The short answer
Yes, in most flat roofs a rooflight can be added to the roof you already have. A new opening is cut, the structure around it is trimmed and supported, a kerb is formed and the waterproof covering is dressed up and over it. What decides whether it is simple or complicated is what lies under the covering: the deck, the joists, the insulation above or below them, and how much of the roof is still sound.
Because the opening cuts through the structure, it is building work. It needs Building Control approval and has to meet Part L, and we handle that notification. If you are weighing a flat rooflight against a larger roof lantern, our guide to flat rooflight or roof lantern sets out how they differ.
What gets cut, and what holds the roof up
A flat roof is a deck laid across joists, with a waterproof layer on top. Cutting a rooflight opening removes a short length from each joist it crosses, so the load those joists carried has to go somewhere else. The usual answer is a pair of trimmer joists or a beam either side of the opening, with the cut joists hung from them in metal hangers.
For a small opening between two joists, that may be a matter of adding short noggins. For a wide one, the trimmers carry more load and sometimes need to be a deeper section or a steel member. It is a structural alteration, so the sizing comes from a calculation for your roof, not a rule of thumb. Where an engineer's input is needed, it is flagged at survey and written into the quote before any work starts.
Warm roof, cold roof and what sits above the joists
Where the insulation lies decides how the opening is built.
- Warm roof
- The insulation sits above the deck, under the waterproof covering. The kerb is formed on the deck, the insulation is cut back to it, and the covering is lapped up the kerb. The ceiling below needs only the lining formed and finished.
- Cold roof
- The insulation sits between the joists, with a ventilated gap above it. The opening interrupts that gap, so the ventilation path has to be kept open round the kerb. Done badly, this is where condensation starts.
- Inverted or green roof
- Insulation or planting sits above the membrane. These are the most involved to alter, because the layers above the waterproofing have to be lifted, set aside and relaid.
Most roofs from the 1960s and 70s onwards are cold roofs in a loose sense, with insulation between joists of varying quality. Extensions built since the 2000s are more often warm roofs. A survey that lifts a corner of the covering, or looks from the ceiling void, will tell us which you have.
The covering you already have
A new rooflight means a new junction in the waterproofing, and it has to bond to what is there. Felt, EPDM rubber and GRP each take a different detail. Our guide to flat rooflights on GRP, EPDM and felt roofs goes through them.
The point to know here is age. If the covering is near the end of its life, a perfectly formed kerb joined to it only shifts the weak point to the edge of the new work. Where the surface is tired, it can make sense to renew the covering in the same visit, so the whole roof ends on one footing. A roofer's opinion on the surface is part of the survey, and we say plainly if the covering needs attention before a rooflight goes in.
Falls, kerb height and the glass
A flat roof is never perfectly level. It is laid to a fall so rainwater runs to the gutter or outlet, and a kerb is a small dam across that fall. Water arriving at the high side of a kerb has to be directed round it, which is why a deep kerb in the wrong place can pond water behind it. The position of the opening on the roof matters as much as its size. For the heights and falls that apply, see flat rooflight upstand heights and falls.
The kerb also sets how the glass sits and how far the unit projects above the roof, which affects the look from the garden and from the road. Where the roof is visible from a neighbour's window or a public place, it is worth thinking about at the start.
Planning, Building Control and your ceiling
On a house, a new rooflight normally falls under the permitted development allowance in Class C, Part 1, Schedule 2 of the General Permitted Development Order, so long as it stands no more than 150mm proud of the roof plane. On a flat roof that limit is measured from the roof surface, which means a rooflight on a tall kerb has to be checked against it rather than taken for granted. The allowance does not reach everywhere: an Article 4 direction in a conservation area can withdraw it, a listed building needs listed building consent, and a flat or maisonette has no householder permitted development rights at all. Where any of that applies to you, ask the local planning authority before work is booked.
Inside the room, the opening is made in your existing ceiling. If it is plasterboard, the cut is clean and the edges are bead and skim. If it is lath and plaster, in an older flat-roofed rear addition for example, the cut needs supporting so the plaster around it does not crack away. The lining is then formed down from the glass to the ceiling, and a deeper roof build can carry a splay. We write all of this on the finish schedule that comes with your quote: the ceiling, the roof build, the shape of the lining and how it is finished.
Access, scaffolding and the day itself
Opening a flat roof exposes the room below, so the work is planned around the weather. The deck is cut and the unit set the same day wherever possible, and the opening is covered if the job runs on. Rooflights are heavy, particularly large glass ones, and they have to be lifted to the roof, so access matters: a rear extension reached through the house is easier than a roof above a first-floor landing that needs scaffold or a hoist. Any scaffold is allowed for in the written quote.
Inside, dust sheets and a sealed doorway keep the cutting dust in one place, and the room is left clean when we finish.
What to do next
Photographs of the room and the roof, and anything you know about when the roof was last recovered, are enough to start. We then survey, measure the roof and ceiling, and send a written quote with the finish schedule. For the full picture of what we fit and how, see our flat roof rooflight installation page, or ask for a quote.