Question answered
Replacing the flat roof on a dormer
The short answer
Replacing the flat roof on a dormer means stripping the covering, checking the deck, laying a new membrane with properly dressed upstands and re-dressing the lead on the cheeks and abutments. The work is mostly detail, and so is the cost. Access over a pitched roof and Part L insulation questions come first, and leaks usually start at a junction rather than in the middle.
A dormer roof is small, but it is one of the harder flat roofs to get right. It is reached over a pitched roof, surrounded by vertical faces and full of junctions where water can enter.
Why dormer roofs leak
A dormer is a small box built out of the pitched roof, with a window at the front, two side walls called cheeks and a flat or gently sloping top. Every one of those parts meets another, and each meeting is somewhere water can get in.
The middle of the flat roof is seldom the problem. The usual culprits are the cheeks, where lead or tiles meet the side walls, the abutment where the dormer top meets the main slope behind it, the junction with the window head, and the outlet or gutter at the front. A stain near the dormer window may be water that has travelled a long way from its entry point.
This is why the survey comes first. We trace the fault from inside and out before we price anything, and we photograph what we find. A dormer that leaks at the cheek will keep leaking if only the top is renewed, however good the new membrane.
Getting up there: access and scaffold
A dormer sits partway up a pitched roof, so reaching it safely usually means scaffold. A working platform at eaves level, extended up to the dormer, protects the roofers and the tiles. A ladder on its own is rarely adequate for stripping and relaying a roof, whatever its size.
Scaffolding that occupies the footway or road needs a highway licence, which the firm erecting it requests under section 169 of the Highways Act 1980. West Sussex County Council grants it for Worthing and Adur and wants a minimum of 10 working days' notice, and Brighton & Hove licenses its own highways. The firm deals with the paperwork, but the notice period can shift when work starts.
Access is a main driver of the price on a small roof, because the scaffold has to go up whatever the size of the job. Whatever quote you hold, see if it states that VAT and scaffolding are included; ours arrives once we have surveyed, as a single total.
Cheeks, lead and the upstands
The cheeks are commonly clad in lead, tile or fibre-cement board. Lead on a cheek is stepped up the slope, with soakers (small folded lead pieces tucked under the tiles) where the side wall meets the main roof. Leadworx describes code 5 as suited to general, vertical and pitched work, calls code 4 the usual choice for general flashing and recommends code 3 for soakers only.
Where the flat membrane reaches a wall, it turns up as an upstand, tall enough to keep standing water from reaching the joint. A dormer top that sags towards the window can pool against that line. IKO, citing the 2018 edition of BS 6229 (since revised), advises designing to a 1:40 fall so that 1:80 is the finished minimum, so the deck is checked and often relevelled.
Window-head details matter too. The membrane must meet the frame without letting water behind it, and this is where old mastic and sealant often disguise a failed junction. Properly dressed lead lasts; sealant buys only time.
Part L and the insulation question
Approved Document L, Volume 1 counts laying a fresh waterproofing membrane on a flat roof as a renovation of the thermal element, the building's term for a roof, wall or floor. Where more than 50 per cent of that element is renovated, or the work is a major renovation of more than 25 per cent of the building envelope, the whole element is upgraded, aiming at the improved roof value of 0.16 W/(m²·K) in Table 4.3.
A dormer roof is small next to the house, but that does not exempt it by itself. Whether it counts as its own thermal element, and what the build-up allows, is something we put to the building control body at the start. The cheeks and front are often insulated at the same time, and the existing construction can leave little room.
Once the roof is renewed, a routine look at the outlet and the lead pays for itself; flat roof maintenance describes what to check. We handle the Building Regulations notification where the work is notifiable.
Planning, wildlife and getting it right
Putting a like-for-like covering back does not usually need planning permission, but a listed building or an Article 4 direction can change that, so check with the Adur & Worthing planning team if your home sits in an area with extra controls.
Roofs are homes for wildlife too. Bats and any nests in use are legally protected, and GOV.UK names reroofing work as something that can disturb bats, so stop and take advice if you notice signs of either.
Dormer roofs added to a house at different times vary a good deal in how they were built, which is another reason the survey looks at the deck and the junctions before anything is specified. The background on why flat roofs appear on so many houses is on why houses have flat roofs. If you want to use the roof for access, see walking on a flat roof. More on the subject is on our flat roofing page.
Dormer roof replacement: more questions
Can the dormer roof be repaired instead of replaced?
Sometimes. A failed cheek flashing or a split at the abutment can be put right where the membrane and deck are sound. If the covering is worn across the whole surface, patching a junction only delays the job.
How long does replacing a dormer roof take?
It depends on the scaffold, the deck condition and the weather. A small roof is not a quick one, because cheeks, lead and upstands take time to detail properly. We explain the programme after the survey.
Tell us about your roof
Describe what is happening and we will call you back to arrange a visit.
- Photographs of what we find
- A written quote with one total price
- Or call 01903 299063
10-year workmanship guarantee, in writing.