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

[Krome10]

Hi all

I think there are a few roofers and folks with roofing experience here? Wondered if anyone could share their thoughts if so please?

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

 

[Spectric]

I had my shed roof done in slate as it was cheaper than the metal sheeting. The first layer was board onto the roof trusses, then a membrane. The first battens ran vertical up the roof and then the battens on which the slates were fixed ran horizontal and this is done to give airflow under the roof so dampness is prevented, if you have wet slates then you have an issue and

No vertical counterbattens beneath. So any water that makes it passed the slates has no path down to the gutters.

and no path for airflow.

 

[Jacob]

Condensation is normal under slates, or any roofing material not highly insulated or sealed off.
There needs to be good ventilation under the slates with a gap over an impervious membrane of some sort over the ceiling structure. Breathable "Tyvek" types etc are most popular. There is usually enough sag/dip between rafters to pick up any drips and take them down under the tile battens to the eaves and the guttering.
Loads of pictures on the net.

 

[Krome10]

@Spectric - many thanks for the reply. Interesting, that makes sense although until now I always thoughts the vertical counterbattens were to give the water a path into the gutter rather than ventilation. With your set up, where does the ventilation come from - do you have it at the eaves and ridge? Interestingly, I've asked elsewhere and been told that condensation on the underside of the slates is to be expected and will form just like it does on other surfaces. Much like Jacob says....

@Jacob - thanks to you also for the reply. In my case the sagging felt between rafters doesn't apply as I have sarking boards below the felt.

Speaking of which, I'm sure I remember reading that in some set ups the slates are fixed directly to the sarking boards with no battens at all, in which case there would be no air gap and nothing to help dry the condensation? Or would no air gap mean to condensation?

 

[Doug71]

I live in an old Victorian school which we converted into a house about 20 years ago, the basic roof structure is exposed trusses with boarding over.

The slate roof needed replacing and we found originally it was just felt straight on top of the boarding with horizontal battens on that for the slates to fix to, which to be honest had done well considering it was 100+ years old.

We needed to add insulation but didn't want to loose the feature beams inside so everything went above the boarding so a bit different to your case, we used the Tri iso insulation to save depth.

I think the construction starting from the boarding working outwards was

Vertical batten fixed to boarding for ventilation

Tri iso insulation

Vertical batten (more ventilation and carry the membrane so it can sag a bit)

Tyvek breather membrane

Horizontal battens

Slates

Working on what I had to do I guess yours would be vertical battens on the boarding, Tyvek, horizontal battens then the tiles, I am no expert on the subject though......

 

[Jacob]

...

Speaking of which, I'm sure I remember reading that in some set ups the slates are fixed directly to the sarking boards with no battens at all, in which case there would be no air gap and nothing to help dry the condensation? Or would no air gap mean to condensation?

Known as "Scottish Sarking".
It's what we did here with our chapel conversion slate roof- copying the existing. No battens , nails straight through the boards
There's still air circulating through the gaps in the slates, and the modern Tyvek felt is "breathable".
The original felt was tar based and brushed off the boards in great clouds of dust. It was probably innovative in 1874. "Tarmac"was a local invention about 1900 but tar was around and used well before that.

 

[Spectric]

The air flows from the edge upto the apex ridge tiles and is needed to reduce any dampness, without the air then things just would remain wet. The guy who did this roof is a local roofer who does both domestic and retail with a good reputation so I did not question his work and let him do the job.

 

[Jacob]

The air flows from the edge upto the apex ridge tiles and is needed to reduce any dampness, without the air then things just would remain wet. The guy who did this roof is a local roofer who does both domestic and retail with a good reputation so I did not question his work and let him do the job.

If the tiles are warped like the typical old "rosemary" or roughish slates then there are enough gaps for air to percolate through. I guess it has to be more designed-in with close fitting modern tiles.

 

[clogs]

this will give a few ideas
slate roof ridge ventilation...google is ur friend.......
u wont have to spend a lot.....
personally I'd aim for a ridge vent hole every meter or so......no worries at the bottom near the gutter.....
Spectrics vertical battens are the way to go.......
u can never have enough ventilation under a norm type roof, slates ,tiles etc....

 

[Krome10]

Many thanks for the replies.

@clogs - Yes, I will be adding ventilation. I'll be putting in a continuous ventilation at both the soffits and the ridge. That was always the plan but was more for the between sarking underside and insulation topside air gap. For the gap between the sarking topside and slates underside I'd need to put in the vertical battens, and that's nothing short of a re-roof; which isn't an option at the moment.

@Doug71 - That's helpful to know, thanks. If we do at some point consider re-roofing that part of the house, I think we'd look to do similar and have a warm roof to keep roof timbers exposed.

For now, I guess what I'm most keen to know 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 slate 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

 

[Titan_uk]

I'd say 1st and 2nd are bad, slates can, and yours looks to have, demlaminate. hard to tell on 3 but I guess it's the same slate so probably. On replacing, slate comes in grades and Welsh slate is 1, with a lifetime guarantee of 75-100yrs. generally regarded as the best, it's harder, more compact and has been squished for about 100 million years longer than say Spanish slate that generally comes thicker to 'pretend' it's the same quality, plus afaik it' only been coming in for about 45yrs so I'd take any guarantee for longer than that with a pinch of salt.
74 years old..

 

[Krome10]

That's all really helpful to know, many thanks.

A fair few are now off the roof for the repair work. Some still ring nicely when tapped with a knuckle, does that mean they might be better than they look at first glance? I've read lots that if they ring they are good, but what if they ring yet look like mine?

What's the slate in the photo?