In reply to SimonMarcYoung:
I,m a joiner,
as already stated you need to use a good quality 18mm WBP Ply.
Sterling board is completly different in the way it is made and more likely to fail when you tighten the holds,also it tends to weaken when you nail or screw it.
WBP on the other hand will remain strong at the nail or screw holes.
dont go to B&Q Go to your local timber merchant. roughly 26 quid per sheet.
Hope that helps,
I,m not on here that often at the moment but you need any other advice private mail me.
cheers Neil