Question · Flat Roof Rooflights
Does a Flat Rooflight Need a Fall?
The short answer
The rooflight itself does not need a fall, but the roof around it does. A flat roof is built with a gentle slope so water runs to the gutter, and the rooflight is set on a kerb that is level or follows the unit's own requirements. The point is to make sure rainwater drains past the unit and does not pond against it.
Flat roofs are not quite flat
A so-called flat roof has a slight slope, called a fall, built into the deck or the insulation above it. It is only enough to see with a spirit level, and its job is to move rainwater to a gutter or outlet. Without one, water sits on the surface, and standing water is hard on any covering.
A rooflight changes that picture in two ways. It interrupts the surface, so water must be guided round it, and it sits on a kerb with a top that needs to be level for the frame and glass to bed properly. Both are matters of careful setting out, which a good installer thinks through before cutting.
Think of a hillside with a boulder placed across a stream. The water finds a way round, but only if there is room. A kerb is the boulder, the fall is the hillside, and the work is leaving enough room for the stream to pass without collecting behind.
Does the unit need to be tilted?
No. A flat rooflight is usually made to sit on a level kerb, or a kerb built to suit the roof, and water runs off the glass because the unit is slightly domed or has a drainage profile of its own. Some glazing is built with a small slope to the pane, and some is flat but sits on a kerb tall enough to keep any ponding beneath the frame.
What matters is that the maker's installation guidance is followed. Different units have different rules for the kerb top, the glass and the upstand. We read those before we measure, and the rooflight kerb guide explains why the kerb height is chosen as it is.
Fixed units and opening units are treated alike here. An opening sash lifts clear of the kerb, so it does not change how water leaves, though it does need the kerb to be true so the sash seats evenly when closed. A kerb that is out of level will show as a sash that sits tight on one side and loose on the other.
Where the rooflight sits on the slope
Position changes things. Water runs towards the low edge, so a unit set across the fall acts as a small dam. The upslope side collects water, and a gap has to be formed to let it round. A common solution is a small raised section, sometimes called a cricket, built behind the kerb to divide the flow and send water either side.
| Position | What it needs |
|---|---|
| Along the fall | Less fuss, as water passes either side |
| Across the fall | A shaped back-fall to divert water |
| Near an outlet | Clear space so the outlet is not blocked |
We look at the layout of the roof, where the gutter or outlet is, and where the water currently goes. Sometimes moving the opening a short distance makes the whole detail simpler. This is part of the decision on size and position that our what size flat rooflight answer describes.
When the roof is level or the fall is poor
Older roofs sometimes have little or no fall, and boards that have dipped over time create hollows. Fitting a rooflight on a roof that already holds water is storing up trouble. If the survey shows ponding, we say so and price the correction as part of the job, usually by laying tapered insulation or a new firring to restore a fall before the covering goes on.
A rooflight can be bedded on a sloping roof without any adjustment.
The kerb must be level at the top and tall enough to clear the water, so the roof's fall is allowed for in the build-up, not ignored.
This work counts as part of the installation, and the new covering is finished to suit the type of roof, whether felt, GRP or EPDM. Our GRP and EPDM roof answer describes how the covering is finished round the kerb.
None of this is a reason to put off a rooflight. A roof with a modest fall is easy to work with, and a roof with a poor one is better found now than after the unit is in. Either way the quote lists what the roof needs, and nothing about the roof comes as news to you later.
How it helps the room below
Good drainage protects the unit and the ceiling, but it also shapes condensation. Water that sits against the frame keeps the surround cold and damp. Water that drains away leaves the kerb drier and warmer, which helps the glass clear. The flat rooflight condensation guide goes into this.
At the free survey we check how water leaves your roof and show the finished level of each kerb on the written quote. For the wider scope of the work, see flat roof rooflight installation.
Slopes, ponding and the rooflight kerb
My flat roof holds a puddle near the skylight. Is that a problem?
It is worth looking at. A puddle that lingers means the fall is poor in that spot, and standing water against a kerb is hard on the covering. Tell us at the survey and we can look at whether the fall can be corrected as part of the installation.
Can a rooflight be fitted to a roof with almost no slope?
Yes, as long as the deck is firm and the kerb is built tall enough to clear any water. A very level roof may call for tapered insulation to create a fall, which would be shown as an option on the quote.
Tell us about the room
A few details is all we need. We'll come out, advise and give you a written quote.
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