Part L and roofing, without the jargon

Part L of the Building Regulations is the part that says your house has to be reasonably energy-efficient. It was last substantially updated in June 2022 with the transitional arrangements running into 2023, and it applies to roof work in more situations than most homeowners — and a surprising number of roofers — realise.

This isn't about "eco". It's a legal requirement, enforced through Building Control, and a re-roof done without Part L compliance can show up at conveyancing when you sell the house and become your problem at exactly the wrong moment. It also decides how much insulation your roof should actually have, which matters for the heating bill.

The piece is for England and Wales (Approved Document L). Scotland and Northern Ireland have their own versions with similar thresholds and different paperwork — the concepts transfer but the specific numbers and sign-off routes don't.

What Part L actually requires

Part L sets maximum U-values for the thermal elements of a building. U-value is a measurement of how much heat escapes through a square metre of surface per degree of temperature difference. Lower is better. A bare single skin of brick is around 3.0 W/m²K. A modern insulated roof is around 0.16 W/m²K.

For roofs specifically, the current targets are:

• New-build pitched roof: 0.11 W/m²K
• New-build flat roof: 0.11 W/m²K
• New element in an extension (pitched): 0.16 W/m²K
• New element in an extension (flat): 0.18 W/m²K
• Replacement or renovation of an existing pitched roof: 0.16 W/m²K
• Replacement or renovation of an existing flat roof: 0.18 W/m²K

Those are the "elemental standards" — i.e. you hit the U-value for that specific roof and you're compliant. A typical re-roof on a 1930s house with no existing insulation at roof level would require somewhere between 140mm and 200mm of PIR insulation (Celotex or Kingspan) to get to 0.16, depending on exact product and detailing.

When Part L triggers on a roof job

This is the part most homeowners get wrong. Part L doesn't apply to every roof job — it applies to specific ones. The four triggers are:

1. New build

Obvious one. A brand-new house has to meet the full new-build U-values from the start. Every detail is designed in from day one by the architect and signed off at each inspection stage.

2. New extensions

An extension is a new part of a building even if it attaches to an old one, and Part L applies to its new thermal elements. A flat-roofed rear extension needs its new roof to hit 0.18 W/m²K. A pitched-roof extension needs 0.16. This usually means a warm-deck design with insulation above the structural deck, or a cold-deck with between-rafter and over-rafter insulation totalling 150–200mm.

3. Renovation of a thermal element

This is the one most people miss. "Renovation of a thermal element" is Part L's phrase for a job that renews or replaces 25 percent or more of a roof element. If you strip and relay a whole roof — a re-roof — you've definitively renovated the thermal element and Part L applies. You have to upgrade the insulation to meet current elemental standards (0.16 for pitched, 0.18 for flat) as part of the work.

Where it gets interesting: a small repair isn't a renovation. Replacing a handful of slipped tiles isn't. Replacing a single valley isn't. But once the scope crosses the 25-percent threshold — a full flat-roof replacement on an extension, a full re-roof on a house, a large-scale repair to more than a quarter of a roof pitch — the insulation upgrade obligation kicks in.

There's an out: Part L includes a "reasonable, practical and technically feasible" test. If the roof can't accept the full insulation depth without raising the roof line, for example — common on a Victorian terrace where the headroom above the rafters is limited — you can document that and aim for the best practical U-value instead of the headline target. Building Control signs off on a case-by-case basis. A good roofer discusses this with you and with building control before the job, not during it.

4. Material change of use

If you convert an existing building into a dwelling (e.g. a barn conversion, an office to flat conversion) the whole envelope, roof included, has to meet Part L standards as if it were new. This is a bigger job than a re-roof and is usually handled by an architect or surveyor rather than a roofer alone.

Cold deck vs warm deck (in thirty seconds)

Two ways to insulate a roof. The terms come up constantly in quotes.

• Cold deck. Insulation sits between and below the rafters, with a ventilated void above it. Cheaper and thinner, but needs careful detailing to avoid condensation on the cold side of the insulation.
• Warm deck. Insulation sits above the rafters in a continuous layer, with the structural timber in the warm zone. More expensive per-m², takes more headroom above the ceiling, but eliminates most thermal bridges and is much easier to detail for Part L compliance on a re-roof.

A warm-deck re-roof typically raises the roof line by 100–200mm, which can have planning implications on tall buildings or those near boundary lines. A cold-deck re-roof keeps the line the same but has to do more work with ventilation and vapour barriers. Neither is wrong — they're different answers to different constraints.

