Where the duty comes from
Approved Document L calls it renovation of a thermal element: providing a new layer, or replacing an existing one, over more than 50% of that element's surface area. Stripping a roof and re-covering it does exactly that. There is a second, wider trigger too — if the works renovate more than 25% of the total building envelope, every renovated element is caught whatever its own percentage.
Note the two duties arrive together. Re-covering more than 25% of a roof is itself building work, so it is notifiable — building notice, full plans, or a Competent Person Scheme registration by the roofer. The Part L thermal-element requirement rides along with it. That is why so many people meet this rule for the first time when the scaffolding is already up.
The numbers
| Element | Threshold — if worse than this… | …improve to |
|---|---|---|
| Roof (renovated) | 0.35 W/m²K | 0.16 W/m²K |
| Roof (newly built, e.g. an extension) | 0.15 W/m²K (Table 4.2) | |
| Rooflight on an upstand | 2.2 W/m²K | |
| Roof window in the plane of the roof | 1.4 W/m²K | |
Table 4.3 for the renovated roof, Table 4.2 for anything new. Source: Approved Document L, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments (England).
A worked example
A 62 m² pitched roof on a 1970s house, 0.60 W/m²K with its original thin quilt, being stripped and re-covered. It is worse than the 0.35 threshold, so the target is 0.16 — reached at ceiling level by topping the loft up to about 300 mm, roughly £900 on this roof:
payback = £900 ÷ £95 = 9.5 years — inside the 15-year test, so the upgrade is required
That is the ordinary case: at ceiling level the upgrade is cheap and pays back quickly, so "it isn't economic" will not wash. Where the argument does bite is insulating at rafter level on a slope that cannot take the depth, or a dormer cheek where the insulation would take the headroom below the usable — technical and functional grounds, recorded with a reason.
Cold roof or warm roof?
Part L sets the U-value, not the build-up. If the loft is unheated storage, insulating at ceiling level — between and over the joists — is the cheap way to reach 0.16, and the roof slope itself can be re-covered like for like. If the loft is habitable, or is becoming habitable, the insulation has to follow the heated envelope up the slope, and a re-roof is the moment to do it: the covering is off, and insulating above the rafters ("over-rafter" or warm roof) avoids both the headroom loss and most of the condensation risk.
Whichever route, check the build-up rather than assuming it. Insulating at the rafters changes where moisture condenses, and the ventilation path and vapour control layer matter as much as the U-value. U-Monkey gives you the U-value and the BS EN ISO 13788 condensation check free, from the same layers.
Flat roofs: the expensive surprise
Re-covering a flat roof renovates the entire element in one go, so the trigger is met by definition. A typical pre-2000 flat roof sits far worse than 0.35 W/m²K, so the target is 0.16 — which in practice means a warm-deck build-up with the insulation above the deck and the membrane over it. Quoted as a like-for-like recover and corrected at inspection, that is a painful conversation. Quoted properly at the start, it is a line item.
What is not caught
- Repairs — slipped slates, a patch of felt, repointing the ridge.
- Re-covering less than 50% of that roof, provided the wider 25% envelope trigger is not met either.
- A roof already at or better than 0.35 W/m²K — though it must not end up worse than it was.
- Solar panels, aerials and rooflight replacements on their own: they are not a new thermal layer (a replacement rooflight is a controlled fitting with its own U-value rule).
Proving it to Building Control
What the building control body wants is simple and specific: which element, how much of it is being renovated, what its U-value is now, what it will be, and — if that is short of 0.16 — why, with the payback arithmetic behind the claim. RenoMonkey is free to use on screen and produces exactly that as a Building Control-ready PDF for £35 +VAT, with a report ID they can verify online.
Frequently asked questions
Do I have to insulate when I have my roof re-covered?
If more than 50% of the roof is being re-covered, yes — provided the existing roof is worse than 0.35 W/m²K. Approved Document L treats stripping and re-covering as renovating a thermal element, and a renovated roof worse than the threshold should be improved to 0.16 W/m²K where that is technically, functionally and economically feasible. A roof already at or better than 0.35 needs nothing beyond not making it worse.
Is a re-roof "notifiable" to Building Control?
Re-covering more than 25% of a roof is building work in its own right under the Building Regulations, so it needs a building notice or full plans application (or a Competent Person Scheme registration by the roofer). That is what brings the Part L thermal element duty into play at the same time — the two arrive together, which is why the insulation question so often lands after the scaffold is up.
Can I insulate at ceiling level instead of at the rafters?
Usually, and it is normally much cheaper. The duty is on the roof as a thermal element, not on a particular position for the insulation: for a cold roof, topping up the quilt between and over the ceiling joists to reach 0.16 W/m²K satisfies it. Insulating at rafter level ("warm roof" or insulated-at-the-slope) is what you need when the loft is, or is becoming, habitable space.
What about a flat roof?
Same rule, bigger consequence. Re-covering a flat roof is renovating the whole element, so a flat roof worse than 0.35 W/m²K should reach 0.16 — which normally means a warm-deck build-up with the insulation above the deck, not a like-for-like recover. It is far cheaper designed in at quotation stage than discovered at inspection.
What if the roof cannot reach the improved value?
Then you go as far as is feasible and record why. The tests are technical (the construction physically cannot take it), functional (it would make the space unusable — headroom lost at a dormer cheek, for example) and economic: a simple payback longer than 15 years. An upgraded element should generally still not be left worse than 0.70 W/m²K. RenoMonkey estimates the payback from your own cost and fuel price and prints the justification.
Does a repair or a patch count?
No. Replacing a few slipped slates, repointing a ridge or repairing a small area is not renovating a thermal element. The trigger is how much of that roof you are actually re-forming — more than half of it.
Does insulating at the rafters cause condensation problems?
It can if the build-up is not thought about, which is why the ventilation path and the position of any vapour control layer matter as much as the U-value. Interstitial condensation risk is assessed to BS EN ISO 13788; U-Monkey does that check free alongside the U-value, so you can see whether a proposed build-up is safe before it is on the roof.
Next: the full Table 4.3 rules · garage and loft conversions · work out the upgraded build-up free.