The short answer
Yes, up to a point. A flat rooflight on an upstand suits a roof that is flat or only gently pitched, because the kerb can be levelled or tapered to the slope. As the pitch steepens, the kerb grows tall on the uphill side and looks clumsy. There, a roof window built for the slope is usually the better unit.
What counts as shallow
A pitch is the angle of the roof in degrees. Pitched roofs at 30 to 45 degrees are the angles the word roof brings to mind. A shallow pitch is far gentler.
A true flat roof has a fall of only 1:80 or so, which is barely an angle at all. Between that and a low-pitched roof there is a long stretch of slopes, from a few degrees up to around 15 degrees. These are the roofs you find on garden rooms, dormer cheeks, porch roofs, bay tops, garages and extensions built in the New Town years. They are covered in felt, rubber, fibreglass, standing-seam metal or large-format tiles, and they are too steep to call flat and too shallow to behave like a traditional pitch.
The unit you choose has to be built for the angle it lands on. A rooflight designed for a flat roof is fixed on an upstand. A roof window is designed to lie in the plane of the slope. Between those two is an overlap where either can work, and that overlap is where this question lives.
Where a flat rooflight works well
On a low slope, the flat rooflight has a clear advantage. It does not depend on the roof covering in the way a roof window does.
The upstand is a box that stands on the roof deck. On a slope of a few degrees, the box can be cut so that its top edge is level or, better, tilted a touch to shed water. The membrane then runs up its sides, and the glass sits on the top. There are no tiles to cut around and no batten lines to match. That makes the flat rooflight a natural fit for felt, rubber and fibreglass coverings, which wrap around an upstand easily. Our notes on fibreglass roofs and rubber roofs show how each finish meets the kerb.
There is also the question of daylight. A flat unit lets in light from the whole sky above it, because the glass faces straight up. On a low slope facing north, that can be a real gain over a window tilted away from the sun.
Glass angle still matters. A pane that ends up dead level holds water, as the page on water sitting on flat glass explains. On a shallow pitch you often get the fall for free, because the roof is sloping in the right direction already.
Where it stops making sense
The trouble begins as the slope steepens. The upstand has to reach the glass level across the whole opening, so its uphill side is lower than its downhill side, or the reverse.
On a slope of only a few degrees, the difference is small, perhaps a few centimetres. As the pitch climbs toward 15 degrees or beyond, it becomes large. The kerb needs to be cut to a tall wedge at the low end. Height is also wasted, since the glass is held level when the roof is not, and the result looks like a box standing on a hill. Flashing the sloping sides of that box is fiddly, because water runs down the roof and against the uphill face. Above the pitch where manufacturers of flat units give their own limits, the unit is simply outside what it was designed for.
A roof window takes over at that point. It lies flat in the plane of the roof and its frame is flashed to the covering. Manufacturers state a minimum pitch for each window, and it is lower than you might expect, with the lowest pitches needing extra flashing or a low-pitch kit. The page on roof window installation goes through how that works, and the choice of unit is covered in VELUX window installation.
| Roof slope | Usual choice | Why |
|---|---|---|
| Flat to a few degrees | Flat rooflight on upstand | Kerb is short, glass sheds water |
| A few degrees to about 15 | Either, by roof covering | Kerb is tapered, or use a low-pitch window |
| Steeper than that | Roof window | Sits in the slope, no tall kerb |
Checking your own roof
You can get a rough idea of the slope before anyone visits. A phone with a level app, laid on a sloping surface, shows the angle in degrees.
At the survey we measure it properly, and we check the covering, the deck, the rafters beneath and the insulation. We look at what lies below the glass, because a rooflight sitting over a bedroom and one over a store have different needs. Noise counts too. A light, thin roof with a low slope gives rain little to soften it, so the glazing build matters. We write the unit, the kerb shape and the glass into the quiet spec that comes with every quote.
The work is installed to current Building Regulations, and we handle the Building Control notification where it applies. Where the roof is a pitched one, the question of which unit suits it is answered in the survey and not guessed from a photograph. A survey quote starts that process.
Questions about low pitches and flat rooflights
Can a dome go on a sloping roof?
A dome can sit on a low slope in the same way as flat glass, on a tapered upstand. Its curved top sheds water whichever way it is tilted. See our dome guide for the limits.
What about a tiled roof with a low pitch?
Tile makers set a minimum pitch for each tile, and a roof below it is not a tiled roof in practice. Where a low-pitch roof has tiles, a roof window with the right flashing kit suits it better than an upstand.
Does a slope change the price?
It can, because a tapered upstand takes more work. The broad drivers of cost are on the cost factors page, and the costs page gives general ranges.
Is a lantern an option on a shallow pitch?
Often, yes, because a lantern stands on its own upstand like a flat unit. It is a bigger feature, and suits a larger opening. The roof lantern installation page explains how they are fitted.
The wider subject is covered on the flat roof rooflight installation page.