Should you re-roof before a loft conversion?

A loft conversion turns the space under the roof into somewhere people live, and the roof above becomes harder to reach and more expensive to fix. A leak that once dripped onto old boxes now lands on a new ceiling, a bed or a wall. Repairs to the covering mean scaffold, and scaffold costs the same whether you want one slate or a thousand.

That is why the roof is the first thing we ask about when someone plans a conversion. If the tiles are slipping, the felt is brittle or the flashings are failing, the roof comes first and the loft beneath it follows. Leaks are also much easier to trace in an empty roof space than behind new plasterboard and a fresh coat of paint. If the roof is sound and has years left, there is no need to replace it and the conversion can go ahead.

Our guide to the signs you need a new roof lists the signs of a failing covering, and they are worth checking before money goes into the room beneath.

The jobs touch in several places. Roof windows or a dormer mean cutting openings into the slope, with every new opening needing its own flashing, and these are far easier to flash properly when the covering is being renewed around them. New insulation goes between and over the rafters, which means the underlay and battens above are naturally exposed. Structural steelwork, new floor joists and a staircase may change the load paths, and the roof timbers may be strengthened.

Doing the conversion first and the roof afterwards means working around a finished room, protecting it, and stripping a covering that already has new windows set into it. Neither is impossible, but both add time and care. Doing the roof first and fitting the windows afterwards means cutting into a brand new covering. The best answer is a plan that treats both as one job, with the roofer and the builder agreeing the sequence.

The main roof replacement page describes the roofing side of that job.

A converted loft room becomes a habitable space, so the roof has to meet the energy rules. Approved Document L (2021 edition) expects a retained roof worse than 0.35 W/m²K to be upgraded to 0.16 (Table 4.3), with 1.4 for new roof windows (Band B is the alternative on the window energy rating scale), and 2.2 for rooflights. From 24 March 2027 the 2026 edition applies, and its Tables 3.2 and 3.3 repeat those values.

Reaching those numbers takes real insulation thickness, so the build-up often changes the rafter depth, the ceiling height and the way the roof is ventilated. Headroom is the usual casualty, and a conversion designer should check it early, long before the first rafter is cut. A designer who ignores this ends up redrawing the stair and the ceiling heights later. It is far easier to specify this when the roof is open than to retrofit it later.

Our page on roof insulation when re-roofing explains how the layers fit together.

A conversion is its own building control application, covering the structure, the stair, fire safety and the insulation. A re-roof alongside it is usually dealt with in the same application, which keeps the inspector's visits together. Where the conversion is notifiable, you need a registered building control approver or the council to sign it off before the room is used.

Planning is separate. Roof windows in an unlisted house often need no planning permission, but dormers are different. They are not permitted development in a conservation area, in the South Downs National Park or around Chichester Harbour, which carries the National Landscape designation, so those addresses need an application to the planning authority. Listed buildings need listed building consent.

Our guide to loft conversion roof windows goes into the windows and planning side, and your council's planning officers can say what applies to your house, since the rules vary with the address.

Start with a survey of the roof. The covering, timbers, flashings and the loft are all looked over, and you get a plain verdict: re-roof first, repair and leave alone, or nothing needed. If it is a re-roof, we agree dates with your builder so the roof is weathertight before the interior work starts.

Ask your loft conversion firm exactly who handles the roof, because some quotes assume a roofer will be found separately, and some leave the weathertightness of the finished slope unclear. Ask also whether the roof windows are included in the roofing price or the conversion price, so that neither party assumes the other has covered them.

You get one total price for the roofing job, written down after the survey. Scaffold that serves both jobs is a cost you only meet once, which is the main saving from combining them. What you pay depends on the size and access of your roof.