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By spearson92
Hi all,

Another question in the workshop build. I've just finished laying OSB over the rafters and I'm now ready to lay the breathable membrane.

The plan is to lay Vent 3 underlay felt, then put mineral felt on top of this. Just looking to confirm if the following method is correct: lay the felt underlay directly onto the OSB overlapping where appropriate and stapling or using clout nails to secure. Then lay the mineral felt directly onto the breathable membrane?

On the vent 3 instructions it says to counter batten after laying the membrane, but this is assuming the use of tiles not mineral felt.

Any suggestions? Should I counter batten on just put the mineral felt straight on top?

By RobinBHM
Cromar vent 3 is a breathable membrane used for underneath roofing tiles.

Mineral felt systems are usually bonded directly to the OSB board.
By Woody2Shoes
I'm not familiar with the particular brand of vapour-permeable underlay, but I'm not at all clear why you're planning to use mineral felt underlay as well. The kind of construction I think you're using is pretty standard in Scotland, where the OSB performs the function of "sarking board".

What part of the country are you in (and what is the level of exposure), the pitch of the roof and the final roof covering?

Here's a link which might be useful background reading - a key question is how permeable is your VPM, and another is should/will your batten space be ventilated? ... le-roofing

Cheers, W2S

PS I've just read the tech data sheet for Cromar Vent 3 and it says it doesn't need ventilation and is suitable for laying directly flat onto insulation - which suggests it should be fine laid flat on sarking boards too. In order to properly drain the batten space (as the above link suggests) you would be best to put counter-battens on top of the underlay (e.g. on top of each rafter), prior to the normal battens - this will allow any snow/rain blown into the batten space to drain to the eaves.

PPS Sorry - I've just read your profile - you're in Inverness!

PPPS I really think that mineral felt on top of the underlay would be pointless.
By spearson92
Hi guys,

Many thanks for the responses. This is my first build so apologies for my ignorance on the subject, and apologies as I don't think I've explained the initial post very well.

I've put down some OSB board on the rafters which is 11mm thick. The shed is a wedge shape, and so the pitch varies from 7 to 13 degrees over about 5m. I'm not sure where I got the idea from, but I thought it would be a good idea to lay breathable membrane (Cromar Vent 3) onto the OSB, then lay the mineral roofing felt on top of that. I understood that mineral felt is just a tougher roofing felt than normal shed felt due to the bigger grain, rather than underlay? The final roof layer would be the mineral felt. I thought that the breathable membrane would offer some extra protection if the felt tears, to prevent moisture going straight onto the OSB.

I've wrapped the sides of the shed with this already which will eventually be clad with featherboard.

Is it best to just lay the felt directly onto the OSB rather than have a breathable membrane in between the felt roof and OSB?


PS. The other option is to use EPDM and be done with it, which is probably what I should do. However a "one-piece" EPDM covering is very expensive. Could I use a 1m wide roll of EPDM and glue it down like I would lay felt with an over-lap?
By Woody2Shoes
Hi - sorry I misunderstood your OP - I thought the mineral felt was to be an underlay for tiles/slates (this sort of thing ... s-felt-5u/ )!

If it were me, I'd go for the EPDM option - it should last several times a felt roof covering.

You can do your own joints - I think you need both adhesive and some kind of mastic (need to follow manufacturer's instructions very carefully). I think a single, pre-formed sheet might be very awkward to get onto the roof and to install if the roof is large/odd-shaped. This would go directly onto OSB (9mm sounds a bit thin TBH, I'd think 18mm better).

Cheers, W2S
By cammy9r
I put breather membrane over the osb sarking then used rooing felt on top of that. No problems with it in 7 years. Its nice to know if the felt cracks that there is another layer there as I only check the roof twice a year.
User avatar
By MikeG.
cammy9r wrote:I put breather membrane over the osb sarking then used rooing felt on top of that. No problems with it in 7 years. Its nice to know if the felt cracks that there is another layer there as I only check the roof twice a year.

No problems, but no point. Breather membrane on top of an impervious layer is pointless.
By spearson92
Hi all,

Thanks for all the advice. In the end I've decided on a felt underlay (sand finish) with a mineral felt capsheet.

Weather's turned a bit pants now, so my next question :D is how damp can OSB3 be to put the felt down with bitumen adhesive and clout nails? I've seen various comments on this on-line, but no definitive answer. Don't want to put it on early and ruin it.