The short answer
It can, and sometimes Part L expects it. Stripping and re-covering a pitched roof is a chance to add insulation, and the Building Regulations can ask for the roof to be brought up to a better standard where that is feasible. New tiles and underlay do not insulate by themselves. Any change must keep the roof space ventilated or correctly designed.
A new roof covering keeps water out but does little for heat loss. Here is what a re-roof changes, what Part L asks for, and the ventilation catch.
What a new covering does and does not do
Tiles, slates and underlay shed water and block wind. They are not insulation. The warmth in a typical pitched-roof house is held in by the layer at ceiling level in the loft, or in the rafters if the loft is a room. A fresh set of tiles over the same loft leaves heat loss as it was.
So the straight answer is that re-roofing gives you the opportunity, not the result. With the old covering off, the roofer can reach rafters and underlay from outside, and the building control body may ask for insulation to be improved as part of the job. A leaking roof also soaks the insulation beneath it, and damp insulation does less, so curing water entry protects what you already have.
Look in the loft before and after. If you can see the tops of the joists through the insulation, there is room to add. If there is a bulging patch or a stain, find out whether the roof has been leaking, and see our answer on loft condensation and roof ventilation.
Spray foam under the roof is a separate decision with mortgage and survey consequences, covered in the last section.
When Part L reaches a re-roof
Approved Document L, volume 1, deals with saving fuel and power in dwellings. Its 2021 edition took effect on 15 June 2022 and lists a new waterproof membrane on a flat roof among renovation works. Work becomes major renovation when over a quarter of the external envelope's surface is being renovated, and an individual element, the roof for instance, is upgraded when over half of its surface area is renovated.
Where it applies, the roof moves towards Table 4.3: a threshold of 0.35 W/(m²·K) and an improved value of 0.16. Where that target cannot practically be reached, or the payback would stretch beyond 15 years, the roof is upgraded to the lowest achievable U-value that still repays itself within 15 years. A newer edition takes effect on 24 March 2027, so building control will say which one covers your start date.
The Planning Portal separately says a re-roof needs no building control application where less than 25 per cent of the building envelope and less than 50 per cent of the roof is affected, so a small repair does not trigger an upgrade. For anything larger, ask your building control body before the roof is opened.
Warm roof, cold roof and the ventilation knock-on
A cold roof keeps insulation at ceiling level, so the space above stays cold and has to breathe. A warm roof puts insulation in or above the rafters, so the whole void stays warm, which suits loft rooms. Which one you have decides what a re-roof can do.
BS 5250 deals with moisture in buildings. With HR underlay, which is not breathable, a cold roof needs an eaves gap of 25 mm on pitches of 15 degrees or less, and 10 mm on steeper ones, says Marley. In some designs a breather LR underlay can depend on the leakage built into ordinary eaves detailing, provided third-party certification backs it.
The catch is that added insulation can choke the eaves. Thick quilt pushed hard into the corner stops air moving, so moisture from the house has nowhere to go and condensation forms on underlay and timbers. A roofer should leave a clear air path from the eaves vents, and may fit trays to hold the quilt back.
Changing the underlay alters how the roof breathes, which is why our answer on breathable membrane vs roofing felt is worth reading before you choose.
Spray foam, VAT and grants
RICS recommends confirming with your mortgage provider that lender policy permits spray foam, and the Energy Saving Trust advises against foaming a leaking roof before the leak is cured. Foam also coats the timbers and makes later inspection harder. Ask your lender and surveyor before agreeing to it.
On VAT, gov.uk gives the standard rate as 20%. Energy-saving materials are zero-rated until 31 March 2027, then revert to a reduced rate of 5%. Whether your job qualifies depends on what is installed and by whom, so ask the roofer, read the gov.uk notice and check whether a quote includes VAT.
On grants, most homes will not get one for a new roof. Some households can get insulation through council-run schemes, among them the Warm Homes: Local Grant for wall, loft and underfloor insulation, which asks for an EPC of D, E, F or G and a household income that usually must be £36,000 a year or less. Check with your council and gov.uk, because eligibility is theirs to decide.
To start, send a photo of your loft and the roof's age if you know it. Our pitched roofing page lists the work involved.
More on this in Pitched Roofing Services in Crawley
Two more insulation questions
Will new tiles make my house warmer?
Not by themselves. Tiles and underlay keep water and wind out, which protects the insulation you already have. Warmth comes from insulation at ceiling or rafter level. A re-roof makes adding or improving it easier, and building control may ask for it, so decide with the roofer before the roof is opened.
Do I need to ventilate the loft after adding insulation?
Usually yes, if you have a cold roof. Insulation must not block the eaves, and the roof needs the ventilation its design calls for. Ask the roofer how air will reach the roof space and keep that path clear. A warm roof is designed differently, so the answer depends on the build-up.