New Posi-joist roof with vaulted ceiling, advice on membrane over sarking?

[boxrick]

Hello folks! In summary I am about to install a posi-joist roof sat on some steel ridge beams. Probably filled with cellulose or rockwool, running MVHR through the posi-joists and having the ceiling vaulted. 

I am currently trying to decide what type of breathable roof membrane to use on top of the OSB3 Sarking Boards. Does anyone have any recommendations of any products which excel at this? 

[JohnMo]

Almost any breathable membrane will be fine, use what's on your drawings. We used Proctor.

 

Would you be better to use sarking boards instead of osb, once installed these will shrink a little and give lots of small slots to let moisture ladden air out.

[boxrick]

On 16/07/2025 at 11:10, JohnMo said:

Almost any breathable membrane will be fine, use what's on your drawings. We used Proctor.

 

Would you be better to use sarking boards instead of osb, once installed these will shrink a little and give lots of small slots to let moisture ladden air out.

I need this slightly structural to prevent the twisting my valley beams. Woodfibre sarking boards don't seem to offer any of that.

Looking at other softwood 'sarking' boards they seem to be basically just fence panels and about 2x the cost. ( I need around 140-160msquare coverage ).

 

[JohnMo]

34 minutes ago, boxrick said:

Woodfibre sarking boards don't seem to offer any of that.

Sorry I was thinking more Scotland and the mandatory use of sarking boards for increased wind loads.

 

Bit different from fencing - but not much. Mine in the process of being installed. Gaps are used to traverse the roof and are back filled when finished.

[boxrick]

9 hours ago, JohnMo said:

Sorry I was thinking more Scotland and the mandatory use of sarking boards for increased wind loads.

 

Bit different from fencing - but not much. Mine in the process of being installed. Gaps are used to traverse the roof and are back filled when finished.

 

 

Which ones did you go with? 

[JohnMo]

11 minutes ago, boxrick said:

 

 

Which ones did you go with? 

Correct sarking boards, from memory 22mm x 150mm

[SBMS]

We are using 22mm steico universal boards with pro clima solitex plus. 

[boxrick]

Price wise the Sarking boards are about £200 more expensive ( for my whole roof ) than OSB3. Roof designer feels they are probably fine, and my builder too. I guess for that much it makes sense to simply get them ordered. 

I got a quote of £1550 for 120 boards delivered (Steico Universal 22mm 600x2230mm wood fibre sarking board) which feels pretty reasonable and I can have them by Wednesday this week so no real delays.

Solitex Plus looks excellent too. I guess its a little higher spec than Proctor Roofshield?