Which VELUX suits a low pitch roof? Match the window to the slope

Every decision on this page follows from one measurement. Get it right and the product choice becomes straightforward.

Pitch is the angle of the slope from the horizontal, measured in degrees. You can find it from inside the loft with a digital angle finder laid along a rafter, or from the outside on a dry day from a ladder you have secured properly. A phone app gives a rough figure, and it is good enough to start with. For a window order, a proper survey confirms it.

Check the roof covering as well. Slates and tiles each have their own minimum pitch, set by the maker of the covering. A very shallow roof is usually covered in sheet metal, felt, or a single-ply membrane, because the joints in slate or tile cannot shed water reliably at that angle. That tells you a good deal before you pick up the angle finder.

A standard VELUX roof window is designed for slopes from a stated minimum pitch up to vertical, and that minimum is commonly quoted at around 15 degrees. Because the range is set by the manufacturer and can vary between products, use the current figure for the model you are buying, not a number remembered from a leaflet.

A roof window is built on the assumption that rain runs down the glass and off the flashing quickly. Take the angle down and that assumption weakens.

Water lingers. On a steep slope a shower clears the pane in seconds. On a shallow one it sits, sheets slowly and carries dirt with it, so the glass streaks and the lower edge stays wet for longer. Leaves and moss collect at the sill, and debris holds moisture against the frame.

The flashing is the critical part. Flashings are cut and folded to shed water at the angles the window was designed for. Used on a pitch below that range, they sit too flat, and wind-driven rain can be pushed up the joints instead of falling away. That is how a window that looks correct ends up letting water in around the frame after a storm.

There is also a comfort side. A shallow roof gets direct sun for much of the day, so the room under it heats up. A window with good solar control, or a blind or awning fitted on the outside, matters more here than under a steep slope.

Three routes cover a shallow roof, and the pitch decides which one applies.

1. A standard roof window where the slope is inside the published range. A loft with a normal tiled roof usually falls here, and the choice of size and glazing follows as normal.
2. A low-pitch roof window from the VELUX range, made for slopes below the standard minimum. These are designed around the slower drainage, with a flashing and frame detail that suit the flatter angle. The range, the lowest pitch and the matching flashing are all set out in VELUX's current product information.
3. A flat roof rooflight where the pitch is lower still, or where the roof is, in effect, a flat roof with a fall built in. Flat roofs are laid to a fall, often written as a ratio such as 1:80, which is much shallower than any pitched slope. Our flat roof rooflight installation service covers that case.

The question of putting a roof window into a genuinely flat roof has its own answer: can you fit a VELUX in a flat roof explains what is offered and what needs a different product.

The model code on the data plate of an existing window helps when you are replacing one. Our page on VELUX GGL, GPL and GGU explains how to read the letters, and the code GBL marks the VELUX low-pitch window, made for slopes of about 10 to 20 degrees, so check the current range before assuming a like-for-like swap exists.

Pitch is the first filter. A handful of other choices then fine-tune the window for a low roof.

Flashing comes next, because it has to suit both the window and the covering. See our guide to VELUX flashing types for how they differ. On a shallow roof, the lower apron and the side gutters carry more water for longer, so this is not the place to economise.

Insulation and air sealing matter too. The collar that fits between frame and roof structure is part of the thermal performance, which our page on the VELUX insulation collar and underfelt collar explains.

And there is sound. Rain on a shallow slope falls at a glancing angle, but it sits on the glass for longer and can drum more steadily. A laminated acoustic inner pane and firm seals help in a loft bedroom. We set out the choice in your quiet spec, and our note on VELUX glazing options for a quieter loft goes through the combinations.

The overall service is on our VELUX window installation page.

We measure the pitch at the survey and write it on the quote. If it is inside a standard range, the job proceeds as a standard installation. If it is below, we say so and recommend the low-pitch route or a flat roof rooflight, with the reason.

The installation is to current Building Regulations and we handle the Building Control notification where the work is notifiable. It carries our 10-year workmanship guarantee, described on the our guarantee page, which covers our fitting of the window and its flashing.

Can I tilt a standard window to make it work on a low roof?

No. The pitch range is a limit on the roof, and the window cannot be tilted to change it. Changing the roof structure to alter the pitch is a much larger job than choosing the right product.

Is a dormer the answer for a very low pitch?

Sometimes. A dormer gives a vertical window and can make a low roof workable, though it is a construction project of its own and needs a separate conversation about planning and structure.

Does a low-pitch window cost more?

Specialist products can differ in price from standard ones. For general guidance, see our costs page, and we give a written figure after the survey.

How do I check my own pitch before calling?

Lay a digital angle finder along a rafter in the loft. Write down the figure and the roof covering. When you are ready, get a quote and send us both.