Things to look out for on your roof
Patios | Asphalt | Dry Rot
We are finding more and more evidence of dry rot in the timbers of old asphalt roofs that haven’t been maintained properly.
This shows an outdoor sitting area with an asphalt covering that was leaking into the flat below.
When we lifted the asphalt we found evidence of two other previous layers underneath. Instead of being 25mm in depth, the three membranes amounted to 80mm, or 3 ¼ inches in thickness. What is more, with warmth, the wetness of the leak and lack of ventilation, dry rot had developed in the decking and cross members.
All the rotten timber had to be stripped out and replaced with new. The old unaffected woodwork was all treated with an anti-dry-rot and anti-woodworm fluid.
The lesson is never try to go over an existing asphalt roof. Asphalt is an extremely good product, but when it is 25 years or more old and begins to leak, strip it back to the woodwork underneath and make sure it is rot free before reasphalting.
Alternatively, use a fibreglass membrane with tiles on top as illustrated here as it was the client’s preferred solution.
Another recent example of rot under an old asphalt roof occurred when we were called out to find the cause of a leak under a recently felted patio.
Felt should never be used if the surface is to be walked upon, but the client informed us this was not the case.
Again, the mistake had been made by the previous roofers of not first removing the old asphalt membrane and checking all was well with the decking underneath. When we cut away the cover of what had originally been the support tray below a large water tank, to our amazement we found 2 inches of water in the roof itself.
On removing the asphalt, we again came across serious dry rot which we had to cut out, treat with chemicals and renew sections of timber before felting.