Sarking price
per m².

Sarking — or external roof insulation — is priced per square metre, but that price per square metre only makes sense if you specify what it includes. Some estimates only cover supplying and installing the insulation system. Others include stripping the old roof covering, the new covering, scaffolding, metal roofwork and repairs to special details — a scope three to four times wider.

This is the first thing to check when you compare two quotes: when the scope differs, the comparison is worthless. A seemingly cheaper quote that excludes scaffolding and verge repairs is no longer cheaper once those lines are added back.

Rather than give you a figure that would not match your roof, we prefer to set out what really makes up the budget — and what makes it vary.

1. Scaffolding and site protection. This depends on height, accessibility and the layout of the plot. On sloping ground or with restricted access, this item climbs quickly.

2. Stripping the existing roof covering, and removing the materials. This varies with the material in place and whether any materials require special treatment.

3. The insulation system itself: supporting boards, vapour barrier, insulation, roofing underlay, counter-battens and battens. This is where the choice of insulation and its thickness weighs most — the target performance is paid for directly in euros per square metre.

4. The new roof covering. Tile, slate or zinc: the difference between these materials is considerable, and it is often the first factor of variation from one quote to another.

5. Repairs to special details: verges, eaves, chimney stacks, roof windows, overhangs. The extra height created by sarking means they have to be dealt with; this item is regularly underestimated in the cheapest quotes.

6. Metal roofwork: gutters, box gutters, valleys and flashings, to be redone to match the new roof thickness.

The condition of the roof structure is the first source of uncertainty. If the timbers are sound, we build on top. If rafters need replacing, sections need strengthening or a damaged purlin needs repairing, the budget changes in nature. That is why we carry out a survey before pricing, not the other way round.

The geometry of the roof comes next. A simple pitched roof with no dormers or valleys can be dealt with efficiently. Add more offsets, dormers, valleys and chimneys, and the labour time on special details can exceed that of the plain areas.

The target performance, finally. Aiming for the regulatory minimum or aiming for the level required to unlock grants does not lead to the same insulation thickness — nor the same price. This last point deserves a calculation rather than a reflex: a greater thickness that unlocks a grant can cost less net than a thinner one without a grant.

Our RGE CertiRénov certification (no. CR-2026-73-0095) is the condition for access to MaPrimeRénov', energy savings certificates and the zero-interest eco-loan for insulation work. The amount depends on your income, the performance achieved and the rates in force. These schemes change frequently: we check the conditions that apply at the time of your project, and we invite you to cross-check them on france-renov.gouv.fr before finalising your financing plan.

The real cost-effectiveness calculation is not “how much does sarking cost”, but “how much does sarking cost on top of a re-roofing job I have to do anyway”. Put that way, the question changes radically: the scaffolding, stripping and new covering are already in the budget. The real extra cost comes down to the insulation system and its installation — to which the grants apply.

That is exactly why we tell all our clients the same thing: if your roof needs redoing within five years, now is the time to decide to insulate. Redoing a roof covering and then wanting to insulate three years later means paying twice for scaffolding and stripping.