We don’t always think about it, but most timber flat roof structures have a diaphragm (typically a plywood or OSB board) to tie the structural components together. Sometimes a pitched roof structure will also require a diaphragm to help provide an interdependence of the structural timbers. The problem is that sometimes these diaphragms can also become the condensation or ‘dew’ point as moisture vapour moves through the construction.
Often pitched warm roof constructions are used to create habitable spaces in the once ‘cold’ roof construction such as a rooftop development or penthouse flat. In some cases, living quarters, kitchens, shower rooms as well as sleeping accommodation are all included in these areas. This can lead to very high moisture vapour pressure especially in the winter months when natural ventilation is kept to a minimum.
In these rooftop constructions it can also be hard to get a complete vapour control layer (VCL) due to the different roof pitches and the fact that we normally fix the internal finishes through the VCL.
Any moisture vapour that penetrates the internal finishes and VCL, along with any moisture from the construction phase is at risk of condensing within the cooler, outer layers of the roof construction. This is quite likely to be interstitial condensation on the underside of the diaphragm and could result in a slowly degenerating structure.
This points up the need to provide ventilation to the underside of the diaphragm in line with BS5250 to ensure any moisture can be ventilated away before it starts to cause trouble. A minimum 25mm ventilation void should be provided between the top of the insulation and the underside of the diaphragm.
Creating the ventilation void is one thing. Detailing the ventilation at the junctions of the roof is another!