A re-roof opens up the part of the house that loses heat most readily, with the scaffold already up and the covering already off. That is why insulation and replacement belong together in the planning, even if the owner first rang about a leak.
There is a legal side too. Part L of the Building Regulations deals with conservation of fuel and power, and under certain conditions a re-roof can amount to renovating a thermal element, and the roof must then meet a stated standard once finished. The conditions are specific and the figures exact, so this guide gives them as published and says who decides how they apply to your house.
It then turns to the practical questions: where the insulation goes, how the roof is ventilated so the insulation does not cause damp, what to know about roof windows and spray foam, and what VAT does to the bill. For the whole sequence of the job, see what happens during a roof replacement, and for the bigger picture the roof replacement page.
When replacing a roof brings an insulation upgrade
Approved Document L, Volume 1, covers dwellings in England, and its 2021 edition took effect on 15 June 2022. It treats the roof as a thermal element. Paragraph 11.2 lists what counts as renovating one: stripping to the structure and rebuilding qualifies, and a flat roof recovered with a new waterproof membrane qualifies as well.
Paragraph 11.3 sets the trigger. The whole element is upgraded where more than 50% of the surface area is renovated, or where the work counts as a major renovation, which Part L defines as over 25% of the external envelope's area. The Planning Portal's re-roofing page puts it in softer terms, saying the thermal element may have to be upgraded.
Notice what the wording leaves open. A full strip and rebuild of a large slope can pass the threshold, whereas patching a few tiles never does. Where a particular roof falls depends on how it is built and where its insulation sits, and the person who decides is building control, either the council or a registered building control approver. We explain how the rules land on your house, deal with the Building Regulations notification where the work is notifiable, and give you the answer in writing.
How this relates to the 50 per cent line
You may already have met the 25 and 50 per cent figures. The Planning Portal uses them to say when a building control application is needed at all, while Part L uses similar numbers to decide when the energy standard must be met. Our page on the Building Regulations for a re-roof separates the two.
A job that stays under the thresholds is unlikely to trigger the upgrade, but it can still be a good moment to improve the insulation by choice.
Reading the U-values without a calculator
A U-value is a rating of heat loss through a building element, written as W/m²K. The lower the figure, the less warmth escapes. Regulations set limits as U-values, and nobody expects you to calculate them, but a few numbers are worth recognising so a quote that mentions them makes sense.
Table 4.2 of Approved Document L gives 0.15 as the limit when a roof is newly built or replaced on a house that already exists. Table 4.3, for existing elements being upgraded, uses 0.35 as the threshold and 0.16 as the improved value to reach. Paragraph 4.8 adds a floor: whatever goes in must perform at least as well as the part it takes out.
Reaching a figure like 0.15 takes a generous depth of insulation, and that has knock-on effects. The rafters may be too shallow to hold it all, the eaves can become tight, and the finished roof may sit slightly higher than before. None of that is a reason to skip it, but it explains why the insulation is settled at survey, before the covering is ordered.
What the quote should tell you
Ask which product is proposed, how thick it will be and what U-value the build-up achieves, because a reputable manufacturer can supply this. The sum depends on the type of insulation, the area, the depth and the access. After our survey you receive the written quote as a single total, priced fairly, and you should check whether any quote you collect includes VAT, since the rules are less simple than they look, as the final section explains.
Do not rely on a figure passed on by word of mouth. The test is the build-up as it is actually laid in your roof, with the depth and the product confirmed on paper before work starts.
Where the insulation goes: cold roofs, warm roofs and flat roofs
How a roof is insulated depends on how it is built, and three arrangements turn up repeatedly on the houses of Worthing and along the coast.
A cold roof has its insulation at ceiling level, laid across the joists, with the loft above left cold and ventilated. Many 1930s semis and post-war bungalows are built this way, and it is often the simplest upgrade, since the covering can be renewed independently of it. With a warm roof, the insulation is placed at rafter level, between the rafters or over them, so the space beneath stays inside the heated part of the house. It suits a loft that is lived in or being converted. A flat roof can be either, and Part L lists replacing its waterproof membrane as a renovation, which is why flat roof jobs so often raise the insulation question.
Between the rafters or over them
Fitting insulation between rafters uses the depth already there, but the rafters themselves carry heat through, and the space may fall short of the target on its own. A layer fixed over the rafters once the old covering is off reduces that effect and can improve the result without costing headroom below. It lifts the roof slightly and alters eaves and verge details, so if the house is in a conservation area, check with the planning team first.
