Understanding sarking and its uses on a roof
Principle of continuous external insulation: from the roof structure to the covering
Sarking is a method of external roof insulation. Rigid insulation panels are installed continuously above the rafters. A roofing underlay is then added, followed by counter-battens, then the covering. The result is an unbroken layer, with fewer thermal bridges.
When to favour sarking: finished attic conversions, heavy renovation, limited height increase
Sarking is often the right choice if your finished attic conversion is already complete. You preserve the living space and limit the works to the interior. It's also logical during a full re-roofing job. Example: tiles, slates, battens, underlay. Finally, when ceiling height is at a premium, external roof insulation avoids losing centimetres.
Points of attention: added thickness, edges, ridge and water drainage
Good airtightness comes down to the details. The added thickness often requires adapting the edges, gutters, flashings, roof windows, and the ridge treatment. Also check the roof ventilation and water management. The goal is simple: keep water out, let the roof breathe.
Preparing the job: survey, substrate and regulatory constraints in 2026
Condition of the roof and structure: essential checks before removal
Before a sarking job, check the roof structure and its supports. Deflection, cracks, moisture, signs of fungi or wood-boring insects. Also check the special points: edges, valleys, chimney stacks, roof windows. The goal: a stable substrate and controlled load transfer.
Compatibility with the covering (tiles, slates, steel sheet) and managing the underlays
Each covering has its own rules: minimum slope, weight, fixings, ventilation. During removal, plan ahead for managing the roofing underlay. High water-vapour permeability (HPV) or not, condition, continuity, junctions at the eaves and ridge. Sarking often comes down to these junctions. Take care with airtightness and the continuity of the vapour control layer on the interior side to avoid condensation.
2026 framework: RGE, performance requirements, financial aid available depending on the case
In 2026, to access MaPrimeRénov' and CEE, the company must be RGE-certified and the insulation must reach a minimum thermal resistance depending on the measure. Also think about the pre-works asbestos survey if the roof dates from before 1997, and about planning procedures if the appearance changes (prior declaration, protected area). To limit administrative holdups, also plan ahead for MaPrimeRénov' file rejections.
Choosing the materials and the external insulation build-up
Insulation suited to sarking: wood fibre, rock wool, PUR/PIR, selection criteria
With sarking, the insulation is installed above the rafters. Wood fibre provides summer comfort and good moisture management, at the cost of greater thicknesses. Rock wool performs well acoustically and in fire reaction. PUR/PIR panels limit thickness for the same level of insulation, but require careful attention to fire safety and end-of-life disposal. Choose based on lambda, compression strength, weight, fire risk, vapour permeability and budget.
Rain barrier, vapour barrier, airtightness: securing against moisture and air leaks
A rain barrier protects the roof structure from infiltration and wind. In renovation, a high water-vapour permeability (HPV) rain barrier is often the target, to allow drying towards the outside. On the interior side, a continuous vapour barrier or vapour control layer, with tapes and sleeves, creates lasting airtightness. The junctions at the edges, roof windows and duct penetrations make the difference.
Target thermal resistance and continuity: limiting thermal bridges at the rafters
Aim for thermal resistance consistent with the aid schemes, often around R 6 to 8 m².K/W on the roof. Sarking makes continuity easier, because the insulation covers the structure. Cross the layers if needed and treat purlins, valleys and dormers to reduce thermal bridges.
Carrying out reliable sarking: steps and details that make the difference
Removal, positioning and panel installation: alignment, fixings, counter-battening
After removing the covering, check the rafters and substrate, then prepare a vapour barrier continuous on the warm side if the build-up requires it. With sarking, the rigid panels are installed in a staggered pattern, with tight joints, and precise positioning to keep an even plane. Fix with long screws and washers into the rafters. Add counter-battens to create the ventilation gap, then battens and the covering.
Treating special points: valleys, dormers, skylights, roof windows
At valleys and around dormers, skylights and roof windows, the rule is simple: no gap in the insulation blanket. Cut the panels as precisely as possible, fill any gaps, then secure with compatible adhesive tapes and flashing pieces. An airtight joint limits powdery water ingress and losses.
Junctions at the perimeter: eaves, edges, parapets and connection with wall insulation
At the perimeter, take care with the eaves, with a free air gap starting point and a drip-edge flashing. At edges and parapets, extend the insulation and membranes upward to avoid the thermal bridge. The connection with wall insulation must be planned before installation, for an unbroken overlap and a clean finish.
Costing, organisation and quality control on site
What weighs on the price: removal, supply, hardware, installation time and access
Costing often comes down to "invisible" items. Removal and disposal, supply (insulation, underlay, battens), hardware (screws, fixings, tapes) and above all installation time. With sarking, the complexity of the roof structure, the special points (edges, valleys, roof windows) and roof access (scaffolding, lifting) can quickly shift the budget.
Planning without stress: protecting the building, weather phasing, waste management
Before opening up the covering, plan clean protection of the building (tarping, storage areas, walkways). Setting a realistic weather-based phasing limits rework and the risk of the insulation getting wet. On the waste side, plan ahead for sorting, suitable skips and disposal certificates to avoid back-and-forth trips that derail the schedule.
Checks at handover: airtightness, ventilation, finishes and documents for aid schemes
At handover, check the airtightness of the covering and the continuity of the rain barriers and tapes, then the ventilation (air inlets, ridge outlet, penetrations). Check the finishes and the absence of visible thermal bridges. For aid schemes, keep a detailed invoice (materials, surfaces, performance), the RGE certificate, and for CEE the required signed documents. To frame the procedure, rely on the quality control points to check at handover.
Key figures
€150 to 250/m²
Price
12 to 24 cm
Thickness
6.0 to 10.0 m²·K/W
R achieved
Frequently asked questions
For MaPrimeRénov' and most CEE data sheets, aim for a thermal resistance R ≥ 6 m²·K/W on the roof (a value commonly required in renovation). Include the brand/reference, thickness and certified R (ACERMI or equivalent) on the quote and invoice: this is a common reason for file rejection.
Pierre-Louis Guhur
CEO of Argile
