Stand in the garden and look at the back of the house. If the slope above the dark room looks more like a gentle ramp than a steep hat, you have a low-pitch roof, and somebody has probably told you that a roof window will not go in. Sometimes that is the right answer. Often it is the quick answer. A roof window is a window that lies in the plane of a sloping roof, and the lower the slope, the less help gravity gives you when rain, snow melt and wind-blown spray try to find the gap round the frame. So the question is never just whether a window can be fitted. It is which window, on which covering, with which flashing and which underlay. This guide sets out how a low pitch is measured, what the manufacturer's own range says, why the covering matters as much as the glass, and where the line falls between a low-pitch roof window and a flat rooflight on a kerb. Whether you call the result a skylight or a rooflight, the survey on your roof settles it.
What counts as a low pitch
Pitch is the angle of the roof slope above horizontal, given in degrees. A steep Victorian slate roof sits at a high angle. A back-addition roof on a terrace, a lean-to over a kitchen, a porch roof or a newer estate roof built with shallow concrete tiles sits much lower. There is no single legal line that says low begins here. The trade uses the word loosely, and the manufacturers draw their own lines for their own products, which is the line that matters when you are choosing a window.
Nobody should guess a pitch by eye. A roof seen from the ground is foreshortened, and a slope that looks flat from the pavement may be steeper than it seems. On the survey we measure it on the roof itself, using a spirit level and angle finder held to a rafter or the tile surface, and we write the figure on the quote. A smartphone level app is fine for a first look from the loft, laid against the underside of a rafter. Do it on a few rafters, because old roofs sag and a single reading can mislead.
Pitch of the rafter or pitch of the covering
On a plain roof the two are the same. On a roof where the covering has been laid over the original, or where a dormer, a hip or a valley changes the plane, they can differ. The figure that counts for the window is the pitch of the plane the window will sit in, measured where the window goes. A roof with several planes can have a steeper face and a shallower one, and the awkward-looking one may be the better place for the opening. That is the sort of thing we look for on awkward roofs.
A low pitch rules out some roof windows. It does not rule out light.
What the manufacturer lists for a low pitch
The most useful single fact in this guide comes from the maker. On its UK site, VELUX sells one window aimed at shallow slopes: a wooden centre-pivot called the low pitch roof window, listed as suitable for 10 to 20 degrees. The trade library page names the model as the GBL. That gives a published floor of 10 degrees for that product. It is the manufacturer's figure for that window, and it applies to the window as supplied and installed to the maker's instructions, not to every roof window on the market.
The same page offers a triple-glazed option for the low pitch window, called Glazing 15, with a quoted Uw of 1.1 W/(m²K). Uw is the heat-loss figure for the whole window, glass and frame together: the lower the number, the less heat escapes. Our guide to U-values and Part L covers how that sits against the figures the Regulations ask for.
We fit VELUX windows, and their published ranges are a clear public statement of what a low-pitch window is built for. Other makers publish ranges of their own, and the same questions apply to them: what pitch, which covering, which flashing. Whichever make goes in, the datasheet for that exact model is the authority, and we read it before we quote. If you are weighing makes, the page on FAKRO, Keylite or VELUX sets out what does and does not vary between them.
| Roof pitch | Usual product |
|---|---|
| Below 10 degrees | Flat rooflight on a kerb |
| 10 to 20 degrees | Low-pitch roof window |
| Steeper than 20 | Standard roof window |
Treat the table as a map of the ground, not a rulebook. Standard roof windows each carry their own pitch range on their datasheet, and some are listed for lower pitches than others. The edges are where a survey earns its keep, because a roof at 11 degrees and a roof at 9 degrees look the same from the road and are different jobs.
Why a shallow slope is harder on the window
A steep roof sheds water fast and the water has little time to sit anywhere. A shallow roof sheds it slowly. Rain runs across the glass, over the frame and onto the flashing at a gentle pace, and in a hard blow it can be pushed sideways and upslope. Snow is the other test. It lies longer on a low slope and melts into the very joint you want to keep dry. Every detail round a roof window is designed to send water down and out, so a shallower fall asks more of every detail.
Three things carry the load. The first is the flashing: the set of sheet components that links the window frame to the covering around it and throws water onto the roof surface. The second is the underfelt collar, a short skirt that ties the window into the waterproof layer under the tiles or slates, so that anything that gets past the covering is turned back out. The third is the covering itself and how well it laps at that angle. Tiles and slates are made to be laid at a minimum pitch for the lap they have. Below that, the covering is working outside its design, and fitting a window into it does not change that.
The but
The roof is shallow enough that the covering itself is near its limit.
The way in
We check the manufacturer's minimum pitch for the tile or slate and the state of the underlay before the window is priced. If the covering is the weak link, the quote says so and sets out the fix around the opening.
The covering decides as much as the glass
Ask two installers about a low-pitch window and one will talk about the glass, the other about the covering. The second is the one worth listening to. Roof windows are fitted with flashing sets made for particular coverings: profiled tiles, flat tiles, slates. VELUX publishes separate flashing families for each, and the profile or thickness limits are set per family. On our flashings guide we go through those families. The point here is simple. The flashing must suit the covering, and at a low pitch there is less room for a near miss.