Carelessly laid insulation causes more trouble than it cures. Gaps at the edges, crushed material and blocked eaves vents all undercut performance, and each is easier to get right with the roof open than to correct later. A loft insulated hard into the eaves with no air path is a common source of the problems in the next section.
Older and historic buildings call for more thought. A listed building has its own rules, and our page on re-roofing a listed or conservation area home explains where consent comes in.
Ventilation and condensation: keeping the insulation dry
More insulation makes a loft colder, and a colder loft collects condensation if damp air from the house can reach it. Warm air from bathrooms and kitchens carries moisture upward, and where it meets cold timber and underlay it turns to water. A well-insulated roof without ventilation can end up wetter than the draughty one it replaced.
The standards handle this with ventilation gaps. Marley's summary of BS 5250 deals with cold roofs under high-resistance underlay: eaves openings run continuously, 25mm wide when the pitch is 15° or less and 10mm above that. A further 5mm continuous opening goes at high level on roofs steeper than 35°, on any roof spanning over 10m, and on every lean-to or mono-pitch design. A low-resistance underlay combined with an airtight ceiling and a breathable covering gets by on far less, as long as the product holds its third-party certificate.
The point for owners is simple. Whoever lays the insulation has to leave those gaps open, usually by fitting eaves ventilators so the material cannot slump across them. Skimp here and the result shows two winters later as black mould along the rafters.
Sealing the ceiling below
The other half of the cure is stopping moist air from getting up there at all. Gaps round loft hatches, light fittings and pipe runs let it through, so sealing them is a small job with a large effect, and a hatch that closes properly with its own seal belongs in any sound upgrade.
If damp appears after work has been done, our page on telling condensation from a leak helps you work out which you have. A moisture problem is rarely solved by adding more material on top.
Roof windows, rooflights and spray foam
If the job includes roof windows or rooflights, Part L has separate figures. For a roof window the ceiling is 1.4 W/m²K, or alternatively a Band B rating under the window energy rating scheme, while a rooflight may not exceed 2.2. Table 4.2 also asks that an upstand built on site reach 0.35. The difference between the two is whether the glass lies in the plane of the roof or stands up from it, and the skylights and roof windows page covers fitting.
Spray foam needs its own caution, because it is often sold as a one-step answer. The Energy Saving Trust (last updated 8 May 2026) warns of firms that offer to mend a leaking roof by spraying foam under it without first curing the leak, and says it does not recommend that. It adds that open-cell foam should carry a much lower risk.
RICS, in its March 2023 consumer guide, recommends that thermal and condensation checks are done by a qualified professional and that you ask your mortgage provider whether their policy permits it. In a listed or protected building the foam needs consent beforehand, and RICS adds that taking it out could damage the structure behind it. Reports from November 2024 had some lenders declining loans on homes with foam, with others judging each one separately. That is a fair reason to think hard before choosing it for a roof you may one day sell.
If a home already has foam and you want it assessed, our spray foam insulation removal page explains the process.
For most re-roofs, board or quilt insulation on a properly ventilated roof is the more straightforward route, and it keeps options open for whoever owns the house next.
VAT on insulation, and what a re-roof does to it
VAT is where a plain upgrade can become confusing. The standard rate is 20%. HMRC's Notice 708/6, last updated 31 January 2024, puts certain named energy-saving materials at the zero rate when installed between 1 May 2023 and 31 March 2027, and from 1 April 2027 these revert to the reduced rate of 5%. The relief covers residential accommodation and charitable buildings, and insulation for roofs or lofts is listed.
That sounds like good news for a re-roof with new insulation, but there is a catch. Where insulation goes in alongside other work, the first question is whether the job counts as one supply taxed at a single rate, or as several supplies. The notice gives an example: an extension project that includes loft insulation counts as one standard-rated supply.
A re-roof itself is standard-rated. Whether a zero rate can reach the insulation element hinges on how the job as a whole is supplied, and we promise nothing beforehand. Raise it at the survey and our written quote will set out the treatment. With any other quote, check whether VAT is included and which rate was applied.
Until you have seen how an invoice is built up, treat a promise of tax relief on insulation within a re-roof as unproven.
Grants and other help
Government schemes shift quickly. The Great British Insulation Scheme required installations to be complete by 31 March 2026, and the Energy Company Obligation is due to run until 31 December 2026. Neither pays for re-roofing. Our roof grants page sets out what exists in this area, and the council's housing grants team is the place to ask on a low income.
