Sarking has no dedicated national standard. The insulation boards that carry the covering are installed under a technical approval issued by the CSTB, system by system, while the covering laid over them stays governed by the roofing standard for the material concerned. Two reference frameworks therefore coexist on the same job, and the more restrictive one applies. On performance, the requirement is set elsewhere: French decree no. 2016-711 of 30 May 2016 makes insulation compulsory when a roof is re-covered, with a roof pitched below 60° to be brought up to 5.2 m²·K/W in zone H1, 4.5 in H2 and 4 in H3 below 800 m. Sarking is the technique that meets those values without eating into the habitable volume.
What actually governs a sarking job
Two reference frameworks on the same roof
The absence of a dedicated standard is not a gap: it is a referral to the technical document of the system chosen. The table below sets out which document applies to what.
| Part of the works | Reference document |
|---|---|
| Insulation board carrying the covering, fixings, permissible pitches | Technical approval for the system, issued by the CSTB |
| Slate, tile or profiled steel covering | The roofing standard for the material concerned |
| Roofing underlay | The underlay's technical document and the system's requirements |
| Thermal resistance to be reached | Order of 3 May 2007 as amended by the order of 22 March 2017 |
| Trigger for the insulation duty | Decree no. 2016-711 of 30 May 2016 |
The practical consequence: screw length, minimum embedment into the rafter (rafters, counter-battens and underlay are placed on our roof-space section, in French), number of fixings per square metre and minimum permissible pitch come from the system's technical document, never from a general rule carried over from an earlier job. Get that document before pricing, and file it with the as-built record.
The insulation duty triggered by re-covering
Since 1 January 2017, re-covering a roof triggers the duty to insulate. The threshold applies to the element plus insulation as a whole, not to the insulation alone. Relaxations exist, in particular where the work would alter the appearance in conflict with the requirements of a protected setting or of planning rules. They are justified in writing, before the works, not after an inspection.
When sarking is the right call
Sarking is justified in three cases: lofts already converted and finished, a full re-covering that opens the roof in any event, and constrained headroom where any internal insulation would cost habitable volume. In other configurations, insulating the rafters from inside is often quicker and cheaper, and its execution is covered in our article on rafter insulation.
Sizing the insulation and the added thickness
From thermal resistance to actual thickness
Thickness follows from the declared lambda, certified by an accredited body. The values below are orders of magnitude: the binding value is that of the product selected.
| Insulant | Indicative lambda | Thickness for R = 4 | Thickness for R = 4.5 | Thickness for R = 5.2 |
|---|---|---|---|---|
| Polyurethane or polyisocyanurate | 0.022 W/m·K | 9 cm | 10 cm | 12 cm |
| Extruded polystyrene | 0.029 W/m·K | 12 cm | 13 cm | 16 cm |
| Stone wool board | 0.036 W/m·K | 15 cm | 17 cm | 19 cm |
| Wood fibre | 0.040 W/m·K | 16 cm | 18 cm | 21 cm |
On top of that thickness come the counter battens forming the ventilated cavity and the tiling battens. Count the total added thickness at verges and eaves, then check compatibility with existing roof windows, chimney stacks and flashings before pricing.
Choosing the insulant on something other than lambda
Lambda decides thickness, it decides nothing else. Compressive strength governs behaviour under foot traffic during installation and under fixings. Decrement delay and heat capacity govern summer comfort under the rafters, a subject developed in our article on summer overheating in converted lofts. Reaction to fire and end-of-life have to be justified to the client. A board with a low lambda but poor compressive behaviour produces an uneven roof plane that the covering will make visible. Those characteristics can be compared without re-keying when the product data is verified and kept up to date in the pricing catalogue.
Added thickness is a planning matter
A change to the external appearance, verge height, overhang or materials, generally requires a planning declaration. In a protected area or within the setting of a listed building, the heritage architect's opinion can impose conditions and extend the timetable. On a building predating 1997, add the asbestos survey: sarking means stripping the covering completely.
Installing without creating a weak point
Substrate, packing and fixing
After stripping, check rafters, purlins and bearings, correct any misalignment and prepare the continuity of the vapour control layer on the warm side if the system requires it. Boards are laid in staggered courses, joints tight, with precise packing to keep the plane even. Fixings pass through the insulation and the counter batten into the rafter, with a minimum embedment given by the system's technical document and a fixing count that depends on the wind zone and the height of the building.
Critical details: valleys, dormers, roof windows
The rule is single: no breach in the insulating blanket. Cut tight, fill the voids with the same material, then handle continuity with the tapes and components the system provides. Around roof windows, the added thickness often calls for specific flashing kits, to be ordered at the same time as the windows and not afterwards.
Perimeter: eaves, verges, junction with the wall insulation
At the eaves, keep the ventilation cavity open and fit a drip flashing that throws water beyond the added thickness. At verges and parapets, carry the insulation and membranes up to avoid the thermal bridge created where the blanket stops. The junction with the wall insulation is drawn before installation: that is where the overlap has to be continuous, and the subject is covered in our articles on thermal bridges and on critical details in external wall insulation.
The particular case of slate
Pitch and rafter length govern the head lap
Slate imposes, under its own standard, a minimum pitch that depends on the climate zone, the exposure and the rafter length, and a head lap that follows from it. Sarking does not change the pitch, but it changes the geometry of verges and eaves and therefore the lap lengths at the bottom of the slope. Redo the lap calculation on the geometry after the added thickness, not on the original geometry.
Ventilating the covering
The cavity formed by the counter battens has to be continuous from eaves to ridge, with effective inlet and outlet areas. On slate, ridge closures and eaves combs are the two points where that continuity most often disappears, usually because insulation or foam has been pushed too far up. A visual check before the slates go on costs a few minutes.
Load and fixing of the covering
A slate covering weighs more than a lightweight tile covering, and that load now passes through the sarking screws into the rafters. The number and length of fixings have to be validated on the actual load of the covering selected, not on a generic figure. Have that point confirmed against the system's technical document before ordering.
Pricing, checks and handover
What drives the price
The price turns on items that are barely visible on a quote: stripping and disposal of the old covering, fixing hardware, adapting the critical details, and above all access and lifting. A complex roof structure, numerous valleys and several roof windows multiply the hours. Price the hardware from the system's technical document, it is the item most often underestimated. Those items stop being forgotten when material, labour and accessories come through from the works plan into the quote.
Weather phasing and protecting the building
Opening a roof exposes the interior. Plan properly sized sheeting, protected storage areas and phasing by slope that limits the open area. Insulation wetted before the covering goes on loses its declared performance and becomes a non-compliance, not just an inconvenience.
Checks at handover
Check the watertightness of the covering, the continuity of the underlay and its tapes, the ventilation of the cavity at eaves and ridge, and the absence of any settlement in the roof plane. Keep the system's technical document, the certification of the insulant's lambda, the areas treated and the photographs taken before closing up. The framework for those checks is set out in our article on quality control points at handover, and for grant applications, on the most common grounds for refusal.
