The minimum fall for a flat roof carrying a rooflight is a finished 1 in 80, meaning the surface drops one unit for every 80 it runs. Because timber deflects under load, firrings are cut on site and deck boards are never perfectly true, the sensible practice is to design to 1 in 60 or even 1 in 40 so the finished result is still at least 1 in 80 after everything settles. Water must fall away from all four faces of the rooflight upstand, not just across the roof overall. A roof laid dead flat, or to 1 in 80 on paper that finishes flatter, will pond, and ponding is the start of nearly every flat rooflight failure.
That figure comes from Building Regulations Approved Document C and BS 6229, which both require a flat roof to have a designed fall so water discharges. The important distinction, and the one that catches people out, is between the fall you design to and the fall you actually end up with.
Design fall against finished fall
Design fall is the slope drawn on paper or set out on site before anything moves. Finished fall is the slope you have once the roof is built, loaded and settled. They are not the same number, and the gap between them is why the design figure is steeper than the minimum.
- Timber deflects. Joists and firrings bend under the weight of the roof build-up, the covering and any snow or standing water, and they bend most in the middle of a span, which is often where a rooflight sits.
- Firrings are cut on site. The tapered timbers that create the fall are cut by hand, and a small error over a long run adds up.
- Decks are not perfectly true. Board thickness and fixing pull the surface out of a perfect plane.
Design to exactly 1 in 80 and any of those will drop the finished fall below the minimum in places, so the roof ponds. Design to 1 in 60 or 1 in 40 and there is enough margin that the finished fall still clears 1 in 80 everywhere after settlement. This is why a competent installer never designs to the bare minimum.
| Fall | Drop per metre | Use |
|---|---|---|
| 1 in 80 | 12.5mm | The minimum finished fall required |
| 1 in 60 | 16.7mm | A common design fall, allowing for deflection |
| 1 in 40 | 25mm | A conservative design fall on longer spans |
| Dead flat | 0mm | Not acceptable, ponds by definition |
Fall on all four faces, not once across the roof
A rooflight upstand is a solid obstruction in the drainage plane, and water has to run past it, not into it. The common mistake is to measure the fall once across the roof, get a good number, and assume the rooflight is fine. A roof can fall correctly overall and still have a flat pocket in exactly the spot the rooflight sits in, especially where the deck has deflected between joists. So the fall is checked on all four faces of the opening individually. Water must run away from each face, or at minimum away from three and clear past the fourth to an outlet. An upstand sitting in a dished area is a bath with a glass lid. Why that happens is on why flat rooflights pond water.
We check the fall with a laser on all four faces before we quote. If your roof already ponds, fitting a rooflight into it is spending money on a problem nobody has solved. Either the fall is corrected with firrings or tapered insulation, or we tell you plainly the roof will still pond, just not against your new unit.
What we do when the fall is wrong
On a retrofit into an existing roof, the fall is often not right, particularly on 1970s and 1980s roofs where firrings were skimped or laid the wrong way. There are two honest corrections, both made while the covering is back during the work.
- Re-firring a local area. Where the roof falls correctly overall and only the area around the opening is dished, the deck comes up in that bay, tapered firrings are cut and fixed, and the deck goes back. The cheapest correction.
- Tapered insulation. Where the deck is flat and cannot be re-boarded, tapered insulation boards are laid to a designed scheme, typically 1 in 60 or 1 in 40, so the falls converge on the outlet and run clear of the upstand. A permanent fix that also lifts the roof’s thermal performance.
The limit on re-firring is height: raising the deck eats into the upstand height above it, so if reinstating the fall would drop the effective upstand below 150mm at the low point, the upstand is rebuilt taller at the same time. The full detail is on ponding water around a flat rooflight.
Suffolk and north Essex specifics
The estate roofs at Kesgrave, Rushmere St Andrew and Chantry, built in the 1960s to 1980s, are where we most often find falls that were skimped or laid the wrong way from new, so the fall check on those is not a formality. On the coast at Felixstowe and Harwich, correct falls matter even more, because standing water holds salt against the covering for longer than it would inland, accelerating corrosion of fixings and degradation of the bond line. A ponding roof near the sea fails faster than a ponding roof inland.
Common questions
What is the minimum fall for a flat roof rooflight?
A finished fall of 1 in 80, which is a 12.5mm drop per metre, as required by Approved Document C and BS 6229. Because timber deflects and firrings are cut on site, installers design to 1 in 60 or 1 in 40 so the finished fall still clears 1 in 80 after settlement. Water must fall away from all four faces of the rooflight upstand.
Can a flat rooflight go on a dead-flat roof?
It can be fitted, but a dead-flat roof ponds by definition, and standing water against the upstand is the start of nearly every flat rooflight failure. The right answer is to correct the fall with firrings or tapered insulation. Where that is genuinely not possible we fit on a taller upstand and tell you plainly the roof will still pond, rather than fitting and hoping.
Why design to a steeper fall than the minimum?
Because the finished fall is always less than the design fall. Timber deflects under load, firrings are cut on site, and decks are not perfectly true, all of which reduce the slope you end up with. Designing to 1 in 60 or 1 in 40 leaves enough margin that the finished result still meets the 1 in 80 minimum everywhere after the roof settles.
Get the fall checked
Every survey we do includes a laser check of the fall on all four faces of the opening, not once across the roof. If your roof already ponds, we tell you what correcting it costs before you commit to a unit. Send us the roof size, whether it is new or a replacement, and where the property sits.
Request a fixed quote in Ipswich, Suffolk or north Essex. The wider job is on flat roof skylight installation.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.