What a survey looks for in an existing roof
Adding a rooflight to a roof that already exists is a question of four things: the frame that holds the roof up, the covering that keeps the weather out, the space between roof and room, and where the light will actually land.
A new roof built with a rooflight in mind has the opening designed in from the start. An existing roof was built without one, so the job is to find a place where an opening can be made cleanly and then to trim, weather and line it so the roof works as well afterwards as it did before. On a pitched roof that means working between or across the rafters and through tiles or slates. On a flat roof it means cutting through joists or decking, building a kerb and carrying the membrane up it.
The survey starts from inside. We look at the room first, because the reason for adding a skylight is the light, and a position that suits the rafters but lights the wrong corner is no use to anyone. Then we go up into the loft or check the ceiling void, measure the spacing and size of the timbers, note the covering and its condition, and look for anything in the way: a chimney breast, a purlin, a water tank, cables or a soil vent pipe.
Pitched roofs: rafters, trusses and the covering
How a pitched roof is framed matters more than its age, because it decides whether the opening can sit between timbers or has to interrupt them.
Victorian and Edwardian terraces in Romsey, Petersfield and around Mill Road, and most 1930s semis, have traditional cut roofs: individual rafters, often with purlins and struts. Many roof window sizes are made to fit between typical rafter spacings, so a single roof window can sometimes go in without cutting a rafter at all. A wider window, or two side by side, means cutting one or more rafters and trimming the opening with doubled timbers to carry the load around it.
Homes built from around the 1960s onward, including many estate houses and newer homes at Trumpington Meadows or Cambourne, usually have trussed rafters. A truss is an engineered triangle, and every member in it is working. Cutting a truss chord or web without a design weakens the whole frame, so any opening wider than the gap between trusses needs a structural engineer, and often the truss manufacturer's input, before anything is cut.
| Roof frame | Opening between timbers | Opening across timbers |
|---|---|---|
| Cut rafters | Often possible for a narrow unit | Trimming needed; engineer advised |
| Trussed rafters | Limited by truss spacing | Engineer design needed |
| Flat roof joists | Depends on joist direction | Trimmer joists; engineer advised |
The covering matters too. Clay tiles, concrete tiles and natural slate each take a matching flashing kit, and on an older roof the survey checks the battens and felt around the proposed opening. Our roof window installation page covers the pitched side in more depth.
Flat roofs: joists, deck and upstand
On a flat roof the new rooflight sits on a raised kerb, so the existing roof has to take the opening in the deck and the weathering up the sides of that kerb.
Flat roofs on rear extensions and garages across Cambridge vary widely: older felt roofs on chipboard, newer GRP or EPDM on OSB, some cold decks and some warm decks. The survey checks which way the joists run, how deep they are, whether the deck is sound and whether the fall is enough for water to clear the upstand. A kerb that stands at least 150mm above the finished roof is the usual guidance, and the covering has to be dressed up it and sealed.
If the existing roof covering is near the end of its life, it may make more sense to renew it at the same time, rather than weather a new rooflight into a membrane that will need replacing in a few years. Our flat roof rooflight installation page explains the kerb and covering in detail.
Planning permission and Building Control
Many houses can add a rooflight under permitted development, but not every house can, and Building Control is a separate question from planning.
For a house, rooflights generally fall under Class C of the General Permitted Development Order, subject to conditions. The main ones:
- The rooflight must not project more than 150mm beyond the plane of the roof slope.
- It must not sit higher than the highest part of the roof.
- A rooflight on a side-facing slope must be obscure glazed and non-opening, unless the opening part is more than 1.7m above the floor of the room.
- Permitted development does not apply to flats and maisonettes, and an Article 4 direction can remove it in parts of a conservation area.
- A listed building needs listed building consent for any rooflight, whatever permitted development says.
Cambridge has many conservation areas and a great many listed buildings, so it is worth checking with the council before you commit. Our guide to planning permission for rooflights covers Greater Cambridge Shared Planning and the neighbouring councils, and the answer on flats and permitted development covers the flat route.
Building Regulations are a separate matter. A new rooflight in an existing roof is generally treated as notifiable work, and cutting rafters, trusses or joists is a structural alteration that Building Control will want to see. The glass also has to meet the current thermal standard. We handle the Building Control notification where the work is notifiable; the Building Regulations guide sets out the parts that apply.
Where the light will land
An existing roof fixes the slope and the orientation, so the choice left is the position, the size and the glass.
A rafter bay on the south slope might be the easiest place to cut, but it may pour summer heat onto a loft bed or throw glare across a desk. A north slope gives even, soft light with little heat. East and west slopes give low morning or evening sun that reaches deep into the room. On flat Cambridge land, with no hills to shade a roof, orientation makes a bigger difference than it would on a hillside town. Our skylight orientation guide compares the four directions across the year, and every quote we give carries a written light plan explaining the position, size and glass we have chosen.
The inside also has to be finished. On a pitched roof with a ceiling below, a light shaft or lined reveal carries the light down, and the insulation around it has to be made continuous so the edges of the opening do not become cold and damp.
Questions people also ask
Is adding a rooflight different from putting one in any roof?
Slightly. The question of which roof types take which products is covered in can you put a skylight in any roof. Adding one to an existing roof is about working with the structure and covering you already have, without rebuilding them.
Can a skylight go in a loft that is only used for storage?
It can. A roof window in an unconverted loft still needs the opening trimmed and weathered, and the insulation detailed around it. Our answer on skylights in a loft without converting it covers what changes.
Will adding a rooflight damage the rest of the roof?
Done properly, no. The disturbance is limited to the area around the opening, which is re-tiled or re-weathered to the new flashing or kerb. The inside is made good and decorated ready for paint.
How do I find out if my roof is suitable?
A survey answers it: we look at the room, the loft and the covering, and tell you where a skylight can go and what light it will give. Book a light survey, or read more about our skylight and rooflight installation in Cambridge.