Listed buildings and conservation areas

Part L allows exemptions for "historic buildings", which includes listed buildings (Grade I, II*, II) and buildings in conservation areas where meeting full Part L would harm the character of the building. The exemption isn't automatic — you have to ask for it, document why full compliance isn't possible, and get building control agreement.

The argument usually runs like this:

"This property is a Grade II listed Victorian terrace. The original roof covering is Welsh slate at an eaves-to-ridge height of X. Raising the roof line to accommodate over-rafter insulation would alter the visual proportions of the facade in a way that Listed Building Consent would reject. We propose a cold-deck solution with full between-rafter insulation, achieving a U-value of 0.22 W/m²K, which is the best practical value for this property without harming its historic character."

Building control accept this more often than you'd expect, because the Historic England guidance on energy efficiency in historic buildings explicitly says harm to fabric and appearance can justify deviation from elemental standards. The key word is "documented". Don't rely on a verbal exemption — get it in writing from building control before the work starts.

Who signs off the work

For a Part L-qualifying roof job, sign-off can come from one of three places.

1. Local authority Building Control. The traditional route. You (or your roofer) submit a Building Notice or Full Plans application, pay the LA fee, and a BC officer inspects at key stages. Typical LA Building Control fee for a domestic re-roof is £300–£650 depending on LA and project value.
2. Approved Inspector (private building control). A privately-run alternative to the LA. Same legal function, often faster, sometimes cheaper. The AI issues the Final Certificate. Typical private fee: £400–£800.
3. CompetentRoofer self-certification.The government's Competent Persons Scheme for roofing. Lets a registered roofer self-certify the work as Part L compliant, issue the compliance certificate directly, and notify the local authority on your behalf. No separate Building Control application. You save the BC/AI fee, and the project runs faster because there's no inspection-booking friction.

Route three is the best for most homeowners on a standard re-roof or flat-roof replacement — ifyour roofer is a CompetentRoofer member and the job is within the scheme's scope. Ask before you sign: "Are you CompetentRoofer, and will you self-certify this for Part L?" If they say no, factor the £300–£800 BC/AI fee into your budget.

What you get at the end

A compliant Part L roof job finishes with paperwork. You should receive:

• The compliance certificate (from CompetentRoofer, the LA, or the AI)
• A statement of the achieved U-value and the method used to calculate it
• Manufacturer datasheets for the insulation product used
• Any condensation risk calculation that was needed for cold-deck designs
• The workmanship warranty from the roofer
• Any insurance-backed guarantee (IBG) paperwork

Put the whole bundle with your house paperwork and keep it forever. When you sell the house, the conveyancer will ask for it. Without it, the buyer's solicitor may require a Regularisation Certificate from the local authority retrospectively, which is painful and sometimes expensive.

What happens if you don't comply

Three realistic consequences, in order of likelihood:

1. Nothing, for years. Building regulations are not policed proactively for most domestic work. You can do an unnotified, non-compliant re-roof and nobody comes checking.
2. A conveyancing problem at sale.The buyer's solicitor asks for the Part L certificate as part of the standard enquiries. You don't have one. Options at that point: indemnity insurance policy (cheap but the buyer may not accept it), retrospective Regularisation Certificate (£300–£1,000+ plus any remedial work the LA demands), or a price reduction agreed with the buyer.
3. An enforcement notice from the LA requiring remedial work or a fine. Rare in practice but legally available for up to 12 months after completion of the works.

None of this is catastrophic on a small repair that never should have triggered Part L in the first place. It's meaningful on a £15,000 re-roof done without a compliance certificate.

How to use this when you get quotes

Questions worth asking every roofer who quotes a re-roof or a flat roof replacement:

• Does this job trigger Part L in your view? (It almost certainly does.)
• What target U-value are you designing to, and how are you achieving it?
• Cold deck or warm deck, and why that choice for this property?
• Are you a CompetentRoofer member? Will you self-certify?
• If not, whose Building Control do I need to notify, and who is paying the fee?
• What paperwork will I get at the end?

A roofer who can answer those without hesitation is the roofer you want. A roofer who shrugs and says "we don't usually bother with that" is a roofer whose work will show up at conveyancing in five years.

Read the hiring checklist for the other questions. The costs guide shows you the realistic price range once compliance work is included. The quote formbrief includes space for your Part L preferences so the roofers who come back know what they're quoting against.