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

Why a rooflight makes it matter more

The unit sits on a kerb standing proud of the surface, which obstructs water crossing the roof. On an adequate fall, water arrives moving and goes around it. On a marginal fall, water slows, spreads and ponds along the upslope face of the kerb.

Standing water against a termination detail is present for days rather than seconds, and it will find any weakness.

Position matters as much as fall

A unit with its long axis running with the fall sheds water past itself. The same unit turned across the fall presents a long dam to the flow.

Where the ideal position for light and the sensible position for drainage differ, that is a real trade and we explain it rather than quietly choosing.

Where there is no fall

Common on older roofs that have settled or were laid without firrings. The answer is to build local falls around the opening with tapered firrings, directing water away from the kerb even though the wider roof is flat.

What we will not do is set a unit into standing water and call it finished.

Checking yours

Look a few hours after heavy rain. Damp is fine. A visible pool, or a tide mark showing where one sits, means the roof is not draining and that needs addressing before a rooflight goes in rather than after.

1 in 80 finished, designed at 1 in 40

The design target for a flat roof is a minimum finished fall of 1 in 80. In practice a roof is designed to 1 in 40, because deflection in the structure, tolerances in the joinery and workmanship variation all eat into the theoretical figure.

Designing to the minimum leaves you with flat spots, and flat spots are where water sits.

Why it matters more with a rooflight

A flat roof without a rooflight tolerates a certain amount of standing water: the membrane is designed to be waterproof rather than merely water shedding.

A rooflight introduces a perimeter seal, and a seal designed to shed water is not designed to be submerged. Sitting in a puddle after every shower works it loose over a few winters, and that is the commonest cause of a leaking flat rooflight.

Which way the fall runs

As important as how much. The deck must fall away from the kerb, not toward it.

A roof with a perfectly adequate overall fall can still pond against a rooflight if the unit sits on the low side of the slope, or if the kerb interrupts the drainage path and creates a dead area behind it.

Where a kerb sits across the direction of fall, the detail should include a way for water to get around it rather than backing up.

The outlet

A fall is only as good as where it drains to. A perfect 1 in 40 discharging into a gulley full of leaves and silt produces exactly the same pond, and clearing it costs nothing.

Check the outlet before assuming the roof or the rooflight is at fault.

Correcting a poor fall

Locally around the kerb, using tapered insulation or firrings under a new deck section. It is normal work, it belongs in the quotation, and it is far cheaper than replacing a unit that has failed because it was sitting in water.

On an older roof where the fall was never built in, this may extend beyond the immediate area of the opening, and that has to be priced rather than discovered.

What to check yourself

Look at the roof after rain. Where the water goes, and where it does not, tells you everything you need before anyone quotes.

Standing water anywhere near the proposed opening is a problem to solve before the rooflight rather than after, and it is why we will not price a flat roof rooflight from a photograph taken from the ground.

Design fall and finished fall are different numbers

This is the distinction that matters and the one most often missed.

Guidance sets a minimum finished fall for a flat roof, and then advises designing to roughly double it, because construction tolerances, timber deflection and long term settlement all eat into the fall you drew.

A roof designed to the bare minimum will not achieve the minimum once it is built and loaded. It will have flat spots, and flat spots hold water.

Why the rooflight makes it more important

The kerb is an obstruction in the middle of the roof surface. Water that was moving across the roof now has something to pond against.

On a roof with a generous fall, water goes round the kerb and away. On a roof with a marginal fall, it collects on the upslope side of the kerb and stays there, which is the exact condition that ends a waterproofing detail.

So the falls need to be arranged so water is directed past the rooflight, not into a dead corner beside it. That is a layout question as much as a gradient one, and it should be drawn rather than left to the roofer.

Achieving it

On a warm roof, tapered insulation creates the fall within the insulation layer. It is the standard modern answer, it does not raise the structure, and it improves thermal performance at the same time.

On an existing roof with the fall lost through deflection, firrings on top of the deck are the usual remedy, and that work happens before the rooflight goes in rather than after.