Breathable / non breathable membrane option

[dangti6]

I am extending width ways an existing garage which currently has felt underlay (80s build) and vented soffits.

 

The intention is to remove all existing tiles from the front elevation, add new membrane, batten, fascias etc etc so that the whole run is replaced.

 

Whereas at the rear, there is an extension which has rafters protruding through the garage roof (pitch change) which I don't really want to disturb. Intention there is to lap the new membrane over the existing where it joins.

 

Therefore about 1/6th of the structure will be original non breathable felt and the majority will be a new membrane. 

 

Would people suggest I buy a non breathable membrane and vent the new UPVC soffits as it was previously to keep it all the same or is it permissible to use/mix a breathable membrane with old?

 

(I really don't want to interfere with the other roof due to the valley etc - appreciate doing so would be a solution for many)

 

 

[Temp]

Are you converting the garage or leaving it as a garage?

 

I would be inclined to go for a breathable membrane AND vent the new soffit.

 

 

 

 

[dangti6]

It's just going to be a garage workshop, with no heating or insulated cavities. For what it's worth, the existing ceiling is plaster boarded with rolled insulation over it which I will probably continue in to the new section, making sure it's not tucked in to the eaves.

 

OK, Thanks so sounds like if I was going to renew the whole lot, a breathable membrane would be the way to go and thus wouldn't be a need to vent the soffit, whereas I can do as you say and use a breathable membrane and vent it as belt and braces to avoid issues with the area that's remaining non breathable. 

 

If I overlap the existing membrane about 600mm (across 2 trusses) / couple of tiles width does that sound sufficient?

 

My research here leads me to going for Protect VP400 membrane. It's like to be exposed for 4-6 weeks whilst I await delivery of my unicorn poo Redland tiles. Fully expecting that delay to increase with the 3rd lockdown this week!

Edited January 7, 2021 by dangti6

[Temp]

28 minutes ago, dangti6 said:

For what it's worth, the existing ceiling is plaster boarded with rolled insulation over it which I will probably continue in to the new section, making sure it's not tucked in to the eaves.

 

Ok so that construction is normally ventilated using vents in the eaves. Air goes in one side, across the "loft" just above the insulation, and out the vents on the other side.  

 

28 minutes ago, dangti6 said:

OK, Thanks so sounds like if I was going to renew the whole lot, a breathable membrane would be the way to go and thus wouldn't be a need to vent the soffit, whereas I can do as you say and use a breathable membrane and vent it as belt and braces to avoid issues with the area that's remaining non breathable. 

 

Yes if you are retaining the impermeable membrane on one side I would retain vents on both sides.

 

28 minutes ago, dangti6 said:

My research here leads me to going for Protect VP400 membrane. It's like to be exposed for 4-6 weeks whilst I await delivery of my unicorn poo Redland tiles. Fully expecting that delay to increase with the 3rd lockdown this week!

 

I used Protect VP400 (Dark blue color) on an out building about 8 years ago. Think I got it from Screwfix. Seemed strong and survived exposed for many months. Used it both under tiles and behind timber cladding.

 

 

 

Edited January 7, 2021 by Temp

[dangti6]

Thanks Temp. That's right, the existing wooden soffits are vented. 

 

Screwfix does appear to come out top price wise for the VP400, £100 for 50x1.5m . I will try my friendly Merchant who have been unbeatable on most stuff to date. 

 

Trusses arrive next week. Measured the existing ones myself to replicate as none of the suppliers I tried were offering surveys. Wish me luck that they match and line up ?