The short answer
A flat rooflight needs very little pitch, because it sits on a level upstand and the roof around it simply drains past. Most flat roofs are built with a gentle fall for water, and that suits a flat rooflight well. Steeper roofs, usually beyond about 15 degrees, call for a roof window or a pitched rooflight instead.
Flat roofs are never quite flat
A roof described as flat is nearly always built with a slight slope, called a fall, so that rainwater runs towards a gutter or outlet instead of standing on the surface. The fall is gentle enough that you cannot see it from the garden, but it is there. A common aim is a finished fall of at least 1 in 40, which is a little under one and a half degrees.
That small slope is the pitch people mean when they ask this question. For a flat rooflight it is the ideal starting point.
Why a flat rooflight copes with a low pitch
A flat rooflight does not sit directly on the roof surface. It is fixed to an upstand, a kerb of timber or insulated material built up around the opening, and the glass is set at the top of that kerb. The top of the upstand is made level, so the glazing is level as well, whatever the roof is doing underneath. Water falls on the glass, runs to its edges and drops onto the roof, where the fall carries it away.
Because the upstand is built to suit the roof, the fall can be a little uneven from one corner of the opening to another. The upstand takes up the difference. Our survey notes the height needed on each side so the finished kerb ends up level and at least 150mm above the finished roof surface.
Where the upper limit sits
Flat rooflights are made for roofs from level up to a modest slope, and makers publish their own limits. As a general guide, a flat glass rooflight suits roofs up to roughly 15 degrees. Beyond that, the upstand would need to be so tall on the low side that it looks clumsy and the glass sits awkwardly against the slope.
On a steeper roof the better choice is a roof window, which is made to lie in the plane of a pitched roof, or a pitched rooflight for a low-slope roof with a particular look. Our roof window installation page explains how those work.
| Level to a gentle fall | Flat rooflight on an insulated upstand |
|---|---|
| Low slope, up to about 15 degrees | Flat rooflight, with the maker's limit checked |
| Moderate to steep pitch | Roof window set into the slope |
| A lantern look is wanted | Roof lantern on a flat roof |
Too little fall causes its own trouble
The opposite problem is a roof with no fall at all, or one that has sagged. Water that stays on a flat roof for days is hard on every surface, and it will sit around the base of a new upstand too. A rooflight does not cause that, but it is a good moment to notice it.
If the roof is ponding, we say so at the survey and talk through what that means for the covering around the new opening. Lifting the fall with tapered insulation is a job for a roofer or a new warm-roof build, and the rooflight can be planned to suit it.
How we measure it
At the survey we check the fall with a long level and a tape, in more than one place, because flat roofs often drain towards a corner. We look at where the outlets are, how the water leaves the roof, and whether the new rooflight will sit across the natural flow of water or beside it. A rooflight placed straight in the path of a drainage run can collect leaves and grit on its upstream side, so we try to keep that in mind when choosing the position.
The written quote itemises every cost separately, including the height of the upstand where it matters, and a plan for each day of the fit comes with it.
A level look from inside
Inside the room the pitch of the roof does not show, because the ceiling is horizontal. The opening is trimmed square, the sides are lined, and the light well is plastered so it looks tidy at ceiling level. Where the roof fall is noticeable, the light well is slightly deeper on the high side, and the finished reveal hides the difference.
Pitch, planning and Building Control
The kerb that levels the glass also feeds into planning. Height above the roof is the figure councils look at, and a flat rooflight on a tall upstand can come close to the 150mm allowance for houses, so we add up the kerb and the frame before anything is ordered. Flats and maisonettes are treated differently and need an application. Our planning guide sets out the conditions.
Cutting a fresh opening also brings Building Regulations into play, for the structure and for heat loss. Where the work is notifiable, the notification is something we arrange for you.
Neither of these rules asks for a particular pitch. They concern height, size and thermal performance, which is why the slope of the roof is a practical question for the fitter rather than a legal one.
A worked example
Picture a rear kitchen extension with a roof that falls from the house wall towards the garden. Standing at the high edge, the roof sits perhaps a hand's width higher than at the low edge, and the rooflight is planned for the middle. The kerb is built so its top edge is level, which means it stands a little taller on the low side than on the high side. From the garden you see a neat frame, and from the kitchen you see a ceiling that has no hint of the slope above.
That is the normal result, and it is why a gentle roof fall is welcome rather than a problem.
Related questions
If you are weighing up what to fit, the flat roof rooflight installation page covers the whole job, and flat rooflight sizes shows how the opening is chosen. Once you know the pitch suits, how a flat rooflight is fitted to a warm roof describes the build-up around it.
Want the fall of your roof checked before you choose a rooflight? We measure it at the free survey.
Get a free quote