We've a wooden floorboard floor in our bathroom and have just had the bathroom done and have some 5mm thick lino to go down. I've read that it's best to line the floorboards first. Most people say use WBP ply. Would a waterproof plasterboard be acceptable too?? My thinking is that the plasterboard will be easy to cut and fit around the many different shapes behind and around the toilet which includes a cast soil pipe within the floor.
I presume you mean "vinyl"?????
Lino is a very old (but still available) product, and also one that is almost a forgotten skill - fitting wise!
sheet vinyl will eventually "mould" itself to any imperfections in the floor beneath it.
If your floor is not perfect then you need to board it first. traditionally this was done with hardboard, today this is done with ply. 4mm will suffice if your subfloor is not too bad, but most fitters will use 6mm. For a bathroom it will not cost too much and NO - do NOT use plasterboard EVER!!!!!!
6mm can be cut with a sharp Stanley type knife blade. you should also be able to create a cardboard template around any porcelain features and transfer this to your ply.
Or maybe you can remove the sink and loo before you ply - this will depend on the flexibility of your pipe connections though!
.....if not keep your templates for use on the vinyl afterwards!
"boarding" should be installed so that any joints are very tight and NOT over a floorboard joint
Fixing methods depend on whether you feel there may be a need to ever lift the floor - are there any pipes/wires/etc that may need future access?
If not cheapest and easiest is ring shank nails - fix every 4 inches around perimeter and every 6inches in middle they should be sized length wise so that there is no danger of hitting any cables or wires - this will depend on thickness of board and thickness of floorboards
If lifting may be required use screws fixed as above and with the same depth warnings
if in doubt - pop back with more questions