The best solution is usually to lay rigid foam insulation boards on top of the weatherproof covering, as this keeps the whole roof structure above dew-point temperature. Paving slabs can be laid on top of the insulation as a weighting layer and to allow access as a roof garden. You refer to your roof as being of “felt and tile construction”, which sounds as though this might be a possibility in your case.
Solving a smelly situation
Q We live in a five-year-old, three-storey house with the second (top) floor being mainly under the eaves in the attic space (bedroom and en-suite WC and shower).
The problem is that when the prevailing wind is from a certain direction, there is an unpleasant smell coming from the shower outlet and it is even possible to feel a draught. I cannot understand how this can happen as I thought that any outlet would require an S-bend or similar, thereby creating a seal. Why is this happening and what is the solution please? MB, north Wales
A You are quite right – all waste outlets should be fitted with a trap or “U-bend” to prevent sewer smells entering the home. However, modern shower trays are sometimes so thin that there is no room beneath them for a traditional S or P-shaped trap, and they are instead installed with a shallow “shower trap”. If the pipe leading away from this is too long or too steep, then the negative pressure created by the water running away can sometimes draw the water out of the trap, leaving it dry. Try running a little water into the shower to see if this solves the problem.