Posting on behalf of a friend. He's renovating an old 1950s terrace and is relocating the boiler. There's a handy cupboard in the kitchen which will take it which is fairly deep. At the moment it has the soil stack running along the back from top to bottom. Rather than mess about rerouting the soil stack he's thinking of boxing that in by creating a new back to the cupboard with plywood, then mounting the boiler on this. Are there any problems with this plan? Would plasterboard be more suitable?