why a sheet
Numbers that have to agree
A rooflight goes wrong when one measurement is treated on its own. A window can match the hole in the ceiling and still be wider than the rafters allow. A flat unit can be the right size and still sit on a kerb too low to keep water out. So we record the readings together, on one sheet, and check them against each other before choosing anything.
The sheet is attached to your quote. Each line of the price traces back to a reading, which is why we can say what the job involves before it starts. Our costs page shows how that works line by line.
pitched roofs
Readings for a sloping roof
Rafter centres
The spacing between rafters, centre to centre, measured in the loft or through an existing opening. It sets the widest roof window that fits between two rafters without cutting one. On a roof with rafters at 600 mm centres, the clear gap is the spacing less one rafter's thickness, and that gap points to a width letter in the maker's range. Our guide to roof window sizes and rafter spacing goes further.
Rafter depth
How deep the timbers are. It decides how much insulation fits round the frame and whether a deeper lining is needed, and it tells us how much the roof can be trimmed if a wider window is wanted.
Pitch
The slope of the roof, read with a digital level. Each roof window model and flashing has a rated pitch range from its maker; the reading tells us which ones your roof falls inside.
Covering and batten gauge
What the roof is covered with and how far apart the battens are. Plain tiles, interlocking tiles and natural slate each take their own flashing, and the gauge decides where the window can sit so the courses finish neatly above and below it.
flat roofs
Readings for a flat roof
Opening size
The clear structural opening, or the one we would form, measured between the joists or trimmers. Flat rooflights and lanterns are sized to the kerb, so this reading leads to the unit's order size.
Roof build-up
What sits between the ceiling and the sky: joists, deck, insulation and finish. We note whether the roof is warm or cold in its construction, and whether the finish is felt, EPDM rubber or fibreglass, because the kerb is wrapped in that same finish.
Kerb height
The upstand the unit sits on, measured from the finished roof surface. Makers set a minimum, commonly about 150 mm. If an existing kerb is lower, the sheet says so and the quote includes raising it. The upstand height guide explains why it matters.
Fall
Which way water runs off, and how much slope there is. Some units build the fall into their frame; others need it built into the kerb. The reading tells us which.
- A common maker's minimum kerb height above the finished roof
- 150 mm
- Part L figure for a new rooflight in an existing home
- 2.2 W/m²K
- Part L figure for a new roof window, or Band B
- 1.4 W/m²K
inside and around
The room, the way up and the permission position
The room inside
Ceiling height, which way the room faces, where the light needs to land and what sits below the opening. A landing over a stairwell, a loft bedroom and a kitchen extension each ask for something different, and a window you need to reach has to be within reach or motorised.
Access
How we will get to the roof safely: ladders, a mobile tower or scaffold, and whether materials can come round the side of the house or have to come through it.
Permission position checked
We look at whether the house is a flat, listed, or inside a conservation area, a National Park or a National Landscape, and which slope the window would sit on, since side-facing windows carry conditions of their own. We write down our view and the reason for it. The council makes the decision, and our planning guide explains how to check the position for your own address.
on the visit
How the survey visit runs
A survey usually takes place with you at home, because we need to see the room as well as the roof. You do not need to prepare anything beyond clearing a path to the loft hatch if there is one.
From arrival to the written quote
- Look and askWe see the room, hear what you want from the light, and look at the roof from the garden.
- Inside the roofIn the loft or through an existing unit we measure the rafters or joists and note the build-up.
- On the roofWhere it is safe, we read the pitch, the covering, the kerb and the fall at the spot itself.
- Check the numbers togetherBack on the ground we compare the readings and choose units that fit all of them.
- Write it upYou receive the survey sheet with an itemised quote. Nothing is ordered until you accept.
yours to keep
What happens to the sheet
The sheet is yours. If you take it to another installer, the readings are still correct. If you go ahead with us, it becomes the order: the unit is bought to those figures and fitted to them. If something on the roof turns out to differ once the covering is lifted, we show you on the sheet what changed.
For a quick overview of the visit, read what happens at a rooflight survey. To book one, use the quote page.