Prefab vs traditional dormers
Short answer
Prefab dormers are built in a factory and craned on in a day. Traditional dormers are built on the roof, plank by plank. Prefab wins on speed and consistency; traditional wins on flexibility for unusual roofs and on the kind of finish a careful site crew can deliver. Both can be excellent — and both can be poor.
What 'prefab' actually means today
A modern prefab dormer arrives as one box: structure, insulation, frames, glazing, lead apron and exterior finish, all factory-assembled. The crew opens the roof, lifts the box on with a crane, seals the connections to the existing roof and dresses the interior. Total time on site: usually one day.
What 'traditional' means
A traditional dormer is built on the roof: structural timbers, sheathing, insulation, frames, glazing, cladding and roofing membrane all assembled in place. Site time is typically 4–8 working days, depending on size and weather. It accepts irregular widths, unusual pitches and bespoke details that a prefab line cannot easily run.
Side-by-side comparison
| Aspect | Prefab | Traditional |
|---|---|---|
| Install time on site | Usually 1 day | 4–8 working days |
| Factory lead time | 6–14 weeks | 2–4 weeks |
| Consistency of quality | High (factory tolerances) | Depends on the crew on the day |
| Flexibility (size, pitch, details) | Limited to the product range | Almost unlimited |
| Need for a crane | Always | Rarely |
| Disruption to the household | One disruptive day | Spread over a week |
| Typical price (like-for-like) | Slightly lower | Slightly higher |
| Lifespan | Comparable when materials are equal | Comparable when materials are equal |
Which one belongs on which house
- Prefab fits best on: standard terraced houses, semi-detached homes built after roughly 1970, modern roofs with good crane access, owners who want minimum disruption.
- Traditional fits best on: older houses with irregular roofs, listed buildings or protected streets where bespoke detailing is required, tight back-of-house plots with no crane access, projects with unusual widths or pitches.