Partial Re-roofing... Slates damp on underside, and questions RE ventilation

[Oxbow16]

Hi all

 

(Background first - the room has a vaulted ceiling (so no loft), is not currently heated, and is only used by one person. There's no kitchen.)

 

I've stripped some slates off my roof over the weekend to do a repair. Most of the slates are quite damp on the underside (a few are even wet, I'd say). They are also a bit flaky - still sound, but with a thin-ish layer flaking off the surface. This is both on the underside and on the top where the slate was covered by the slate above.

 

First and foremost, is this normal and to be expected? If it is, then great I'll replace the bad ones and get on with it. If it's a sign of a problem, what's the most likely cause and - dare I ask - remedy?

 

If it is a potential problem, there are a few factors which might be contributing and which might also solve the problem once put right. The trouble is, with the slates back on I'm not going to know in the future whether there's been an improvement. Here's those factors:

 

- There is currently no ventilation. The make up from inside is: plasterboard, rockwool/glassfibre, no air gap, sarking boards, felt (1f), battens, slates. I'll be putting in ventilated soffits and vented ridge, a 50mm airgap between the sarking and top of insulation, and replacing the insulation with PIR. But will adding ventilation beneath the sarking boards and felt do anything to help ventilate above the sarking boards and felt?

 

- Only horizontal battens were used. No vertical counterbattens beneath. So any water that makes it passed the slates has no path down to the gutters. That being said, the felt was pretty much all dry and and the battens look fine. But I didn't know if this was significant from a ventilation point of view (or whether there even needs to be ventilation between the sarking boards and slates?).

 

- There has been a section of plasterboard ceiling (10 foot wide 3 foot long) missing for the last year or two, along with 10" a hole in the sarking and felt. So from inside you can see the underside of the slates through that hole (the hole is from a removed flue). Given that the hole is only 10" and the removed plasterboard 10x3, could this contribute to the much larger area outside where the slates are damp/wet underneath?

 

Sorry for the length of the post. Any thoughts - as ever - would be most welcomed and appreciated.

 

Many thanks

[Oxbow16]

An update....  

 

What I'm most keen to know now is how bad the flaking is on my slates. It's actually worse on the upper section of the topside (the sections covered by the slates above). I'll attach some photo examples, and wondered if anyone familiar with slates can say whether they look bad and if so whether that is likely caused by the lack of ventilation, or perhaps just because they're old.

 

I should add that the battens all seem fine. Would they not show signs of mould or rot if lack of ventilation was a problem?

 

Cheers

 

Link to photos. Sorry, they're not the best. Can take more if needed

 

https://drive.google.com/drive/folders/1qlaU8o5W_reDlT86soxY7D3n9w5GTaO2?usp=sharing

 

[Oxbow16]

Any thoughts form roofers or those familiar with slates?  Sorry to bump, but keen to know more about the slates and the situation we're in...  

 

Many thanks