Insulation when re-roofing

Do you have to add insulation when you re-roof?

Insulation depends on the share of the roof being renewed. Below: the Part L trigger, the figures, the feasibility test and the ventilation that goes with it.

The short answer

Yes, once more than half of the roof surface is renovated: Part L of the Building Regulations, in Approved Document L (AD L), then expects the whole roof to reach a U-value of 0.16 W/m²K, if that can be done and repays its cost within 15 years. A U-value rates heat loss through the roof, and the lower the number, the less heat escapes. Replacing a few tiles sits well below that line.

4 min read · Part of Roof replacement

A hipped red-tile roof on a white-rendered bungalow — Insulation laid between the rafters of an open roof

How the renovation trigger works

AD L 2021 para 11.3 says the whole of a thermal element, which means a wall, floor or roof, should be improved to Table 4.3 column (b) in either of two cases.

More than 50% of the surface of that individual element is renovated.
The job counts as a major renovation, because over 25% of the external building envelope is renovated.

Para 11.2(d) adds that on a flat roof, replacing the waterproof membrane counts as renovation. Windows, roof windows and rooflights fall outside the definition of a thermal element, so they have figures of their own. A repair that touches only a small patch does not set this rule off.

How a particular job is counted is for building control, whether the council team or a registered approver. We measure the area at the survey, so the share is known before the quote is written, and we ask building control early where a roof sits close to either line. Measuring first also avoids the awkward position of finding out halfway through that the job has crossed a threshold.

The U-values in plain figures

These come from AD L 2021, the edition in force in England until 24 March 2027. Units are W/m²K throughout.

Roof figures in AD L 2021
Case	Figure	Table
Existing roof, threshold	0.35	4.3 (a)
Renovated roof, improved	0.16	4.3 (b)
Roof as a new element	0.15	4.2
Roof window	1.4 or WER Band B	4.2
Rooflight	2.2	4.2
Upgraded under the para 4.13 fallback	Generally no worse than 0.7	Para 4.13

Read the tables as two separate questions. Table 4.2 is for new elements, such as a roof window put in. Table 4.3 is for existing elements, and column (b), the improved figure, is the one a renovated roof is held to. Para 4.12 also reaches retained elements above the threshold, such as a roof kept in a loft conversion.

When you read a data sheet or a quote, look for a stated U-value, not just a thickness in millimetres. Thickness alone says little, because the heat loss of a build-up depends on the material as well as its depth. The figure worth seeing in writing is the one the roof will achieve as built, set against 0.16 where that applies.

When the full standard cannot be met

The 0.16 figure has a release valve. Para 4.13 allows a fallback where reaching column (b) is impractical for technical or functional reasons, or where the upgrade would take longer than 15 years to repay in a simple payback. The roof is then upgraded to the lowest figure that is achievable on both counts and pays back within those 15 years. A simple payback means the cost of the upgrade measured against the saving each year.

So the standard bends only for a reason that can be shown, such as rafters too shallow to hold the thickness needed. We record the reason at the survey and agree it with building control, not on the day. Where the roof is being opened up anyway, the time to settle insulation thickness and ventilation is before the covering goes on, because the build-up and the eaves gap are fixed together.

AD L 2026 starts on 24 March 2027 for use in England on work outside the higher-risk category, with the tables renumbered 3.2 and 3.3. For the wider test a re-roof must pass, see our guide to building regulations approval to re-roof.

Ventilation, foam and what sits above

Insulation changes how moisture behaves in a roof, so ventilation is planned with it. Take a cold roof under an HR underlay: Marley asks for 25 mm of eaves ventilation up to a 15° pitch and 10 mm beyond it. BSI lists BS 5250, the moisture code, as current and under review. The other underlay route is on our page about the breathable roof membrane.

Spray foam sits in a different category from board or quilt:

RICS cautions that once foam covers the roof timbers, nobody can inspect them fully.
In March 2023 RICS reported concern in some parts of the lending market, so check with your lender first.
The Energy Saving Trust points out that closed-cell foam lets no vapour through, and says of open-cell foam that the rot risk should be much lower.

Whichever route is chosen, keep the maker's data sheet for the insulation and the underlay, since building control may ask for both. Foam already in the roof is a separate job, covered on our spray foam removal pages, and the roof replacement page follows the whole job.