Insulating a tiled roof at rafter level: what's required

Part L of the Building Regulations deals with saving fuel and power. Its current guidance for homes, Approved Document L Volume 1 (2021 edition), has been in force in England since 15 June 2022. Under it, stripping a roof down to bare rafters and rebuilding on them counts as renovating a thermal element.

Renovate more than half of the roof surface and Table 4.3 becomes the standard for the entire roof. The envelope has its own trigger: renovating over 25% of the external surface of the building is a major renovation. Refixing some slipped tiles or repairing a ridge falls far short of either figure, so nothing more is asked of the insulation. Smaller re-roofing jobs, under 25% of the envelope and under 50% of the roof, do not normally need building control approval at all, which patch repair or re-tile takes further.

A U-value tells you how quickly heat escapes through a building element, so the lower it is, the better the insulation. For an existing roof, Table 4.3 sets a threshold of 0.35 W/m²K and a target of 0.16 W/m²K after renovation. Table 4.2 sets 0.15 for a newly built or replacement roof element.

A note to the table makes room for awkward cases. Where reaching the full figure would reduce head room, the note accepts a lesser standard, which is relevant under the sloping ceilings of a loft room. The same note says the insulation and any air gap together should at least equal the depth of the rafters. Technical feasibility is judged by your building control body, so we raise the question with them before a build-up is chosen.

Pack the space between the rafters and you can cut off the air that a cold roof depends on. BS 5250, as Marley summarises it, expects air to enter at the eaves and, where the covering and membrane call for it, to travel above the insulation. How far a breathable underlay and an airtight ceiling reduce that need is covered in our guide to roof ventilation and condensation.

When this goes wrong, warm room air reaches cold timber and condenses inside the structure, out of sight. RICS sounds the same warning about spray foam: poor installation can lead to dampness and uncontrolled condensation, so thermal calculations and condensation checks belong with a qualified professional beforehand. Whatever product is used, the rafters, battens and underlay need to be sound before anything is fitted between them.

Spray foam has attracted particular worry. RICS, in its consumer guide of March 2023, does not regard it as reversible and thinks it unlikely to get consent in a listed building. Today's Conveyancer reported in November 2024, on the back of a BBC investigation, that some lenders said they would decline homes with spray foam and others would assess each application individually. Check with your mortgage and insurance providers before settling on it.

Older construction often suits materials that breathe, which is relevant to clay roofs of the kind in our pantile roofs answer, and to buildings in a conservation area. We lean towards products that can be inspected and lifted later, and the quote names exactly what has been specified for your roof.

Rafter depth caps what fits between them. On an older house the rafters can be shallow, and filling them leaves little or no air gap above the insulation. Two answers are common: a further layer beneath the rafters, which costs head room, or a thinner board with a better rating, which costs more per square metre.

Roof pitch matters as well. A shallow roof copes less well with moisture and with the limits of the tile itself, and our answer on low-pitch tiled roofs explains the minimum pitches for common tiles. We look at both before proposing a build-up, because a layout that works on one roof can trap damp in another. Where timber is already damp, it is dried and repaired before it is covered.

Most roofing work attracts standard-rate VAT. Supplying and fitting insulation to a roof or loft at home is zero-rated between 1 May 2023 and 31 March 2027 and then drops back to the 5% reduced rate, according to VAT Notice 708/6. Your quote shows VAT, and we give no advice on mixed jobs.

First comes a survey, with photographs of the rafters and eaves. The written quote takes in stripping, insulation, membrane, battens, labour, waste and making good. Where Part L applies we explain it and manage the Building Regulations notification. Our workmanship guarantee lasts 10 years and comes in writing. A survey is typically £150 to £500 in MyBuilder's 2026 inspection guide, and the rest of our tiled-roof advice sits under pitched roofing.

Insulating between the rafters gives a warm roof, in which the insulation follows the slope and the loft space becomes part of the heated house. The alternative is a cold roof, with the insulation on the loft floor and the roof space left open to air. The choice shapes everything else: the ventilation, the head room and the way the roof is built.

A warm roof suits a loft you want to use or convert, and it is often the natural choice when the tiles are coming off in any case, since the rafters are open to work on. A cold roof is simpler, and cheaper to add to when the loft is only storage. Both can meet the regulations if the details are right. We explain which is practical for your roof, why, and what each does to the quote.