Dormer materials - is sheathing ply acceptable/meet regs?

[STEVEROG]

I am in the middle of some building work. The contractor would, with my permission, like to change the 'sarking' from Marine Ply to sheathing grade ply. The specific areas where they want to change the material are on the roof and sides of the dormer. The build-up of the sides is apparently like this:-

Paint
Built- up render
Stainless steel EML
Vapour barrier (used to keep the cavity clear of snots of render)
50mm timber battens to form a cavity for ventilation
Breather membrane
Sarking
Studwork
Insulation between and over the warm face of the studs
Plasterboard

I am concerned that the sheathing ply is clearly NOT designed for exposure to moisture. Should I allow the change?

Thanks in advance

 

[freddiemercurystwin]

WBP board is not really designed to do this either. Continual periods of damp weather can leave the render saturated and the ply continually wet. I have used a product called Rendalth in the past to overcome this problem. http://monocouche.co.uk/pdf/RENDALATH INSTALLATION GUIDE - July 2009.pdf

Untitled

• freddiemercurystwin
• 19 Jul 2010
• 2

The insulation would probably need to be beefed up slightly now the new Part L regs have come into force before anyone says.

 

[^woody^]

The contractor wants to change the grade of ply purely for price reasons.

Sheathing ply is for limited use for site hoardings and shuttering etc, not long term structural use

 

[chappers]

as woody says he only wants to change it for pricing reasons, though non Lloyds rated marine ply isn't much more than WBP anyway.

to be honest osb or wbp ply would be sufficient, ask him if he is going to reduce the price accordingly.