This type of roof is most commonly used in loft conversions as any type of membrane can be used and because either there is no original roof membrane or it is a non-permeable one. This system should incorporate effective use of a vapour control barrier, such as Permavent DRY, installed directly underneath the insulation.
Air flow must be maintained and we recommend that:
- Eaves protectors are used above over fascia ventilators to prevent the membrane from 'sagging' behind and blocking the flow of air.
- Through ventilators, which are used over the wall plate (such as roll out rafter tray).
- The loft insulation to the ceiling is not allowed the block the flow of air at the eaves.
When insulation between the rafters for the hybrid roof, special care should be taken:
- To ensure that 50mm of air flow is maintained above the insulation.
- Air flows across the hip ridge and valley boards and around roof windows.
- Air flows under the ridge or through a ridge ventialtion system.
- Rafter length above 10mtrs long require increased or calculated air flow.
Cross Ventilation (passive or natural ventilation) must be maintained and should be addressed at the design stage. The building will ventilate by using the positive and negative pressure that is created as air moves around it (this pressure is shown when a front or back door slams when there appears to be no wind). The negative pressure will draw air through the roof space as a draft. Complex buildings or roof designs can create voids that air will not be drawn through such as flying gables or dormer roofs.
Condensation can form in these voids and they are now a detailed part of the approved documents and must be ventilated under the building regulations.
Wind Uplift (zonal fixing). On rare occasions a strong negative pressure can form on the rear side (lee side) of the roof plane. If the roof is covered with tiles that allow air to move through them (such as plain tiles) then the negative pressure can draw air from the front ventilators, across the roof space and through the laps of the roofing membrane. This can cause the membrane to 'flap' and the tiles to 'chatter'. In extreme cases it can pull the tiles from the roof and has lead to the zonal fixing method being introduced. The correct taping of the lap joins using 'Permavent Tactape' during instillation will virtually eliminate the risk of this problem.