The problem of ventilation and flat roofs

We often speak to architects who struggle to effectively detail flat roof structures, either as part of a pitched roof or as a separate flat roof, because of the issue with ventilation.

Generally, flat roof constructions using wood fibre insulation can be constructed without any ventilation so long as an intelligent vapour control membrane is fitted to the interior. This type of construction can be thinner with wood fibre insulation than using synthetic insulation simply because you can fill between the joists as well as insulating above. This forms a structure with high thermal mass, reducing overheating, and excellent acoustic insulation creating very comfortable internal spaces.

The typical construction would have the UdiSTEAM 10 plus variable VCL membrane fixed to the underside of the joists. All the joints are taped and the perimeter is well taped to the air tightness layer in the walls. Between the joists UdiFLEX wood fibre wool is used to fully fill the void and a layer of UdiTOP wood fibre sarking board is laid, creating a surface on to which the ply deck can be fixed. The ply should be fixed down through the UdiTOP layer into the joists to secure it. Finally a single ply or torch applied membrane can be laid to finish the structure. Click here to see a section drawing.

There are however a few situations where you do need to ventilate. If using a standing seam finish you do need to lay counter battens over the UdiTOP wood fibre sarking boards to prevent condensation on the rear of the metal. Typically this would be a 50mm ventilated cavity.

Similarly, for ‘Green’ roof constructions you also have to ventilate as for standing seam roofs, but for different reasons. The first example above works because during the warmer months the heat from the sun dries the small amounts of accumulated moisture within the construction towards the interior. When you lay a thick layer of plants above this the capacity to dry is very much reduced. Therefore you have to rely on ventilation to keep the structure dry to avoid long term moisture accumulation.

If you have roof designs that you think could benefit from using wood fibre insulation please feel free to send them to us and we’ll help create a specification that works for you.

Chris Brookman
Chris Brookman

Author