What is sarking (over-rafter insulation)?

Sarking (in the continental sense, also called the sarking roof or sarking method) is a way of insulating a pitched roof on the outside, with the insulation boards laid as one continuous layer on top of the rafters, trusses or roof boarding. The roof structure thus stays on the warm side and remains visible from inside, and the insulation is nowhere interrupted by timber members. In Germany this is called Aufsparrendämmung. Note: in the United Kingdom, sarking usually means something else, namely the roof sheathing boards or the roofing underlay. This article covers the continental meaning: continuous insulation over the rafters.

Build-up from inside to outside

1. Loadbearing structure: trusses, rafters or purlins with roof boarding on top.
2. Vapour control layer: sealed airtight to walls and penetrations.
3. Insulation boards: usually foil-faced PIR, or wood fibre boards. Joints are closed with tongue and groove or tape.
4. Vapour-open, water-resistant membrane: often already bonded to the board, or as a separate roofing underlay.
5. Counter battens: screwed through the insulation into the loadbearing structure with long screws.
6. Tile battens and roof tiles, or another roof covering.

Advantages and disadvantages

Advantages: no thermal bridges through the timbers, no loss of internal space, an exposed roof structure and a continuous airtight layer.

Disadvantages: the roof covering has to come off, the roof becomes thicker (watch the junctions at the gutter, the gable and the neighbours) and it may raise planning permission questions if the roof height visibly changes.

Applications

• New build with an exposed roof structure.
• Renovation where the roof tiles are being replaced anyway: that is the moment to insulate on the outside.
• Historic roofs where you want to leave the inside untouched.

Tips

• Calculate the Rc value using the thickness and lambda value from the product documentation. In the Netherlands, new-build roofs must reach at least Rc 6.3 m²K/W; lower requirements apply to renovation, but insulating more pays off.
• Use the screws and screw spacing specified by the system supplier. They determine whether the counter battens and tiles stay in place, including under snow and wind.
• Work out the gutter and gable junctions properly on a drawing beforehand. That is where things most often go wrong in practice.

Related terms

• Roof sheathing
• Vapour control layer
• Vapour-permeable membrane
• Rc value
• Thermal bridge

Want to know more about building terms? Visit our knowledge base at fredsdiyplans.com.