The answer
A roof window needs a pitch inside the range the manufacturer rates it for, and that range has a floor. Below it, rain and snow do not clear the glass and the flashing cannot keep the opening watertight. On a shallower roof the answer is a flat rooflight, raised on an upstand and built for low angles.
What pitch means and why it matters
Pitch is the angle of the roof, measured in degrees from the horizontal. A steep slate roof on an older house has a high pitch. A modern extension roof might be nearly flat. A roof window is a sloping window that sheds water down its own glass, so it relies on the roof to give it enough fall.
Every manufacturer states a range of pitches for each model, and the floor of that range is the minimum pitch for a roof window. Pushing below it risks water pooling at the sill and creeping beneath the flashing. We fit within the stated range every time.
For the wider picture of types, see our roof windows hub.
A useful way to picture it: a puddle forms where water has nowhere to go. On a roof window the glass is the surface, and the pitch is the slope that sends the water away. Too little slope and the surface holds it, whatever the glass quality.
The two ways of lighting a low roof
When a pitch is too shallow for a standard roof window, there are two honest options. The first is to raise the angle: a roof window can be fitted with a kerb or a tilt-up frame that lifts the glass to a better angle. The second is to change the type. The flat rooflight is raised on an upstand, its glass set to a gentle fall, and it is designed for exactly this situation.
Which suits you depends on the room. A skylight in a pitched loft room lights the sloping ceiling and gives a view straight up. A rooflight on a low extension roof pours daylight into the middle of the floor.
Our flat roof rooflights hub explains the upstand and how the fall works.
Neither route is a compromise. Each is the right product for its roof, and the choice follows the angle, the room and the view you want from the space below.
How we find your pitch
You do not need to measure the angle yourself. At the free survey we check it on the roof or from inside the loft, using a level and the rafter line. Having the true number matters because a roof that looks steep from the garden can be shallower than it seems, especially across a long run.
If you want a rough check beforehand, look at the gable end or a side view of the house. A gentle slope that you could imagine walking up is nearer the low end than a steep one that you could not.
We write the measured pitch and the model it allows into the fixed quote, so the numbers are on paper before work begins.
| Roof angle | Suits | Why |
|---|---|---|
| Within the rated range | Standard roof window | Glass sheds water on its own fall |
| Slightly below range | Kerb or tilt frame | Lifts the glass to a better angle |
| Low or flat | Flat rooflight | Upstand with a slight fall |
Pitch, flashing and the roof covering
The flashing is metal or lead work that seals the join where the window frame meets the roof covering. Its job is to carry water over the frame and onto the tiles below. On a steeper roof the water moves quickly and the flashing has an easy task. On a shallow roof water moves slowly, and that is where leaks begin.
The covering matters too. Concrete interlocking tiles, plain clay tiles and slate each need a different flashing kit, and each has its own minimum pitch. We match the kit to your roof at the survey rather than applying one rule across the house.
The right flashing, set properly, is what makes a roof window watertight. It is also what a replacement unit needs: a new window in an old opening should come with fresh flashing, not the tired old set.
Where a roof covering is worn, replacing the tiles around the opening is part of a sound fit, and we tell you so at the survey before it reaches the quote.
Low pitch and what to check
A low pitch does not close the door on daylight from above. It only changes the product. Before you decide, a few questions help the survey go faster.
- Is the roof a loft slope, a dormer cheek, or an extension?
- Does the roof covering look old or worn?
- Which way does the slope face?
- Would you like the unit to open for fresh air?
Bring those answers and we can tell you on the day which route is open. For a loft room, our guide on roof windows for loft conversions covers head height and position.
Before the survey
- Which part of the house is the roof?
- What covering does it have?
- Which way does it face?
- Should the unit open?
The planning and cost side
Pitch does not decide planning. Whether you need consent depends on the building and the area, and the answer to that is in do you need planning permission for a roof window. Cost depends on size, glass and the work at the roof edge, and what moves the price sets that out.
Ask us for the survey on WhatsApp 07880 284133 or at info@vertexskylightsmiltonkeynes.co.uk. We measure, advise on the type, and give a fixed written quote.
More on roof angle
Can a roof window go in a very shallow roof?
A standard roof window cannot, but a flat rooflight can. It is built for low roofs, standing on an upstand whose slight fall clears the water.
Does a steeper roof limit the window?
Each model also has an upper limit in its rated range. A very steep roof may need a different model, which we check at the survey.
Will a low pitch affect the flashing?
Yes. A shallow roof needs a flashing kit chosen for it, and the kit has to suit the tile or slate on the roof.
What if my roof has two different slopes?
Each slope has its own pitch, so we measure the one the window will sit in. A house with a main roof and a rear extension can need a roof window on one and a flat rooflight on the other.
