EPDM edge trim and breathable gap

[jonnypron]

Been looking for weeks but cannot find any specific details on this;

I'll try to explain but apologies if I waffle. I'm building a garden studio, timber frame, without overhang. The top of the cold roof has 25mm air gap above celotex insulation then osb roof, then will be epdm. The walls have Rockwool insulation then, breathable membrane, battens, then will have cladding.

I understand the bottom cladding will have a gap for air to run into and up in the gap, but my confusion is the top edge. The cladding will fit neatly upto / under the edge trim of the roof, but how does the air escape again? I've seen from diagrams a tiny air gap behind the front of the edge trim, but is this enough for air to flow from the bottom and out????

I am aware with soffits you put an ventilation hole but I've got flush joists.

My solution could be the fact that the roof air gap runs in line with the walls gap so the air could effectively run from bottom cladding gap of the front of the structure, up the wall, through the roof gap front to back, down the back wall and out the bottom of the cladding... or vice versa! Although as the joists run upto the sides edge i would have to drill some holes in the side of the roof joists to join the roof gap!

Any advice greatly appreciated. Sorry if this makes no sense!

 

[^woody^]

You need to vent the cladding with a continuous gap along its top edge, and the obvious place for this is at the roof/fascia junction.

A soffit would normally deal with this and allow venting whilst keeping driven rain out. If you are not having a soffit, then you will need to design another suitable detail to do this.

You can't really expect air to vent up the wall and then across the roof to the other side. It will hold, and condense somewhere within that.

Not having a soffit or roof overhang is a poor design in any case, as then there is no protection for the cladding, and it will quickly weather at the top edge

 

[geraldthehamster]

Sorry, I realise this won't be much help at this stage, but why didn't you build a warm roof? Celotex on top of OSB deck, OSB on that, EPDM over? Easier and more effective, and no need for ventilation.

 

[jonnypron]

Sorry, I realise this won't be much help at this stage, but why didn't you build a warm roof? Celotex on top of OSB deck, OSB on that, EPDM over? Easier and more effective, and no need for ventilation.

I would have loved that seeing as celoxtex is such a horrible product to cut into joist gaps, but height restrictions would have been passed with warm roof!

 

[vinty]

I did a Firestone EPDM course some years ago, the instructor told us that one of the differences between EPDM and other rubber membranes such as Butyl was that EPDM was vapour permeable which allowed the roof to breathe therefore avoiding condensation.

 

[jonnypron]

I did a Firestone EPDM course some years ago, the instructor told us that one of the differences between EPDM and other rubber membranes such as Butyl was that EPDM was vapour permeable which allowed the roof to breathe therefore avoiding condensation.

thanks for the info; that's all good to hear. This is now done, but my issue was more with the air gap where the roof meets the walls, but this is fine on our building now as we've used open joint cladding so there's little spaces for air to flow easily between each cladding batten anyhow. If the epdm breathes then any extra flow of air out this way is an added bonus.