A large roof can take some looking after. One way which has been promoted to help keep slates in place is foam, sprayed on the underside of the slates and laths.
many don’t like it…it just seems wrong. Especially in this case when it’s been used in an 18th century grade 2 listed building in North Yorkshire.
I was called in to check the roof for a potential buyer. The RICS chap had condemned the entire roof on the basis that this foam had been used.
I found some light infestation by common furniture beetle but generally the roof timbers were fine. However, there was a bit of staining here and there and moisture meter readings from the exposed rafters confirmed some of these were leaks. Externally there was some evidence of the odd slipped slate and this did correspond to the stained and damp areas.
However, I though I’d do some investigation work on the unstained and apparently dry areas, as a way of checking out people’s opinions; that the spray foam can trap moisture in, which on face value seems a logical position to take.
Using a combination of deep probes and eventually by picking some foam away, to confirm what I was finding, the following results were found.
The rafters and trusses were dry, where they were exposed (11 to 16%)
The upper sections of the rafters and the slate laths were almost all on the damp side, ranging from 19 to 28%.
Now then. The roof is old so I don’t think the laths are heavily treated with a salty preservative so why the high MC?
As can be seen the roof void was being assaulted by twin bathroom extractors venting into the void..never good. So maybe that wasn’t helping. Could it be though that a high MC is there due to the reduced ability of the laths to dry out after getting wet, through inevitable condensation on the underside at night? This seems likely.
Anyway, I didn’t agree that a new roof was in order, but I have put the new owner on notice that regular planned maintenance is an essential part of looking after this building and maybe the life of the roof may be shortened by the presence of the foam.