Is this slate roof ‘porous’ ?

[Donna M]

Hi all, 

This slate roof is 150 yeas old. The structure appears to be sound and well built, the rafters are wet when it is stormy weather, tiles seem to get wet at corners of tiles as in picture. Is this porous slate or condensation, or something else ? Any advice on next steps is a reroof inevitable? 
 

Thanks all

 

D

[markc]

I reckon this is just wind blown water with you saying stormy weather. 

[Redbeard]

10 hours ago, Donna M said:

the rafters are wet when it is stormy weather, tiles seem to get wet at corners of tiles as in picture.

 

Is this a problem over the whole roof, or in discrete areas only?

 

I suspect, given your description, that it is rain blowing up the laps. Although I suggested in another recent thread that sarking felt is not always 'noticeable by its absence' this may be an exposed situation where it would help. It's a bit older than mine, but not much, and I would have little hesitation re-laying at least 60% of my slates as a new roof . I have seen an example in an area of very high industrial pollution where natural slates resembled 'porridge', but if your slates look OK from the top it may well be 'cause they are.

[Donna M]

Hi, i was only able to access one area to see the underside of the roof like this. There were other areas across the roof where I noticed drips, and two valleys have damp issues, but there is no sagging in the timbers as far as I could see. 

 

elsewhere there has been stiff insulation installed under the rafters (as in loft side) with a foil backing (facing into the loft space) and then a layer of plasterboard. The plaster board is getting wet in some locations but not uniformly across the whole roof. There did seem to be a gap between the insulation and the roof slates so I think it is ok ventilation wise.

 

if it is getting wet on the inside of the slates periodically with stormy weather is that essentially ok provided he timbers dry out when it is a sunnier day? If it is a problem is there a rule of thumb that says how long timbers can be getting wet like this? Does rot set in pretty quickly?

 

thanks for your responses!