Do flat rooflights need to be sloped?

Flat roofs are not flat either. A well-built warm-deck or cold-deck flat roof is laid to a designed fall so surface water is carried to an outlet rather than left standing. Current guidance for flat roofing works to a finished fall of around 1 in 40, roughly 1.5 degrees, once you allow for deflection and any unevenness in the deck. The rooflight over that roof follows the same logic. It needs somewhere for water to go.

Glass does not soak up rain, so anything landing on a level pane stays there until it evaporates or the wind pushes it off. When it evaporates it leaves behind whatever was dissolved in it: dust, pollen, salt off the estuary air, and the fine grit that traffic and building work put into the atmosphere. That is the tide mark you see on a neglected flat pane. A fall of a few degrees turns that from a puddle that dries in place into a film that runs off and takes most of the dirt with it.

There is a drainage reason and a longevity reason, and they point the same way. Standing water sits against the edge seal and the glazing tape for hours after every shower. Sealed glass units are rated to cope with occasional wetting, not to be partly submerged for a working lifetime. Keep water moving off the glass and the seal stays dry between showers, which is the condition it was designed for.

There are two common ways to give a flat rooflight its fall, and it helps to know which one you are being quoted.

• A pre-formed sloped unit. Many flat-roof rooflights are made with the fall built into the frame or the glass itself. The frame sits square on a level upstand, but the pane is angled inside it, typically by 5 degrees on the fixed flush-glazed units and sometimes more. You get the drainage without having to build a tapered kerb.
• A tapered upstand. The alternative is a level rooflight sitting on a builder’s kerb that has been built up higher on one side, so the whole unit tilts. The upstand is timber or an insulated proprietary kerb, and the height difference across it sets the angle. This is the usual route for a large lantern or a run of glazing where the units themselves are level.

Either approach works. What matters is that the finished glass has a fall and that the fall runs the right way, towards the low edge of the roof and its outlet, not back towards a parapet or a wall where the water has nowhere to escape. A rooflight that slopes towards a wall abutment simply moves the puddle to the worst possible place.

The upstand itself does a second job. Lifting the glass 150mm or so above the finished roof surface keeps the perimeter detail clear of any water lying on the membrane, and it gives the roofer a proper edge to dress the waterproofing up against. A rooflight bedded straight onto the deck with no upstand is asking for trouble regardless of the glass angle. Most flat-roof failures trace back to a poor upstand rather than to the unit sitting on it.

Fit a flat rooflight with no fall and the problems arrive in a predictable order.

First is ponding. Water collects in the middle of the pane or against the lowest corner and sits there. On a large flush unit even a millimetre of deflection in the frame is enough to hold a surprising volume. Second is staining. Every pool dries to a ring, and those rings build into a permanent haze that no amount of cleaning from inside will touch, because it is on the outer face. Third is the drip-line nuisance: water that should have run off in one direction finds the lowest actual point, which may be over a join or a corner, and works at the seal there.

None of that is a fault you can correct later by adjusting the glass. The fall has to be designed in before the opening is cut and the upstand is built. Getting it wrong means taking the unit back out and rebuilding the kerb, which is why the slope is a specification decision, not a detail to sort out on the day. If your existing flat rooflight already ponds and stains, the cure is to replace it as part of a properly falled installation. Swapping a failed level unit for a sloped one is replacement work, and correcting the upstand geometry at the same time is the only way to make it stick.

The fall solves drainage. It does very little for heat, and on a south-facing roof in Southend that distinction matters more than anywhere. A flat rooflight, even one tilted 5 degrees for drainage, is still close to horizontal, so it faces the high summer sun almost square on through the middle of the day. Orientation barely helps you here: a near-flat pane collects strong overhead sun whichever way the house is turned.

Southend-on-Sea faces south across the Thames Estuary with an open horizon and bright reflected light off the water, so a flat rooflight over a rear extension gathers sun from mid-morning until late afternoon with nothing shading it. The 1930s semi belt across Westcliff, Thorpe Bay and out towards Hadleigh was largely built with the garden, and the back of the house, facing south. Put a flat-roof extension on the back of one and glaze it, and you have built a solar collector over a kitchen that already makes its own heat.

That is why the drainage slope and the glass specification are two separate questions on the same unit. You need the fall so the pane sheds water, and you need a solar-control glass, a G-value of around 0.3 or below, so the same pane is not pouring heat into the room every clear afternoon. Getting the slope right and leaving the glass as standard clear double glazing solves the puddle and leaves you with the greenhouse. Both belong in the brief from the start, which is the whole basis of how we approach a flat roof skylight installation, and it feeds directly into our energy-efficient glazing work.

Before anyone quotes you, settle three things about the roof. Confirm the flat roof has a designed fall and a working outlet, because a rooflight cannot drain onto a roof that does not drain itself. Confirm the rooflight will have a fall of its own, whether that comes from a sloped unit or a tapered upstand, and that it runs towards the low edge. Then ask which way the roof faces, because if it is south or anywhere near it, the glass specification matters as much as the angle.

Ask the supplier to tell you the fall in degrees and the glass figures in the same breath: the G-value, the light transmittance and the U-value for the build-up you are being quoted. If either answer is vague, keep asking. If you would rather we worked it out, tell us the rough opening size and the orientation and we will come back with both the drainage detail and the glass specification. Ring 01702 898232 or request a quote, and we will look at the whole roof rather than just the hole in it.