any imposed load, and progressively over years through creep in the timber.
Structural design typically limits deflection to span over 360 or fourteen millimetres, whichever is less. On a four metre span that permits eleven millimetres of sag at mid span. Against a designed drop of one hundred millimetres over four metres, eleven millimetres in the wrong place is a meaningful fraction of the gradient. Where the fall runs across the span rather than along it, that mid span dip sits directly across the drainage path and becomes a permanent shallow trough.
Firrings, tapered insulation and how a fall is built
There are three common ways to get a gradient into a flat roof, and they suit different situations.
- Firring pieces are tapered softwood strips fixed along the tops of the joists before the deck goes down. Cheap, standard, and the usual approach on a cold deck or a timber warm deck. The taper runs with the joist direction, so the fall direction is fixed by the way the joists span.
- Tapered insulation is a factory cut board system laid over a level deck, designed to a layout drawing with the falls, cross falls and any hips set out. It gives complete freedom over which way the water goes and it is the right answer where the fall has to run against the joist span.
- Sloping the structure by setting the joists themselves at a gradient. Used on new build where the ceiling below can follow or be levelled separately.
Setting the direction of the fall around the kerb
This is the decision that gets made badly most often. A rooflight kerb is a solid obstruction rising off the roof plane, and water arriving at it has to go around.
The fall should run so that the kerb sits across the flow as little as possible, and so that the upstream side of the kerb is not the low point of a catchment. On a rectangular unit that usually means running the fall parallel to the long axis, so water travels along the two long sides rather than damming against one. Where the unit is wide relative to the roof, cross falls are formed to split the flow and take it around both sides. Tapered insulation makes that straightforward to set out; firrings do not.
The back-fall trap behind an upstand
The specific failure to design out is a back-fall on the uphill side of a kerb. It happens when the kerb is set into a roof where the fall runs towards it, so the covering arrives at a vertical face and stops.
Water then sits in the corner between the roof surface and the kerb, permanently, and everything about that corner is now working under a head of water rather than under running water. Silt collects, moss establishes in it, and in freezing weather the ice expands into the angle fillet. The correct detail sheds water away from the kerb on all four sides, formed either with tapered insulation or with a purpose made cricket, which is a small triangular fall built on the uphill side to divide the flow.
| Gradient | Drop per metre | Status |
|---|---|---|
| 1:40 | 25mm | Recommended design fall in BS 6229 |
| 1:60 | 16.7mm | Acceptable design fall where tolerances are tightly controlled |
| 1:80 | 12.5mm | Minimum finished fall |
| Below 1:80 | Under 12.5mm | Ponding likely, treated as a zero fall design |
| Zero fall | 0mm | Only with a covering warranted for permanent immersion |
Checking the fall on an existing roof
You do not need instruments to get a useful answer. A two metre straightedge and a spirit level with a measurable gap at one end gives the gradient directly: a twenty five millimetre gap under a two metre straightedge is 1:80.
The more revealing test costs nothing. Look at the roof the morning after heavy rain. Where water is still lying, the roof is either flat there or falling the wrong way, and the outline of a dried pool tells you exactly where the low spots are. Do it before the rooflight is designed rather than after it is fitted, because a fall that is short is corrected during the covering work and not afterwards.
Zero fall roofs, and what they ask for instead
Some contemporary designs genuinely want a level roof, usually where the finished surface is a terrace with paving on pedestals or a walk-on deck. That is a legitimate design decision, and BS 6229 treats it as a distinct case rather than a failure.
What it requires is a covering system warranted for permanent standing water, which in practice means a hot melt or a specific single ply or liquid system with the manufacturer’s written agreement, plus more drainage outlets than a falling roof would need. The rooflight kerb in that situation is set higher, because the design water level is higher, and the upstand allowance is measured from the top of any standing water rather than from the membrane.
How the fall changes the kerb height
Fall and upstand height are two halves of the same calculation. The one hundred and fifty millimetre figure quoted across the industry assumes a roof that drains, and is set out in full on how high a rooflight kerb should be.
Where the fall is shallow, exposed, or historically prone to standing water, the kerb goes up rather than the fall coming down, because water depth against the upstand is what the height is resisting. On the Harlow and Basildon new town stock, and on the older flat roofed rear additions across Chelmsford, the fall is very often short of 1:80 by the time it is measured. That is a survey finding, not an obstacle, and it changes the detail rather than the answer.
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