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Dormer flat roof replacement

A dormer flat roof is the small flat roof on top of a dormer window or loft conversion, and replacing it means rebuilding that roof and its edges from a scaffold. The area is small, but the height, the tiled roof around it and the bedroom below make careful detailing more important than on most flat roofs.

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Rear box dormer with a grey GRP flat roof and anthracite cladding on a Victorian terraceComputer-generated example image
Guide price
£1,400–£2,650
Survey
Free for new and replacement roofs
Quote
Fixed, in writing
Coverage
England, Wales, Scotland, NI

What does replacing a dormer flat roof involve?

Replacing a dormer roof means putting up safe access, stripping the old covering, renewing the deck, insulating above it where the room below is heated, and laying a new membrane with trims and abutments that tie into the main pitched roof. Most of the skill goes into the edges.

  • Scaffold to the dormer, with edge protection at the working level.
  • Strip the old felt and check the deck and the timber at the dormer cheeks and front edge.
  • Insulate as a warm roof with a vapour control layer, because the room below is almost always a bedroom or bathroom.
  • Set the fall, usually towards the front or one side, so water discharges into a gutter rather than onto the tiles.
  • Detail the back edge where the dormer meets the main roof, with an upstand under the tiles and lead or a membrane flashing.
  • Fit new drip trims at the front and sides, and a gutter where one is needed.

How much does a new dormer roof cost?

An 8 m² dormer roof in GRP, including scaffold, is typically £1,400–£2,650 including VAT. Scaffold is a large share of the cost on a small roof, so the per-square-metre rate is less useful here than the whole-job figure.

ItemGuide price
8 m² dormer in GRP, with scaffold£1,400–£2,650
EPDM rubber rate£65–£115 per m²
GRP fibreglass rate£75–£140 per m²
Torch-on felt rate£60–£105 per m²
Scaffold, two-storey or dormer£450–£950
Warm-roof insulation£35–£60 per m²

Insulation and new deck boards (£20–£35 per m²) are added where they are needed, and new edge trims are £12–£22 per metre. These are guide prices; the fixed price follows a free survey. Use the guide-price tool or the cost guide to compare.

Is GRP or EPDM better on a dormer roof?

GRP fibreglass is often the first choice on a dormer, because it is laid as a liquid resin over a mat and forms neat, rigid edges, corners and drip details on a small roof. It cures to a hard surface that copes with occasional foot traffic during window cleaning or maintenance.

EPDM rubber works well on a plain rectangular dormer and can usually be laid as one sheet, with no field joins. Unlike GRP, it does not depend on resin curing conditions on the day. Torch-on felt can be used, but working a torch close to timber cladding, fascias and tiles needs extra care, so a cold-applied system is often chosen instead.

We recommend the system after surveying the dormer, its edges and how it meets the main roof.

Do I need building regulations approval for a dormer roof?

Usually, yes, if you are replacing most of the roof. A dormer roof sits over a heated room, so it counts as a thermal element, and renovating more than half of it normally means upgrading the insulation where feasible.

In England the renovation target is 0.16 W/m²K for homes, approved through building control or a competent person scheme. Wales usually needs approval for most re-roofing work. Scotland works through the council's building standards and, where required, a building warrant. Northern Ireland asks you to contact district council building control before starting.

Insulating above the deck raises the roof slightly. On a dormer, that extra height has to fit under the main roof tiles at the back edge, so the build-up is planned at survey. Re-covering a dormer that looks the same afterwards does not usually need planning permission, but a listed building or conservation area may bring extra controls. More on our building regulations page.

How long does a dormer roof replacement take?

The roofing itself typically takes one to three days, with scaffold going up before and coming down after. Allow a few extra days overall for the scaffold.

The room under the dormer stays usable, though it will be noisy during the strip-out, and dust can fall through gaps around the ceiling. Moving furniture away from the window helps. Scaffold usually needs space on the drive or garden, and sometimes a pavement licence. GRP has a strong smell while it cures, so keep the dormer windows shut on the day it is laid.

Why is scaffold and tile detailing so important on a dormer?

Because a dormer roof is high, small and surrounded by a pitched tiled roof, most failures happen at the edges rather than in the middle. Safe access is what allows those edges to be done properly.

