- Thermal insulation horizontal across ceiling.
- Ventilation at ridge level is required as per NHBC guidelines.
Typically a Scottish practice, the roofing underlay is laid directly over a series of sarking boards fixed to the roof rafters with the roof slates nailed directly to the sarking boards. A sealed roof system is achievable using a vapour permeable underlay such as Monarperm 700, if a minimum 2mm gap can be guaranteed between the sarking boards which should be a minimum of 150mm wide.
The sarking board itself is a barrier to the migration of water vapour and in most situations will require the provision of ventilation to the roof space in accordance with BS5534 (BS5250:2011).
The moisture content of the sarking will vary with the seasons and the underlay being in direct contact needs to allow moisture to pass through it. This is achieved either with a vapour permeable or a specially designed hybrid underlay which allows moisture to migrate away from this intersection.
To avoid damaging the integrity of the underlay by direct nailing of the roof slates, a counter batten and a tile batten configuration should be adopted.
Underlay Selection - Resistance To Wind Uplift
One of the functions of a roof underlay is to reduce the wind load generated under wind gusts acting on the slates or tiles.
This issue is addressed in the BS5534 Code of Practice for Slating and Tiling and the selection of the correct underlay is dependent on the exposure of the roof to wind uplift, the type of roof construction specified and roof tile batten gauge. It may also be necessary to seal the lap of underlay.
Underlays and breather membranes fully supported by sawn timber sarking boards are subject to the wind uplift zonal requirements of BS5534 Click here to find the appropriate underlay and fixing requirements for your roofing project.
Note: Boarded or sarked roof construction with breather membrane fully supported by Plywood or T&G boards (Scottish practice) impose no wind uplift conditions on the roofing underlay and as a consequence the underlay selected by the specifier is not subject to the wind uplift pressure zonal classification requirements of a BS5534 Code of Practice for Slating and Tiling.