Where the 15 degree figure comes from
The 15 degree minimum is set by the manufacturers, and it marks the point at which their window, its flashing kit and the underlay collar have been tested to shed water reliably.
A roof window in a pitched roof works like a large tile. Water running down the slope meets the top of the window, is turned sideways by the flashing, runs down the side channels and is delivered back onto the tiles below. Each of those steps depends on gravity. At 40 degrees water moves quickly and has little time to find a gap. At 15 degrees it moves slowly, wind can push it uphill under the flashing and snow can sit against the head of the frame. Below that angle the design margins run out, so the major roof window makers publish 15 degrees as the lower limit for their standard products, and fitting below it usually falls outside the product warranty.
The upper figure matters less in houses but is worth knowing. Centre pivot windows are commonly rated to 90 degrees, so they can go in a very steep slope or even a vertical face. Top hung windows often have a narrower range with standard springs, commonly up to around 55 degrees, with different springs for steeper roofs. The types of roof window guide covers how each opening type behaves.
The tiles may set a steeper limit than the window
A roof window rated from 15 degrees does not make a 15 degree roof suitable, because the covering around it has its own minimum pitch, and that is often the higher number.
Every tile and slate has a minimum pitch from its maker, based on headlap and exposure. The window flashing is designed to sit into that covering, so if the tiles are already below their rated pitch, the roof around the window is marginal before any opening is cut. As a guide:
| Roof covering | Typical minimum pitch |
|---|---|
| Plain clay or concrete tiles | About 35 degrees |
| Natural slate | About 20 to 25 degrees, by size and exposure |
| Clay pantiles | About 22.5 to 30 degrees |
| Interlocking concrete tiles | About 12.5 to 22.5 degrees, by product |
These are typical figures only: the maker's data for the actual product on your roof is what counts. The flashing kit also has to match the covering, and the roof window flashings guide explains how kits for tiles, slate and pantiles differ. For slate in particular, see can you fit a roof window in a slate roof?
Which Cambridge roofs come close to the limit
Main roofs on Cambridge houses are almost always comfortably steeper than 15 degrees; the shallow slopes tend to be on rear additions.
The slate roofs of Victorian and Edwardian terraces in Romsey, Petersfield and around Mill Road, and the tiled roofs of 1930s semis in Cherry Hinton or Arbury, are generally well within range. The pitches to check are lean-to roofs over side-return extensions, single-storey kitchen additions, garage conversions and some porch or bay roofs. These are often built as shallow as the tiles allow, and a few are shallower than they should be. On newer homes at Trumpington Meadows or Great Kneighton the main roofs are fine, but a shallow monopitch over a ground-floor room can sit right on the boundary.
A roof at 17 degrees with interlocking tiles rated to 17.5 degrees is a different job from one at 17 degrees laid with plain tiles, even though a roof window is technically rated for both. The survey checks the pitch and the covering together.
What to do on a roof shallower than 15 degrees
There are three workable routes below 15 degrees: a low-pitch roof window kit, a flat rooflight on a kerb, or a sun tunnel where the goal is light rather than a view.
- Low-pitch kit. At least one major maker offers a low-pitch installation for its roof windows, typically covering roofs from about 10 degrees. It pairs the window with a dedicated flashing, an extra underlay and drainage collar and its own fitting rules. Check the current product data, as ranges change.
- Flat rooflight on an upstand. If the roof is shallower still, or the covering is a membrane rather than tiles, the right product is a flat rooflight on a timber kerb. The kerb lifts the glass above the roof surface and gives it its own fall.
- Sun tunnel. For a corridor or small bathroom under a shallow roof, a tunnel with a dome can bring daylight down without a large opening.
The roof windows for low-pitch roofs guide compares these options in more detail.
How the pitch changes the light in the room
A shallow roof window faces the sky more directly than a steep one, so the same size of pitched roof skylight gathers more daylight and more summer sun.
At Cambridge's latitude the midday sun stands at roughly 61 degrees above the horizon in late June and only about 14 degrees in late December. A south-facing roof window at 20 degrees takes the summer sun almost square on, which is bright and warm, and in a small room under a lean-to it can be too warm. The same window at 45 degrees meets the high summer sun at a more glancing angle and catches more of the low winter sun, which gives a more even year. A steep pitch also lets you see out towards the horizon when you stand beneath it, while a shallow one shows mostly sky.
The will a roof window make my loft too hot? answer and our light planning page explain how we work this out for a specific slope before anything is cut.
Questions people also ask
How do I measure the pitch of my roof?
From inside, hold a digital angle finder or a phone level app against the underside of a rafter. From outside, measure the rise over a known horizontal run: a 15 degree roof rises about 268mm for every metre of run. We confirm the figure at the survey.
Can a roof window go in a near-vertical roof?
Yes, within the window's rating. Very steep faces, such as the lower slope of a mansard, suit centre pivot windows rated to 90 degrees. Top hung windows need checking against their rated range and spring type.
Does a lower pitch need a bigger roof window?
Usually the reverse. A shallow window sees more sky, so a smaller pane can light the same room, and oversizing on a low south slope brings heat and glare with it.
What if my extension roof is exactly 15 degrees?
Then the tiles decide. If the covering is rated for that pitch and the flashing kit suits it, a standard roof window can work. If not, a low-pitch kit or a flat rooflight is the safer choice. Our roof window installation service in Cambridge starts with that check, so book a light survey and we will measure it.
