The short answer
Every conservation rooflight has a stated pitch range from its maker, and your roof has to fall inside it. Shallow cottage roofs and steep slate or stone roofs can need different units, because pitch decides how water sheds around the frame and how the sash hangs. We measure the pitch on site, then choose the unit whose range fits.
Pitch is an angle, and the range belongs to the product
Roof pitch is the angle between the roof slope and the horizontal, measured in degrees. A conservation rooflight is made for a stated span of those angles, and outside it the unit is the wrong one.
The lower limit exists for water. Rain runs off a steep slope fast. On a shallow one it moves slowly, it can back up against the upstand and flashing, and wind can push it upwards under the covering. The maker sets the lowest pitch at which the flashing detail is expected to keep water out.
The upper limit is about the unit itself. On a very steep roof the glass, bedded at that angle, shifts its weight onto the frame, and the sash of an opening version hangs differently. The range is printed in the maker's data, and a competent survey reads your angle against it before anything is ordered.
So there is no single number that covers every conservation rooflight. A shallow pitch may suit one range of unit and rule out another, and a steep one the other way round. Quote any pitch from a friend's roof and you may be quoting the wrong product.
Why shallow cottages and steep slate roofs need different answers
The roofs around Crawley and across Sussex come in a wide spread of pitches, and the covering is usually a clue to the angle.
| Covering | Typical pitch | What it means for the unit |
|---|---|---|
| Natural slate | Moderate, often laid flatter than tile | Check the lowest pitch in the range |
| Plain clay tile | Steeper, around 40 degrees or more | Most ranges cover it |
| Horsham stone | Traditionally steep | Check the upper limit and the bedding |
| Outbuilding or lean-to | Shallow | May need a flat roof unit |
These are general patterns, not a rule for any one house. Slates can be laid quite low when the laps are generous, which is why a slate roof may be shallower than the tiled house next door. Plain clay tiles need a steeper pitch because the water has to run off across many overlapping edges. Horsham stone slabs are heavy, and the roofs that carry them were built steep to support them.
Our pages on conservation rooflights in slate roofs, clay tile and tile-hung roofs and Horsham stone roofs cover the covering itself. This page is about the angle underneath.
How the pitch is measured on your roof
A rough guess from the ground is never enough. The survey takes the angle directly.
- From inside the loft, a digital level is laid along a rafter at the planned position, so the reading comes from the structure, not the surface.
- On the outside, the angle is checked along the roof slope, because old roofs sag and rafters may differ from one end to the other.
- The figures are compared with the range printed for the shortlisted unit.
- If the roof sits near the edge of a range, we choose a different unit rather than squeeze one in.
Where the roof meets the edge of the range, the frame width and the flush-fit detail come into it too. How flush fitting conservation rooflights sit in the roof explains why the detail changes with angle.
Pitch changes how the unit opens and how much light it gives
The same pane delivers a different amount of daylight at a different angle, and an opening sash behaves differently too.
On a shallow slope, the glass faces the sky more directly and collects more overhead light. On a steep one the glass catches the horizon and less of the zenith, so the room is lit from the side more than from above. Neither is better. They are different kinds of room.
A steeper pitch also matters for any unit that opens. The sash swings out and up from the slope, and the clear gap you get depends on the angle. Read do conservation rooflights open for the styles, and can a conservation rooflight be used as a fire escape if the opening has to serve as a way out of a loft room. For size, the page on choosing a conservation rooflight size shows how the pitch feeds into the measuring.
When the roof is too shallow for any pitched unit
Some roofs fall below every conservation range. A lean-to, a porch or a flat-roofed extension may have only a slight fall, and a pitched-roof unit simply cannot be used.
A flat roof is given a fall so water drains, expressed as a ratio such as 1:80. That is far below the pitch a sloped unit needs. On such a roof we fit a flat roof rooflight set on an upstand, and the options are on flat roof rooflight installation. A taller glazed feature, built for a low pitch, is a roof lantern. Either can be finished in a slim, dark frame to keep the restrained look a conservation setting wants.
Pitch questions from cottage and barn owners
Can I find my roof pitch myself?
You can get a rough idea with a phone level laid on a rafter in the loft. It is a fair starting point, but the survey measures it properly before anything is ordered.
Does a barn conversion have a different pitch range?
Barn roofs are often steep and long, which suits some units and rules out others. Conservation rooflights for barn conversions goes into the rest.
Will a planning officer care about the pitch?
They care how the unit sits in the roof plane. A correct match lies flush and looks right, while a wrong one stands proud. That is a planning matter too, described on planning consent for conservation rooflights.
What do I send to get started?
A photo of the roof and your address through the quote form or on WhatsApp is enough. Our hub, conservation rooflight installation in Crawley, shows the range.