The short answer
A flat rooflight is not fitted dead level. The glass needs a slight slope so rain runs off rather than lying on it, and each maker states the minimum pitch for its own unit. We take that figure from the manufacturer and check it against your roof at the free survey, then build the kerb to suit.
The short version of the minimum pitch
When people ask about the flat rooflight minimum pitch, they usually mean one of two things: how much slope the glass itself must have, and how much fall the roof around it needs. Both matter, and the numbers come from the makers rather than from a single rule that fits every product. Treat any figure you see quoted online with caution, because it may belong to a different product. A glass unit with a smooth, flat top needs a little slope to shed water and keep itself clean. Some units are designed with a built-in slope or a curved profile, and others rely on the kerb being formed to give it. We read the technical data for the exact unit we are quoting and build to that.
Why a dead-level pane is a problem
Water that cannot run off will sit, and standing water on glass leaves dirt rings, grows algae and puts a constant load on the seals at the edges. A flat roof is meant to shed water to an outlet, and the rooflight should help rather than fight that. A pane with a slight fall clears itself in the rain, stays clearer for longer and puts less stress on its seals.
The same logic applies to the roof around the rooflight. The rooflight sits on a kerb, and the roof covering rises up it as an upstand. If the roof falls toward the rooflight, water gathers on the high side of the kerb. If it falls away, the kerb is kept clear. The way that junction is built is set out in our kerbs and upstands guide.
Where the pitch comes from
On a flat roof, the pitch of the glass comes from one of three places.
- The rooflight is a domed or pyramid-shaped unit that slopes in its own form.
- The kerb is built with a slight fall across the top, so a flat pane sits at an angle.
- The roof itself is laid to a gentle fall and the kerb follows it.
Flat glass rooflights, sometimes called flat glass roof windows, are the type most affected. Many are sold as a pane set into a low-profile frame, and the maker's instructions state the lowest pitch at which the product is guaranteed to drain and seal properly. A roof lantern or a pyramid unit, by contrast, has pitched sides and sheds water by its shape, which is why it behaves differently on a very shallow roof.
How we check it on your roof
At the survey we measure the fall of the roof rather than assuming it. A roof that looks level to the eye can be falling gently toward a gutter, or can have sagged and be holding water in a dip. We note both, because the answer decides how the kerb is detailed and whether the roof needs levelling or re-falling around the opening before anything is fitted.
If the roof is shallower than the maker allows for the rooflight you want, there are straightforward answers. We can choose a unit designed for a lower pitch, form the kerb to give the glass its fall, or lay the roof covering to a firmer fall around the opening. The right one depends on the roof and on how you want the rooflight to look from inside.
Signs the pitch is wrong on an existing rooflight
If a rooflight is already in place, the glass will tell you. Water that lingers on the pane for hours after rain, a green or grey film along the lower edge, and dirt that gathers in a line where the water stops all point to too little fall. A damp stain on the ceiling near one edge can mean water has been sitting against a seal. These signs do not always mean the whole rooflight must be replaced, since the cause may be a sagging roof, a blocked outlet or a kerb that has settled. We look at all three when we survey.
Pitch and Building Regulations
A new or replacement flat rooflight is building work, and the unit must meet Part L. The figure to meet is 2.2 W/m²K for a flat-roof rooflight, and the few degrees of fall on its kerb do not change that, because the category follows the roof, not the glass. A unit in a pitched roof is a roof window with its own, lower limit. We confirm the figure at the survey, and we cover the wider rules in our answer on Building Regulations for a rooflight. Building Control is told about it by us.
What it means for the finished room
Inside, none of this should be visible. The kerb, the fall and the upstand are worked out on the roof, and the lining inside is shaped, plastered and made good. If you are also deciding on the style of the glazing, our page on frameless or framed flat rooflights covers how each sits on the kerb. When you are ready, we survey for free, put a fixed price in writing first, and stand behind the installation with a 10-year workmanship guarantee.
