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Over Rafter Sarking
A warm pitched roof is created by placing insulation directly over the rafters. Continuous ventilation of the roof void is not required with this method of construction and the risk of condensation within the roof structure is eliminated. In new buildings, extra value can be added to the property by creating an additional bedroom, study or dry warm storage area. This method of insulating is also ideal for existing buildings where the tiles/slates need replacing. For this we would recommend the use of Seconds Sarking Board.
Fixing details
- A preservative treated stop rail, the thickness of insulation should be fixed at eaves level.
- Lay boards either following the rafters or across rafters. The boards should be lightly butted with staggered joints.
- All joints running from eaves to ridge must occur aver the rafters. There is no requirement to tape board joints (see fig. 2).
- Boards are held in position with counter battens (38mm x 38mm). Secure with Inskew fixings.
- A breathable sarking membrane is draped over the counter battens and held by tiling/slating laths.
- Tiling and slating over the boards should be carried out in the same way as on an un-insulated pitched roof, fixing slate laths into the counter-battens instead of the rafters (see fig.4).
- If exposed rafters are required inside the building, plasterboard can be laid over the rafters before the insulation is fixed. The length of the fixings should be increased accordingly. Where a greater thickness of insulation is required, or to reduce the roof build up height a double layer of insulation can be used.
- Nailable sarking clips are driven into the top of rafters at 1m centres up the roof slope. This provides support for an addition thickness of insulation between rafters.
- If two different board thicknesses are required the greatest depth board is placed over the rafter to prevent condensation occurring between the two layers.
- To prevent a cold bridge, a vertical section of insulation should be installed between the rafters above the cavity closer.
- Boards should be cut using a sharp knife or a fine toothed saw.
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