Sarking and roof underlay

Frequently asked questions

If a specification says sarking, what should be ordered?

Ask, because the word alone cannot settle it and the three answers differ by an order of magnitude in cost. A Scottish drawing saying sarking usually means timber boarding fixed over the rafters, which is a structural deck with its own thickness, grade and fixing pattern. An English drawing saying sarking usually means a roll of underlay membrane. An Australian one usually means a reflective foil laminate. Ordering boards against a membrane specification puts several thousand pounds of timber on a job that wanted rolls; ordering rolls against a boarding specification leaves the roof without the deck it was designed around. The tell is normally elsewhere on the drawing — a thickness in millimetres means boards, a roll width and a lap dimension means a membrane, and a reference to emissivity or an R-value means foil.

What is the difference between a breather membrane and a traditional felt?

Whether water vapour can pass through it, which decides how the roof space gets rid of moisture. A traditional bitumen felt is effectively closed, so moisture rising from the building has to leave through deliberate ventilation openings at the eaves and ridge, and a roof built with felt and no ventilation path collects condensation on the underside. A vapour-permeable breather membrane lets vapour out through the sheet itself while still shedding liquid water, which is why it can allow a roof to be built with less ventilation — but only when it is installed the way its certification assumes, including the right way up, supported correctly, and usually with an air gap beneath it. Substituting one for the other without revisiting the ventilation is a common cause of a roof that drips in winter.

Does the underlay do anything structural?

More than most people expect, which is why it is specified rather than chosen on price. On a tiled or slated roof the underlay is a designed part of the wind resistance: air pressure inside the roof space pushing up against the covering is partly balanced by the underlay, and a sheet that tears, sags too far between rafters or is fixed with too few battens changes the uplift the tiles have to survive. British practice treats it that way explicitly — the underlay's resistance class and its fixing are part of the tiling specification, not an accessory to it. It is also the roof's second line of defence against wind-driven rain and snow, which is why the lap dimensions and the detailing at eaves, valleys and abutments matter as much as the sheet itself.