- Thermal insulation horizontal across ceiling.
- Ventilated roof space.
This traditional roof construction uses movement of air through the roof void to prevent the occurrence of harmful condensation. Ventilation of the roof void should be provided in accordance with BS 5534 (BS 5250: 2002) using proprietary eaves and ridge ventilation fixed in accordance with manufacturers instructions.
Thermal insulation is installed horizontally at ceiling level. Nominally a thickness of 300mm of mineral wool is required to comply with Part L of the Building Regulations.
The underlay is draped over the rafters with a minimum of 10mm gap at the centre of the rafters between the underlay membrane and the underside of the tiling battens to allow water to drain effectively from the roof.
Building Regulation require that the ceiling below the roof void is well sealed.
Well Sealed Ceilings
The following measures are essential for achieving a well sealed ceiling:
- Access hatches should be adequately sealed and not sited in kitchens or bathrooms.
- The heads of wall cavities should be sealed.
- The ceiling should be sealed to the external walls.
- Penetrations for services should be permanently sealed with Monosleeve.
- Recessed light fittings should be airtight or be hooded or boxed.
(See BS 5250 section 18.104.22.168 for further guidance).
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.
Click here to find the appropriate underlay and fixing requirements for your roofing project to comply with the wind uplift zonal requirements of BS5534.