A warm roof is a type of roof construction which has an insulation layer above the rafters, and immediately below its weatherproof membrane. A warm roof construction has many benefits over a traditional ‘cold roof’, essentially it is a ‘breathable roof construction’, which allows moisture to escape which in turn prevents damp and any associated decay problems. A warm roof construction allows heat to be conserved within a property – without the need for a ventilation system.
A warm roof is thermally efficient and cost effective, and is commonly accepted as the best roof option for our climate within the UK. There are however, some technical challenges to consider when installing and fixing a warm roof.
Structural loads on a pitched warm roof must be considered differently from those on a traditional ‘cold roof’, where the weight of the roof covering is supported directly by the rafter. On a warm roof the weight is taken up by counter battens, which are laid on top of the insulation, a non-structural element. The counter batten effectively forms part of a composite rafter above the insulation and it is essential therefore that it becomes a structural member to which the tile batten can be fixed and, at the same time, hold down the insulation against wind suction.
As insulation thicknesses have increased over time, so have the sliding or bending loads that are imposed on the counter batten. The laid weight of the roof covering and the roof pitch both affect the sliding load, as the heavier the tile and the steeper the roof, the greater the load becomes. However, the snow load, which is a considerable factor in the overall assessment, starts to decline at approximately 40º.
A consequence of the sliding load is that under the weight of the roof covering the insulation could compress, impairing its thermal performance.
The counter batten (usually 25mm-50mm thick and 38mm-50mm wide) must be thick enough to grip the roof covering fixing method and enable it to resist the wind uplift. It must not buckle under the sliding load between fixings and must be wide enough to meet the manufacturers requirements for clamping the insulation. Wind suction will be affected by the anticipated wind speed, the height of the building, the site exposure and the local topography.