Breathable membrane installation (Scotland)

[lc155]

Hi,

Got a dormer window in a bay shape that needs work done to the roof framing of it, as the flashing on the sides has failed over the years as well as the mortar that sealed between the slates at the peaks. Will involve ripping the slates off and repairing any sarking/framing that requires it along with replacing flashing. At the same time, I figured putting down a breather membrane would be a decent idea while I'm at it, as due to the roof age, it's just bare slate nailed into the sarking boards.

Question is, can I get away with installing the membrane straight onto the sarking boards? Was thinking of using Cromar Vent3 Pro (I'm in climate region 4 for wind uplift) but all the installation instructions I look at when it comes to membranes always seem to just talk about using battens on top like how they do in England.

So, can I just staple/nail (which is preferable?) the membrane directly to the sarking boards, tape the laps as what seems to be required for my region, and then just nail the slates on top? Is that sufficient or will wind uplift still be a problem there?

Thanks.

 

[robinbanks]

Difficulty you have doing that is you then need to walk on a "plastic" sheet (because you don't walk on the slates) that is stapled to a potentially smooth wood surface, many meters above the ground. When you step on an end, carrying 50kg of skates, and every staple rips out cartoon stylée and you're dumped off the side of the building you're injured or dead

I'd batten, and counterbatten, it for no other reason than to give myself something to walk on, while preserving the integrity of the felt.

If you're in a terrace with neighbours and hence cannot have a change in height, I'd consider either glueing it down (spray adhesive) or don't bother installing it ; it'll be full of holes by the end of the job, having been tromped all over, had slates rested/dropped/cut on it, and two nails per slate banged through it.

I see often people trotting out some "wisdom" that a membrane is needed to make a roof watertight; it ain't true - keeping water out is the slate's job

 

[lc155]

Well, while it won't make it watertight, it does help against wind driven rain which we seem to be getting a lot more of over the years, which is what I wanted it for since rot required the removal of a few dormer rafters and new ones installed. Damaging the membrane by walking all over it wasn't a concern - it was a small dormer that could be leaned over to on both sides from the scaffolding. I did look up the manufacturer BBA certificate and it did state it could be installed on sarking boards directly, so we stapled it on and then nailed in slates.

The only thing I'm thinking about now is ventilation. I know some breathable membranes will not require the usage of additional ventilation, as they are considered open air enough. I don't know if Vent3 Pro fits this, as the BBA certificate doesn't explicitly state if extra ventilation is required or not either way. It just gives vapour permeability figures.

https://www.cromarbuildingproducts.com/wp-content/uploads/2022/04/4748ps3i2.pdf

It gives a figure of 0.151MN, which I am not sure how to translate into real world practicality. Do note that this is just a dormer window rood, the rest of the roof is completely open air ventilation - just not sure how much of that would now reach the dormer. I do plan to board the ceiling below it with vapour check plasterboard as is usually advised for loft rooms in order to prevent any potential leaks ruining the plasterboard. Just not sure if that's all enough.

 

[lc155]

Ok, maybe I can just install one of these (I haven't finished the front dormer peaks slating yet due to the weather, only sides are done. I heard the Ubink vent is one of the best flush ones you can get.

Doing one on each side would have probably been best but if I can do one at the front and then open up the sarking a bit at the back (where it connects to the open ventilated roof space) that feels like it would be enough, yes?

 

[robinbanks]

I'd say so; what's your insulation buildup?

 

[lc155]

I'd say so; what's your insulation buildup?

For the dormer, I'm planning 200mm of fibreglass over the joists, and in between the joists I will fit 75+50mm rockwool. Avoiding PIR mainly because it has zero sound insulation properties and the walls used to be lath and plaster, so there is significant sound transmission to the room. I could have put 25mm on the bottom but then I'd have issues with the tilt and turn window opening inwards - it's pretty close to the ceiling as is, so any curtain rail would be a disaster with loss of height.


Attached a very crude drawing showing the plan. Green squiggles are how I expect airflow to behave with one vent into the dormer and a path for air to reach the main attic ventilated space.

 

[lc155]

Well, haven't gotten the slates all on before all the snow got dumped on everything. I sure hope these membranes can survive a bit of snow and ice in the meantime!

 

[robinbanks]

It'll be fine