Question answered
What roofing material is most energy efficient?
The short answer
No roof covering is the most energy efficient on its own, because the heat you pay for escapes through the insulation layer, not the tile or slate. Slate, clay, concrete and flat roof membranes all keep the weather out; insulation and ventilation decide the warmth. Part L expects a rebuilt roof to lose no more than 0.15 W/(m²·K), and a re-roof may bring an insulation upgrade with it.
On a UK pitched roof, the tile or slate on top makes little difference to your heating bill. The insulation beneath it and the ventilation around it do nearly all the work.
Why the covering matters least
A tile or slate is a rain screen, nothing more. A well-insulated loft under any covering will outperform a poorly insulated one under the supposedly best tile. Its job is to shed water, and it does not hold heat in. Heat leaves a house through the ceiling and loft, which is why insulation sits at the loft floor or between the rafters, not in the covering itself.
Much online advice about energy efficient roofing comes from hot climates, where reflective roofs reduce cooling. That logic carries over poorly to a UK house that needs to keep warmth in for most of the year. Choosing between slate, clay and concrete on energy grounds is a distraction, and so are claims that one colour of tile saves money.
The covering should be chosen for the pitch, the look, the load on the structure and how long it lasts, which the page on what roofing material lasts the longest covers. Energy performance is settled separately, in the build-up beneath it.
What Part L asks of a re-roof
Approved Document L, Volume 1 (2021 edition incorporating 2023 amendments) sets the thermal rules for dwellings in England. Table 4.2 of that document caps heat loss through a new roof element at 0.15 W/(m²·K). That figure is a U-value, and the lower it is, the less heat escapes.
Renovating a roof can bring the whole element up to standard. Taking a roof back to its rafters and building it again counts as renovation, and so does a fresh waterproof membrane over a flat deck. The upgrade follows where work touches more than 50 per cent of the roof surface, or more than 25 per cent of the external building envelope. For existing elements, Table 4.3 gives a roof threshold of 0.35 and an improved figure of 0.16 W/(m²·K).
In practice, a full re-roof often means better insulation. The page on insulation when re-roofing goes through the options, and the roof replacement page sets the job in context.
Glazing set into the roof
Part L treats glazing in the roof separately. Table 4.2 gives roof windows a ceiling of 1.4 W/(m²·K) (the alternative is a rating of Band B), and rooflights 2.2 W/(m²·K). They differ in how they are installed: a roof window is set flush with the slope, while a rooflight sits on a raised kerb, and any upstand built on site should reach 0.35 or better.
These figures matter if you add or replace a window as part of the work. A poorly specified one loses more heat than the insulation around it can save, and a re-roof is a convenient moment to put it right.
Whether a new glazed opening is allowed without planning permission depends on the house and the area, so ask before you order; the skylight guidance on this site covers that.
Ventilation: the other half of a warm roof
Insulation without ventilation can leave a cold roof damp. Marley's reading of BS 5250 for a high-resistance underlay is an eaves gap of 25 mm on roofs at 15 degrees or less and 10 mm on steeper ones. A continuous 5 mm gap at the top is added where the pitch passes 35 degrees, the span tops 10 m, or the roof has a single slope.
Roofs with a low-resistance underlay, a sealed ceiling below and a vapour-open covering above can rely on roughly 3 mm at ordinary eaves, provided the certification conditions are met. Deciding which case applies is a technical call for the survey, not a guess.
The warmth of a loft is only as good as the ventilation that keeps its timbers dry, a point that matters most when work is done in colder months, covered in roofing in winter.
Foam and other shortcuts
A leaking roof should not be sprayed with foam from below before the leak is fixed, according to the Energy Saving Trust. Open-cell foam lowers the risk, but the fault still comes first.
The same logic applies to any insulation. A dry, sound roof structure comes first, with a covering in good order and the leak traced. Insulation is then added to a roof that can still be inspected from below, so any fault that develops later can be found.
For the order of work in a full re-roof and when the insulation goes in, see what to expect during a re-roof and how long a re-roof takes.
Ask any contractor how they will keep the roof space ventilated after the insulation goes in, and make sure the answer is in the written quote.
Insulation, tiles and heat
Do light-coloured tiles lower my heating bills?
Not in any way that matters for a UK house. Colour affects how much heat a surface absorbs in sunshine, which is a cooling question. Heating bills depend on how much heat escapes through the ceiling and roof, and that is down to insulation and airtightness.
Will a re-roof mean new insulation?
Often, though not every time. Where under 25 per cent of the building envelope and under 50 per cent of the roof is touched, the Planning Portal says building control is not normally needed. Past those limits, the thermal upgrade rules in Part L can apply to the work.
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