Dormer Roofs Explained: Types, Costs & Common Problems
Dormer Roofs Explained: Types, Costs & Common Problems
A dormer is a structure that projects out of a pitched roof, with its own walls, window and roof, to add headroom and light to a loft. Most UK dormers are flat-roofed box dormers on the rear of the house. Replacing a dormer's roof covering typically costs £1,200 to £3,500, while building a new dormer as part of a loft conversion usually runs from £35,000 to £60,000.
Here is how the main types of dormer differ, what each one costs to build or repair, why dormers are one of the most common sources of roof leaks we get called to, and what the planning rules say.
01/The main types of dormer
Flat roof (box) dormer
Most commonA box that runs across most of the rear roof slope with a near-flat roof. It gives the most usable floor space and is the standard choice for loft conversions on terraces and semis.
Gable-fronted dormer
TraditionalA small pitched roof with a triangular front, tiled or slated to match the main roof. Common on older houses and often used on front elevations where appearance matters.
Hipped dormer
Softer lookLike a gable dormer but with three sloping sides. It blends into a hipped main roof well, though it gives slightly less headroom inside.
L-shaped dormer
Maximum spaceTwo connected dormers — one on the main roof and one on the rear offshoot. Popular on Tyneside terraces and Victorian houses to create a large room and a bathroom.
Less common types include shed dormers with a single shallow slope, and curved eyebrow dormers on period or thatched properties. Whatever the style, every dormer adds the same thing to a roof: more junctions where water can get in.
02/Dormer costs at a glance
| Job | What's involved | Typical cost |
|---|---|---|
| Flashing repair | Re-dress or replace lead at a cheek or apron | £200 – £600 |
| Flat dormer re-roof | Strip and replace felt, EPDM or GRP on a typical dormer | £1,200 – £3,500 |
| Re-clad dormer cheeks | Replace tile hanging, lead or uPVC cladding on the sides | £800 – £2,500 |
| Fascia, soffit and gutter | New uPVC boards and guttering to the dormer | £400 – £1,000 |
| New dormer (loft conversion) | Structure, roof, window and cladding as part of a conversion | £35,000 – £60,000 |
Scaffold is usually needed for dormer work and typically adds £500–£1,200 on a two-storey house. Doing dormer repairs at the same time as other roof work saves paying for scaffold twice.
Why dormers leak
A dormer turns a simple roof slope into a set of abutments, upstands and edges, and each one relies on a detail being done properly. When a customer tells us water is coming through a loft room ceiling, the dormer is the first place we look.
- Flat roof coverings that have reached the end of their life — old torch-on felt blisters, cracks and splits at the edges after 15 to 20 years.
- Failed lead at the cheeks (the side walls) where the dormer meets the main roof tiles or slates.
- A missing or short apron flashing below the dormer window, letting water run behind the tiles.
- Flat roofs laid with too little fall, so water ponds and eventually finds a way in.
- Blocked or undersized dormer gutters overflowing onto the main roof or down the cheek.
- Condensation inside a poorly ventilated or uninsulated dormer roof that looks like a leak.
That last one catches a lot of people out. If the staining appears mostly in cold weather and there is no obvious defect outside, it may be condensation rather than rain. Our guide to loft condensation versus leaks explains how to tell the difference.
04/Choosing a covering for a flat dormer
EPDM rubber
A single sheet with no seams on most dormers. Flexible, UV-stable and typically lasts 25 years or more. A good all-round choice.
GRP fibreglass
Laid in place and very hard-wearing, with neat moulded edges. Suits dormers that are walked on for maintenance or need a crisp finish.
Torch-on felt
Modern high-performance felt in two or three layers. Cost-effective and reliable when laid well, with a typical life of 15 to 25 years.
Whichever covering you choose, the details at the edges matter more than the material. We cover the options in more depth on our flat roofing page, and the lead work around dormers on our lead roofing page.
05/Do you need planning permission for a dormer?
Many rear dormers fall under permitted development, so no planning application is needed, provided they meet the limits. The main ones in England are:
- No dormer on a roof slope that faces the road at the front of the house.
- Total added roof volume of no more than 40 cubic metres on a terraced house, or 50 cubic metres on a semi or detached house.
- Materials similar in appearance to the existing house, and the dormer set back at least 20cm from the eaves where practical.
- No higher than the highest part of the existing roof.
Flats, listed buildings and houses in conservation areas — including parts of Jesmond, Tynemouth and Gosforth — usually need planning permission. Building Regulations approval is needed for any new dormer regardless. Check with your council before you commit.
Common questions
How long does a dormer roof last?
A flat dormer roof in EPDM or GRP typically lasts 25 years or more, and modern torch-on felt 15 to 25 years. A pitched dormer covered in slate or tile lasts as long as the main roof, although the lead at the cheeks and apron may need attention sooner.
Can a leaking dormer be repaired rather than re-roofed?
Often, yes. If the flat roof covering is sound, the leak is usually at a flashing or an edge detail, and a targeted repair is the sensible option. If the felt is blistered, split or ponding across the whole surface, replacing it is better value than repeated patching.
Can you add a dormer to an existing loft conversion roof?
It is possible, but it is a structural job that needs Building Regulations approval and often a structural engineer. For new dormers we can work alongside your architect or loft conversion company on the roofing element.
Getting your dormer looked at
We survey dormers across Gosforth, Newcastle and North Tyneside, take photographs from the roof, and give you a fixed written price for the repair or re-roof. See our re-roofing and roof window pages for related work.