Why a shallow pitch is awkward for a roof window
A roof window sits in the plane of the slope. Gravity does the work of getting rain off the glass and round the frame, and the shallower the slope, the less help gravity gives. Water moves slowly. Snow melt lingers. Wind can push spray up the slope, under the top of the flashing.
So a roof window on a shallow slope depends more on its flashing, its underlay and its seals than the same window on a steep one. That is why makers state a range of pitches for each product, and why fitting instructions set a lower limit. The firm fitting it has to work inside that range.
Plenty of ordinary roofs are shallow: a lean-to over a kitchen, a back addition behind a terrace, a porch, a garage, a roof built with large-format concrete tiles. Leicestershire has all of them. The shallow slope often sits over the dark room, so the question comes up.
What the maker's own range says
The firmest fact comes from a manufacturer. In its technical library VELUX shows a wooden centre-pivot roof window made for low pitches of 10 to 20 degrees. That is a published figure for that one product, fitted to the maker's instructions.
It does not mean every roof window works at 10 degrees. Other windows have their own ranges, and the figure depends on the roof covering and the flashing kit as well as the glass. Treat 10 to 20 degrees as an example of what a maker publishes, and let the specific product on your quote carry its own figure. If a firm cannot tell you the pitch range for the unit it is quoting, ask again.
What matters is that the product, the covering and the pitch all line up on paper before anyone cuts a hole.
How the pitch is measured
A roof seen from the pavement looks flatter or steeper than it is. Pitch is measured on the roof, with a digital level laid along a rafter or on the tile surface, and it is worth taking a few readings because old roofs sag and one reading can mislead. From the loft, a level laid against the underside of a rafter does the job for a first look.
Two things to watch. First, the pitch that counts is the one at the exact spot where the window will sit, not the average of the roof. A roof with a dormer, a hip or a valley can have steeper and shallower faces, and the unfashionable shallow face may still be the best place for the light. Second, the roof covering matters. A pitch that suits slates may be too shallow for a tile that is rated for a steeper slope, and then the covering is the real limit, not the window.
At the free survey we measure it, write the figure on the quote and name the product that suits it.
When a flat rooflight is the better answer
Below the pitch range a roof window is rated for, the sensible answer changes character. Instead of fighting the slope, treat the opening as a flat-roof detail: a kerb is built up round the opening, the waterproof layer is carried up its sides, and a flat rooflight sits on top at its own gentle tilt. Water is led away from the unit by the kerb and the covering, not by the pitch of the slope.
On a shallow back addition behind a terrace, this can be the neat solution, because it gets daylight into a room that has had none, without asking a roof window to work outside its design. The kerb stands up above the roof surface, so there are things to check: how far it projects, how it looks from the garden, and the planning limits. Rooflights and roof windows are usually permitted development in England, with limits on how far they stand proud of the roof slope. The planning and Building Regulations guide covers it, and a flat or listed property has its own rules.
Whichever route suits, the inside matters too. A shallow slope often has a low ceiling beneath it, so the depth of the reveal, the angle of the light and the lining all change how bright the room feels. We talk that through at the survey, standing in the room, so the choice is made with the ceiling in view.
A sun tunnel is a third route where the slope is very shallow or the opening very small. It brings in daylight through a tube and a small dome, and suits hallways and bathrooms.
What to ask a firm quoting for a shallow roof
Four questions sort the careful from the careless.
- What is the measured pitch where the window goes, and how was it measured?
- What is the pitch range for the unit you are quoting, and where is that written?
- Is the roof covering suitable for that pitch, and what flashing and underlay are included?
- What happens if the roof is below the range? Is there an alternative on the quote?
A firm that answers with numbers and product names has looked at the roof. A firm that says it will be fine has not.
We do this survey as a matter of course. If a skylight can go in, we tell you which kind and why. If the honest answer is a different product, a different slope or a different position on the roof, we say that too. Our approach to difficult roofs in general is set out on the awkward roofs guide, and the whole range of what we fit is on the home page.
To have your own roof measured and a rooflight or roof window matched to it, book the free survey. A fixed written quote follows.