Concrete and clay tiles on a shallow roof
Many shallow estate roofs and extension roofs carry large interlocking concrete tiles. Interlocking tiles carry their water in channels, and the channels are designed around the pitch the maker gives. Check the tile maker's stated minimum for that tile and for that rafter length, because the minimum can change with exposure and length of slope. A window dropped into a roof whose tiles are already at their limit has nothing in hand. If the tile is a known make, the label on the tile back or the batch information in the loft sometimes names it. If not, a sample goes off for matching.
Natural slate
Slate laps over itself in a different way from a tile, and the headlap, the amount each course covers the one two courses down, is what keeps the roof dry at a low angle. A slate roof with a generous headlap can carry a lower pitch than one laid with a short lap. The headlap has to be read from the roof, and on an older roof it varies between slopes. A conservation-minded job may also need a flush, low-profile unit, covered in our pages on conservation rooflights.
Sheet and membrane roofs
Some low-pitch roofs are not tiled at all. A lean-to or an extension may carry a standing-seam metal sheet, a single-ply membrane, or a bonded roofing material laid to a gentle fall. A flat roof is not truly flat either, and a lot of roofs described as flat sit at a few degrees. If your slope is covered in sheet material, a roof window made for tiles and slates is the wrong tool, and the flat rooflight route is the natural one.
Underlay, battens and the build-up round the opening
Cutting an opening in a roof means cutting the rafters or trimming round them, and in a low pitch roof the build-up around the cut is the part that stops leaks. The sequence runs from the inside outwards. Trimmers are fitted to carry the cut rafters, so the opening is framed in timber. The underlay is cut and dressed up to the frame, and the underfelt collar is tied into it. The battens are cut back and the flashing is laid course by course with the covering. Insulation is returned round the opening so there is no cold edge inside.
On a shallow roof, the underlay matters more than it does on a steep one, because it is doing more of the work. We look at it from the loft. An old felt that has gone brittle, a sagging sheet that holds a pool of water against a batten, or a tear near the ridge changes what we do around the opening. We would rather tell you at the survey than discover it with the roof open.
What we check on a low-pitch survey
- Pitch measured on the roof, on more than one rafter
- Covering type and the maker's minimum pitch for it
- Condition and lap of the underlay, seen from the loft
- Rafter spacing and where the trimmers would go
- Flashing family that matches the covering
- Which way the window opens and what is outside it
Building Regulations apply to the opening. Cutting structural timber, the thermal performance of the window and the weathering all fall under them. Where the work is notifiable, Building Control is told by us. For the short version of the planning side, read the planning and Building Regulations guide.
Light, headroom and how it opens
Pitch also changes how the window works from inside. On a steep slope the glass faces the horizon, and you get light across the room and a view of the sky and treetops. On a low slope the glass faces more directly upward, so you get light that falls steeply into the room and a view mostly of sky. In a kitchen that is good news for the worktop and the table. In a bedroom it can mean the sun lands on the pillow, which a blind then has to answer. Think about the blind when you pick the window, not afterwards.
A centre-pivot window turns around a bar across its middle, and the bottom edge swings outwards as the top swings in. On a low slope, the sash comes down towards the room when it is turned for cleaning, and the geometry gets tight. Allow room above the window inside for the sash to turn, and keep furniture and any ceiling light clear of its path. Choosing between a top-hung and a centre-pivot opening is a separate decision, covered in top-hung or centre-pivot.
Headroom is the fourth thing to think about. A low-pitch roof leaves less room under the rafters than a steep one, so a roof window may be lighting a space that does not have standing height. That is fine for a landing, a utility room or a stair, and it is a question for a room meant to be lived in. A window does not create headroom. If the room is a loft you want to convert, the structure and the stair come first.
When the roof is too flat for a roof window
Below the pitch a roof window is listed for, there is still a way in. A flat rooflight sits on a raised upstand, the kerb, built up from the roof deck, and the covering is carried up over its outside so rain is kept on the roof. It does not rely on the fall of the slope to shed water round the frame, because the kerb lifts the glass clear of the water and the roof membrane does the waterproofing. That is why flat rooflights suit lean-tos and extension roofs at shallow angles. The glass itself still needs a slight fall to clear rain and keep the surface clean, and the kerb detail depends on what the roof is built from. Our pages on minimum pitch for a flat rooflight and kerbs and upstands go through it.
Between the two lies a band of roofs that could go either way. A roof at 12 degrees might take the low-pitch window if the covering and underlay are sound, or might suit a flat rooflight better because the covering is tired and the roof is due for work anyway. We lay out both options in the quote when it is close, with the reasons, so the choice is yours and not a shrug. Our page on a skylight in a shallow-pitched roof takes the same question from the homeowner's side.
What to bring to the survey, and what you get back
You do not need to prepare much. Tell us which room the light is for, send a photo or two of the roof from the garden by WhatsApp on 07880 284133, and mention anything you already know: the age of the roof, a tile make from an old invoice, past leaks above the room. We come to the house, climb up, measure the pitch and the opening, look at the covering and the loft, and talk through the options on the spot.
You pay nothing for the survey. What comes back is a written quote with one fixed figure, which lists the window, its size, where it goes and how the inside is finished, plus a note on the covering we found. If the honest answer for your roof is a different product from the one you asked about, the quote says that too. The work carries our 10-year workmanship guarantee. Skylight, roof window, rooflight: the words change from person to person, and the survey turns them into a drawing and a figure. Ask for a free survey and we will take it from there.
At a shallow angle the covering is the question. The glass is the easy part.