What is the minimum fall for a flat roof with a rooflight?

Why the design figure is twice the finished one

The standard asks for a finished fall of 1 in 80 as the minimum that will actually shed water. Nobody can build to a minimum, so the design figure is set at 1 in 40 to leave room for everything that eats into it.

What eats into it is predictable. Joists deflect under their own weight and under snow. Timber shrinks as it dries. Firrings cut on site vary by a few millimetres. Insulation boards compress slightly under a walked-on covering. Add those together across a four metre span and you can lose half your fall without anybody doing anything wrong. Designing at 1 in 80 and hoping is how roofs end up dead level in the middle.

What 1 in 40 looks like on a real extension

The arithmetic is worth doing before anybody agrees a ceiling height. A fall of 1 in 40 is 25mm of drop for every metre travelled.

On a typical 4m deep rear extension that is 100mm of build-up to find between the high end and the low end, on top of the insulation and the deck. On a 6m roof it is 150mm, which is a real constraint on ceiling height and on how the roof meets the existing wall under a first-floor window.

Three ways of forming the fall, and what each costs you

Firrings are tapered timber strips cut from softwood and fixed along the top of each joist. They are cheap and they work, but they put the fall below the insulation, so the insulation and deck follow the slope and the ceiling below is either sloped too or dropped separately.

Tapered insulation is a cut-to-falls scheme delivered as a numbered board layout. It costs more and it is the better answer around a rooflight, because it can form cross falls and diverters that timber firrings cannot. Sloping the structure itself, by setting the joists at an angle or using a firred beam, gives the deepest fall for the least build-up but has to be decided at design stage.

The rooflight sits in the fall, not beside it

Here is where a rooflight changes the exercise. A kerb standing 150mm proud of a roof is a dam across a slope. Water running downhill meets it, spreads sideways, and gathers at the two upslope corners.

So the fall has to be designed to take water round the unit rather than into it. On a modest unit that means the tapered scheme is cut with diverters: two shallow ridges running from the upslope corners out to either side, so the flow is split and pushed past. On a wide unit, or one close to a parapet, a formed gutter behind the upslope face collects the water and runs it away with its own fall. Neither of these happens by accident. They are drawn before the insulation is ordered.

Measuring the upstand from the water, not the deck

The fall also decides how tall the kerb has to be, and this catches people out. If the roof falls at 1 in 40 and the unit is 1.5m deep in the direction of fall, the deck at the upslope face of the kerb is around 38mm higher than the deck at the downslope face.

A kerb built at a uniform height off a sloping deck therefore gives you 38mm less clearance on the upslope side, which is the side water actually reaches. The height that matters is measured from the highest level water will ever stand at, not from the nearest bit of deck. That is why how high a rooflight kerb should be is answered with a survey rather than a catalogue figure.

Zero-fall roofs, and when they are legitimate

A zero-fall roof is a deliberate design, not a mistake, and it exists. Certain single-layer systems, mastic asphalt in particular, are specified to be laid without fall on the understanding that the roof will hold water and that every detail is designed to be permanently submerged.

That is a specification decision with consequences. Everything at the perimeter, including the rooflight upstand, has to be higher, all laps have to be fully bonded rather than partly, and the roof has to be structurally checked for the weight of retained water. It is a legitimate approach on a terrace or a podium. It is not a way of rescuing a roof that somebody forgot to put a fall on, and treating it as one is where the trouble starts.

Existing roofs that were built level

Plenty of Oxford extensions from the sixties and seventies have flat roofs with no measurable fall at all. If you want a rooflight in one, the fall has to be dealt with as part of the work.

Usually that means a tapered insulation overlay laid on top of the existing deck, which adds build-up at the high end but avoids stripping back to joists. Sometimes it means the roof is recovered properly first. What we will not do is set a kerb into a level roof, weather it, and let the client find out the following winter that water stands 20mm deep against the upslope face all through February. If the fall cannot be made to work, that is worth saying before anybody spends money.

What we check on the survey before pricing

A laser level and a tape answer most of it in twenty minutes. We take levels at the four corners of the roof and at the proposed opening to find the actual fall rather than the intended one. We find where the roof drains, whether that is an outlet, a gutter or an open edge, and whether there is an overflow. We check the joist span and direction, because the fall runs with or across it. And we look at what happens at the low end, since a perfect fall that discharges into a blocked outlet achieves nothing.

Whether the glass itself needs a slope is a separate question, answered on do flat rooflights need to be sloped. We install and replace rooflights across Oxford and up to twenty five miles by road, we have been in the trade fifteen years, and the workmanship carries a ten-year guarantee.