Should You Re-Roof Before a Loft Conversion?

The sensible order depends on what the conversion needs from the roof. A conversion with roof windows set into the slope, or a dormer built out from it, means cutting into the covering anyway. If you re-roof first and then open a slope for a dormer, you pay for tiles that are about to be stripped in part. Where the covering is in good order and the conversion adds only roof windows, a good roofer can fit them neatly into the existing roof.

When the covering is old, and the rafters will be exposed or strengthened for the conversion, the better route is usually to plan one programme. The structure is opened up, any rotten timber is dealt with, new insulation and membrane go in, and a single covering goes over the lot. That also avoids damaging a brand-new roof with the heavy building work that a conversion involves.

There is one rule of thumb. Never leave the question of the roof's condition until the builders are in. Ask for a survey of the roof before you price the conversion, so you know whether the covering will last the conversion's life. Our guide to the Building Regulations for re-roofing gives the background, and the wider job is laid out on our roof replacement page.

Access is a big slice of what any roof job costs. Checkatrade's October 2026 roof replacement guide gives £700 to £1,300 for a new roof's scaffold as a national figure, which is only an example. Putting the roofing and the structural work under one scaffold hire period can save a good deal compared with paying for it twice.

The timing matters, though. The scaffold has to stay up long enough for both trades, and the programme needs planning so the roofer, the builder and any dormer fabricator are not waiting for each other. A scaffold on the public pavement brings in the highway authority, whose licence has to be in place first. In West Sussex the council asks for at least 10 working days of notice from application, so the start date has to allow for it, and the licence is sorted between the scaffolder and us.

Ask each contractor to price against the same scaffold and to say who erects it, who pays for extra weeks and who is responsible for making the roof watertight each night while the structure is open. The answers should be in writing before anything starts.

Class B of the permitted development rules covers a dormer, with limits. It allows up to 40 cubic metres added to a terrace house and 50 cubic metres for any other dwelling; a dormer cannot go beyond a slope that makes up the principal elevation where it faces a highway; and Class B does not apply if the house is on article 2(3) land, which includes conservation areas and National Parks. Roof windows are dealt with under Class C, with a limit on how far they stand out from the slope and a condition that any window in a side roof slope is obscure-glazed.

That is the outline, not advice for your house. A design that goes beyond those limits needs planning permission. Ask Crawley Borough Council's planners, or the council for your own town, before the drawings are fixed.

None of this changes the roofing detail. A dormer cheek and roof need lead or another waterproof covering, and the junction with the main roof is where leaks start if it is rushed, so the roofer should be involved early. Our answer on the building control certificate for a new roof covers the paperwork.

A loft conversion is building work, so it needs approval from building control, and the same body will look at any re-roof that goes with it. Two parts of the Approved Documents matter most to the roof.

Approved Document L, on the conservation of fuel and power, treats a retained roof element in a conversion under Table 4.3 of the 2021 edition, with a threshold of 0.35 and an improved value of 0.16 W/(m²·K). New or replacement roof elements are held to 0.15. Paragraph 4.13 allows a lesser upgrade where reaching that figure is not technically or functionally feasible, or would take longer than 15 years to pay back. The 2026 edition takes effect on 24 March 2027, with the same roof figures in Table 3.3. Your building control body will say which applies to your job.

Approved Document B, on fire safety, covers the escape route. For a house of two storeys the conversion usually calls for a protected stair; Approved Document B also accepts other arrangements, including a second route out of the top floor, or sprinkler protection in the open-plan areas below. That is the designer's job, but it affects the roof, because the escape window or roof window must meet minimum sizes. Ask early so the new roof is built around it.