Best course of action for a roof with no ventilation

[K Wimberley]

No gap at all. Cemented in. That is the problem. Builder friends have taken a look and wondered too.
The roof has a second gradient to slow down water as it runs down the roof. It is to these rafters that the ply for the overhang is fixed. How about lifting the ply, raising the height of these lower level rafters with some extra wood on top and replacing everything to create a continuous gap above the granite to allow ventilation. Not cheap in terms of working hours and quite disruptive, but the only solution my brain can think up so far.

 

[freddiemercurystwin]

Thought you were gonna do a doodle?

 

[theprinceofdarkness]

No gap at all. Cemented in. That is the problem. Builder friends have taken a look and wondered too

Thats odd, you say you have a sarking cloth, so where does this end? My stone built cottage has this sort of roof. A fillet of cement was put on top of the outer skin of stones, the sarking cloth lapped over this into the gutter, then the stone flags laid as tiles. If the tiles are actually cemented to the top of the blocks where does the sarking cloth end?. if its inboard of this fillet then any blown in water will dribble down on to the top of the wall and it can't get out cos' there is a fillet of cement in the way.
If it such a problem then just cut some holes in your gable wall, you can then get all the ventilation you want. if your loft conversion has got skeilings (sloping ceilings at the side), then you can use the area behind the low vertical wall at the bottom of the skeiling to provide ventilation, so it would be good if the holes were cut within this area. This volume would then be at the outside temperature so it would need insulating on both its vertical and horizontal surfaces.
Frank

 

[K Wimberley]

Sorry all, I've been out with a nasty flu. Here's a quick sketch. I am pretty sure of most of it.
Here are the less sure bits:
I assume the felt goes over the ply sheet at the base, but don't know. I can check this. We haven't had any issues at this point anyway. The water goes in the gutter!
Not entirely sure if the ply is consistently this length (just at the perimeter as opposed to a full sarking board), but I can't see/feel any ply above this point in the roof-space wherever I have access (just the felt and the feel the tlles behind). The exact point it begins is hard to ascertain.

Frank, as you can see from the sketches, there are places upstairs where that would be the case and you could diamond drill vents through the thick granite to get some ventilation in the roof void, but at other places the 'skielings' go right down to the base, so the void is not continuous (why I describe them as channels). I have also heard vents at the gable ends can be problematic for air flow (irrelevant anyway due to the lack of continuity in the voids).

Tricky! As mentioned I wondered if you could raise the last section of tiling to add a continuous strip of ventilation the length of the roof. But it is a big job.

https://app.box.com/s/n5se2vtcok4wvlccnf8sj7h9v3c2pwj7

Back to my original question - given the search for a solution may take longer than the time to clad inside to make the house at least a bit cosier (at the moment some rooms upstairs are stripped back to the wooden structure) - should I temporarily clad in ply or osb and then remove at a later date to add wool once we find a ventilation solution, or should I add a proper VCL vapour control layer to reduce the amount of warm damp air entering the unvented roof space?

 

[K Wimberley]

We found a solution to the ventilation question.
So we are now looking at the following two options under the rafters in the roof space:

(from the outside)
existing tiles and impervious felt
ventilated air cavity - minimum 50mm throughout
100mm sheeps wool insulation (or another wool if we run out of money/wool)
vapour control layer - either a membrane such as intello OR taped and glued osb
plasterboard

I have read a lot on OSB v membrane (if osb is sufficient or not).
Climate and location: exposed but relatively moderate (south England)
We are sometimes out for a month or so due to work, so conditions inside vary (in terms of heating and moisture levels)

Hope somebody can offer advice!
Thanks,
Kaspar

 

[Adam182]

Might be a long shot but did you ever figure out a solution for this?