Ladders alone do not give a safe working platform at that height, and every roof is treated as fragile until a competent person confirms otherwise. Scaffold with edge protection lets the team work along the front and sides of the dormer, and reach the junction with the main roof.

The back of the dormer is where most leaks start. The new membrane has to turn up under the tiles or slates, with a flashing that sheds water over it. Where the old detail used lead, we reinstate or replace it. Small roofs also leave little room for error at the drip edge: the front trim must throw water clear of the window frame and fascia, and any gutter has to be big enough for the area it drains. Dormer cheeks with tile hanging or cladding are checked for gaps that let water behind.

If you are also planning insulation upgrades or work on the main roof, tell us at survey so the scaffold can be shared.

What's included when we do the work?

  • Free survey of the dormer roof, cheeks and main roof junction
  • Scaffold with edge protection, priced in the quote
  • Strip-out of the old covering and deck repairs
  • Warm-roof insulation and a vapour control layer
  • New membrane, drip trims and back-edge abutment under the tiles
  • Written workmanship guarantee, with building control handled where needed

What do guide prices look like?

Guide prices including VAT at West Midlands rates; regional pages adjust these for your area. The fixed price comes after a free survey for new and replacement roofs.

JobGuide price inc VAT
Dormer (8 m²), GRP, scaffold£1,400–£2,650
Single garage (18 m²), EPDM rubber£1,600–£2,800
Extension (20 m²), GRP, new deck + insulation£3,000–£5,400

Does anything differ outside England?

  • Wales: In Wales, most re-roofing needs building regulations approval, so check before starting if more than 25% of the dormer roof is being replaced. The renovation U-value target is 0.16 W/m²K.
  • Scotland: In Scotland, check with the council's building standards service whether the dormer roof work needs a building warrant. Alterations and structural work usually do, and insulation is assessed under Section 6 of the technical handbooks.
  • Northern Ireland: In Northern Ireland, contact the district council's building control before work on a dormer roof begins. Roof insulation is considered against Technical Booklet F1/F2.

How does the job run from start to finish?

  1. Survey from safe access

    We inspect the dormer roof and its junction with the main roof, measure up and check the room below.

  2. Quote and scaffold plan

    Your fixed written quote covers scaffold, insulation, membrane and trims, and we agree where the scaffold will stand.

  3. Strip and rebuild

    Once scaffold is up, the team strips the dormer roof, insulates, lays the membrane and forms every edge and abutment.

  4. Check and strike

    We check the edges and drainage from the scaffold before it comes down, then hand over your paperwork.

Questions about dormer roofs

Can a dormer roof be replaced without scaffold?

Rarely. A tower or full scaffold is usually needed to give a safe working platform and edge protection at that height.

Will insulating my dormer roof make it higher?

Yes, by the insulation thickness. We plan the build-up so the new roof still sits correctly under the main roof tiles at the back edge.

Why does my dormer leak at the back where it meets the roof?

That junction is the most common leak point, usually because the upstand or flashing under the tiles has failed. A proper re-roof rebuilds it.

Can you replace the lead on my dormer at the same time?

Yes, where lead forms part of the dormer abutment or cheek flashings, we include renewing it in the roof job.

Does the room below need emptying?

No, but moving furniture away from the window and covering the floor is sensible. The room stays usable throughout.

How long does a GRP dormer roof last?

As an indicative range, GRP lasts 20–40 years with good specification and maintenance.

Is a dormer roof cheaper than a garage roof?

Not by as much as the size suggests. Scaffold and the extra detailing at height mean a small dormer roof often costs about the same as a single garage roof.

Related services

GRP fibreglass roofing

Hard-wearing laminated fibreglass with moulded trims, for dormers, extensions and garages

EPDM rubber roofing

Single-sheet synthetic rubber for garages, extensions and porches, laid cold with no flame

Warm roof insulation upgrades

Insulation laid above the deck during a re-roof, to cut heat loss and condensation risk.

Flat roof repairs

Patch, seam, upstand and outlet repairs on felt, EPDM, GRP, liquid and asphalt flat roofs.

Where do you offer dormer roofs?

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