Measure the pitch before anything else
The roof angle decides which kind of VELUX can go in, so it is the first number we take on a lean-to survey.
A lean-to is a single slope that runs down from the house wall to a lower wall or row of posts. Across Cambridge you find them everywhere: the rear kitchen additions behind gault brick terraces in Romsey and Petersfield, utility rooms added onto 1930s semis, and brick outshuts on village cottages. What varies widely is the pitch. Some older ones were built steep with plain clay tiles; plenty of later ones were built as shallow as the upstairs window sill would allow, sometimes at 10 degrees or less.
VELUX pitched roof windows are designed for roofs from 15 to 90 degrees. The flashing kit relies on gravity to carry water over and around the frame, so on a shallower slope it cannot be trusted to shed rain and melting snow. The table shows how the pitch steers the choice.
| Lean-to pitch | Usual VELUX option | What to check |
|---|---|---|
| Under 15° | Flat roof window on an upstand | Roof covering and deck |
| 15° to 20° | Roof window, low end of range | Tile minimum pitch |
| 20° to 45° | Standard roof window | Rafter length, headroom |
| Over 45° | Roof window, taller sizes suit | Handle reach |
The covering matters too. Interlocking concrete tiles are often rated for pitches in the high teens or low twenties, natural slate typically needs more, and plain clay tiles far more again. If the roof is already close to the minimum for its tiles, the window flashing will be working at its limit as well. Our guide to VELUX flashing types explains which flashing goes with which covering.
When the roof is too shallow for a roof window
A lean-to below 15 degrees is treated as a flat roof for glazing purposes, and the VELUX answer is a flat roof window rather than a pitched one.
The VELUX flat roof range sits on a raised kerb and carries its own glass top, so rain runs off the glazing instead of relying on the slope of the roof. On a shallow lean-to covered in felt, EPDM or a single-ply membrane, this is often the neater route. Tiles on a very low slope are a different conversation: they may already be below their own rated pitch, and the survey will say so plainly rather than cutting a new hole into a roof that is struggling. The flat range and its glass tops are set out in our guide to VELUX flat roof windows.
Polycarbonate sheet lean-tos are their own case. The sheeting is not built to carry a framed window, so fitting one normally means replacing that roof with a solid, insulated one first.
Fitting a window into a short slope
Lean-to rafters are often short, so the window has to fit between the wall at the top and the gutter at the bottom with roof covering left on both sides.
A lean-to might have only 2 to 3 metres of slope. Above the window you need enough tiles, and enough distance from the lead or wall flashing where the roof meets the house, for the head of the window flashing to tuck under the covering properly. Below it you need a few courses before the eaves. That leaves a working band in the middle, and the height of the window has to fit within it.
This is why shorter sizes such as the 04 and 06 heights (980mm and 1180mm) turn up so often on lean-tos, and why two windows side by side can light a wide kitchen better than one tall one. Width runs across the rafters, which in these roofs are commonly around 400mm apart, so a wider window means trimming rafters and adding timber to carry the load. How the codes translate into sizes is in VELUX window sizes and codes, and the extra work of going wide is covered in whether you can fit a bigger VELUX window.
Headroom plays a part as well. At the low end of a lean-to the ceiling can be well under 2 metres, and a top hung window swinging outward from there is awkward to reach. A centre pivot window placed higher up the slope keeps the handle within reach and lets a worktop or table sit beneath.
Where the light will land
A shallow roof window looks at more of the sky than a steep one, which brings more light and, on the wrong slope, more heat.
The flatter the glass, the more directly it faces the high summer sun. On a south facing lean-to in Cambridge, where there are no hills to break the sun, a low pitched window can put a hot, bright patch across the kitchen table from late morning well into the afternoon. North facing lean-tos, common behind terraces whose gardens face the other way, give steady, cool light that suits a kitchen or a home office very well.
Position changes the effect as much as size. A window near the house wall throws light deep into the old back room, which is often the darkest part of a terraced ground floor. A window near the eaves lights the extension itself and the garden view. We plot the sun patch for both before settling on one, as described in how we plan the light.
Structure, insulation and the paperwork
Cutting a new opening in a lean-to is structural work on a small roof, so the timbers, the insulation around the frame and the notification all need handling properly.
Many lean-to roofs were built lightly, with slender rafters and little or no insulation. When rafters are trimmed for a new window the load is carried by doubled timbers either side and new trimmers above and below. The frame is then sealed into the roof build-up with an insulation collar and vapour barrier so the reveal does not become a cold, damp edge, which our guide to VELUX insulation collars and kits explains.
A new roof window is normally notifiable under the Building Regulations because it alters the roof structure and its thermal performance, and we handle the Building Control notification where the work is notifiable. Planning permission is not usually needed on a house under permitted development, provided the window does not project more than 150mm from the roof plane, but conservation areas, listed buildings and flats change that. The detail is in whether you need planning permission for VELUX windows. For the full range of pitched windows we fit, see our VELUX window installation page.
Questions people also ask
What is the lowest pitch a VELUX roof window can go on?
Standard VELUX pitched roof windows are made for roofs of 15 degrees and steeper. Anything shallower calls for a flat roof window on an upstand, which carries its own glass top to shed water.
Can a VELUX go into a lean-to with a polycarbonate roof?
Not directly. Polycarbonate sheeting cannot support a framed window or take the flashing, so the usual route is to replace it with a solid, insulated roof and fit the window as part of that work.
Is one large window or two small ones better on a lean-to?
On a short slope, two smaller windows often fit better and spread the light more evenly along a wide room. One larger window gives a single stronger pool of light and fewer rafters to trim, if it fits the height available.
Does a lean-to roof window need Building Control?
A new opening in the roof is normally notifiable because rafters are cut and the insulation changes. A like-for-like replacement in the same opening is treated differently, which the survey will confirm for your roof. To see what your lean-to could take, book a light survey.
