Can a rooflight be fitted to a flat roof?

How it sits

Not in the roof plane. A flat roof rooflight sits on a kerb, a raised timber upstand built up above the roof surface, which lifts the frame clear of the water line so the covering can be dressed up its face and terminated under the frame.

That is the whole difference between this and a pitched roof window, and it is why the two are not interchangeable.

What the roof needs

A fall. Flat roofs are laid to at least 1:80 finished so water leaves. A rooflight interrupts that flow, so it needs to sit where water reaches it moving.

Sound structure. The opening is trimmed and the load carried to the surrounding joists, or to a steel on a wide opening.

A sound covering. Cutting into a covering near the end of its life is spending money on something you are about to replace.

Room around it. Enough flat roof left around the kerb for the covering to terminate properly.

New build or retrofit

During a build it is straightforward and cheap, because the opening is formed as the roof goes on. Retrofitting into a finished roof is a bigger job: the covering is opened, structure altered, kerb built, covering made good and the ceiling below opened and finished.

Detail on the flat roof rooflight page.

Yes, and it is one of the commonest jobs we do

Flat roof rooflights are a mature product designed specifically for the situation. They sit on a kerb built up from the roof deck, with the covering dressed up the kerb and the unit overlapping it.

What decides whether the installation lasts is the kerb rather than the unit, and that is worth understanding before comparing quotations.

The kerb, in four parts

Height. At least 150mm above the finished roof surface. Every flat roof holds some water, and a unit bedded straight onto the deck sits in it after every shower.

Fall. The deck falling away from the kerb, so water cannot pond against the base. On an older or self built extension this is the thing most often missing.

Insulation on the outer face. A 1200 by 1800 rooflight has six metres of perimeter, and an uninsulated timber kerb punches straight through the roof insulation around all of it. A sheet of rigid board and an hour, and it disappears under the covering, which is precisely why it gets skipped.

A continuous vapour control layer sealed to the frame, so warm moist air cannot reach the cold side of the construction.

Fixed or opening

Fixed is cheaper, marginally better thermally and has nothing to maintain. Opening lets accumulated heat leave at the highest point of the room, which on a south or west facing extension is what makes it usable in summer.

Above reach that means motorised, with a rain sensor, because a horizontal opening left open in a shower admits considerably more water than a sloped one.

Sizing and glass

Glazed area around 15 to 20 percent of floor area gives a bright room without turning it into a greenhouse. On a long narrow room, two or three units in a row light it more evenly than one large one, cost less, and fail independently.

Solar control glass belongs on any substantial flat rooflight, because a near horizontal pane has no orientation to shelter behind and collects the midday sun directly.

Which coverings

Felt, single ply, liquid applied and green roofs all take a rooflight. The detail at the kerb varies with the system, and compatibility between the membrane and the rooflight materials is a specification question worth confirming.

Why we survey

Whether a kerb exists, how high it is, whether it is insulated and which way the deck falls are the four things that decide the cost, and none of them is reliably visible in a photograph from the ground.

What to send us

A photograph showing how the flat roof meets the house, one of the room below, and if you can do it safely, one of the roof surface after rain showing where the water sits.

That last picture answers most of the question, because the fall on the existing roof and whether there is already a kerb are the two things that decide the cost.

We will give an indicative range by email from those, free and without an appointment, and we will not fix a price without a survey, because a kerb that is too low or a deck that falls the wrong way is not something to discover on the day.

Yes, and it is a different product from a pitched roof window

A flat roof rooflight sits on a kerb built up from the deck rather than being flashed into a covering. The waterproofing is dressed up the kerb and terminated into the unit, and the unit sits on top.

A pitched roof window cannot be used this way, and using one below its minimum pitch is outside its warranty and outside what it was designed to weather.

The four things to establish first

The build up. Warm roof or cold roof, established by inspection rather than assumption. A cold roof has a ventilated void that the kerb interrupts, and the air path around it has to be maintained or a damp pocket forms beside the unit.

The fall. A kerb is an obstruction in the water path. On a roof with marginal fall, water ponds permanently against it. Correct the fall first.

The covering age. Cutting a new opening into a membrane with a few years left means the rooflight comes out when the roof is recovered. Do both together where the covering is due.

The structure. Trimming an opening in a flat roof deck is designed work beyond a modest size.

The kerb details that matter

High enough to clear any water the roof can hold, including with a blocked outlet. Insulated on its outer face, continuous with the roof insulation. Square and level, because the unit is made to the finished dimensions.

The insulation is the one most often omitted, and it produces condensation on the internal face that gets reported as a leak every winter.

Slope on the glass

A modest pitch keeps the pane clear. A truly horizontal unit holds water, dulls over the years, and defeats self cleaning coatings entirely.