Short answer
Yes. A flat rooflight, or flat skylight, is not meant to sit dead level. Manufacturers specify a minimum fall so rainwater runs off the glass and the roof instead of lying on it, and that figure is typically a few degrees. The fall usually comes from the roof build-up and the kerb. Some units also have a slight slope built into the glass, and the data sheet for the unit tells you which. A roof with no fall at all needs one created before the glass goes on.
Why a flat rooflight needs a minimum pitch
The name misleads. A flat roof is flat in the way a table is flat to the eye, but it is built with a gentle slope towards a gutter or an outlet. A flat rooflight on that roof is designed with the same idea. Water that cannot drain from the glass collects in the low corner, dries to leave a ring of dirt, and sits against the seals for longer than they were meant to cope with.
A minimum pitch also keeps the glass cleaner. Rain running across a slightly angled pane carries some of the dust with it, and a level pane holds its puddle until the sun lifts it. None of this causes a sudden failure. It shortens the life of the surface and makes the glass look dull in a way no amount of cleaning changes.
What the manufacturers specify
Each manufacturer publishes a minimum pitch for its flat rooflights, typically a few degrees, and some units are approved for a lower figure when a certain detail is used. We work from the maker's sheet for the specific model, not from a general rule, because a frameless unit, a lantern and a hinged unit do not all ask the same thing.
Our survey measures the fall of the existing roof with a level and records it. If the fall meets the manufacturer's figure, the kerb goes on as it is. If it does not, the job plan says how the shortfall will be made up, and the fix is part of the written quote before any order is placed.
| Source of the fall | What it means for the rooflight |
|---|---|
| The roof deck | The kerb sits on a roof that already slopes, and the kerb top is levelled |
| Tapered insulation | The slope is built into the roof layers, and the kerb is made to match |
| The kerb itself | One side is built taller so the glass sits at an angle |
| The unit | The glass has a slope built in, and the manufacturer says so |
Does the fall come from the kerb or the unit?
It can come from either. On most flat roofs the deck already falls towards the outlet, and the kerb is built so its top edge is level. The glass then sits level on a roof that slopes under it, and the manufacturer's minimum is met by the angle of the pane relative to the horizontal only when the kerb is built to give it.
The cleanest result is a kerb whose top face carries the fall, so the unit sits at the manufacturer's angle and the upstand heights on the uphill and downhill sides differ by a few millimetres. The answer on flat rooflight upstands explains how those heights are set and why the membrane has to be dressed to the higher side first.
What happens to standing water on flat glass
Standing water on glass does not make a roof fail on its own. The risks build slowly. Water that sits behind the frame edge tests the sealant for longer, the dirt line that forms is hard to wash, and a wet corner in winter can freeze and lift a seal slightly each time. Over years that is how a unit loses its finish ahead of its time.
Another effect is cosmetic but real. A rooflight over a kitchen or living room is looked at from below every day, and a ring of grime on the outer pane shows against the sky. Giving the glass a fall, the flat skylight fall the maker specifies, is the simplest way to keep the view clear. If you are choosing between a rooflight and a lantern for a roof with a slight pitch, roof lantern or flat rooflight sets out how each sheds water.
Flat roofs that really are flat
Some older roofs have lost their fall. A deck that has sagged between the joists leaves a shallow dish, and a rooflight dropped into the dish inherits the problem. In that case the job is to create the fall before the glass goes in, either by tapering the insulation, packing the kerb, or adjusting the deck if the joists allow.
That is a decision for the survey, not for the day of the fitting. We record the fall, compare it with the manufacturer's minimum, and say in the written job plan how it will be met. A pitched roof asks a similar question, and the minimum pitch for a roof window answers it for sloping roofs.
- Measure the existing fall with a level at survey
- Check the figure against the data sheet for the exact unit
- Decide where the fall comes from: deck, insulation or kerb
- Record the answer in the job plan before ordering
How the job plan records the fall
In the job plan we write for your house, the unit row names the model and size, along with the minimum pitch the maker gives for it. The notes beneath say where the fall comes from on your roof and what extra work, if any, that involves.
Because the fall is settled on paper before the unit is ordered, nothing is found out when the old covering is lifted. The 10-year workmanship guarantee covers the kerb and the finishing around it. For the full range of units on a flat roof, go to the hub on flat roof rooflight installation.