Laying plywood over a timber floor
A floor which is subject to movement will disrupt tiles laid over it so if you intend tiling over a suspended wooden floor you will first have to make the surface as firm as possible by covering it with a layer of man-made boards. Water-resistant resin-bonded plywood is a suitable material as it will resist penetration by the damp adhesive you will be spreading over it and you will avoid the problem of rotting boards. The boards should be at least 12mm (1/2in) thick. To prepare the floor to take the plywood you should punch any protruding nails below the surface at the same time checking that the floorboards are firmly secured. You can then go ahead and fix the sheets of plywood to the floor using nails spaced at 225mm (9in) intervals across the middle of the sheets and at 150mm (6in) intervals round their perimeter. You will have to cut the boards to shape round any recess or alcove , and where there is a pipe run, fix narrow strips of plywood over the pipes to make access to them easier. Make sure you stagger the joints; this will prevent any floor movement causing the tiles to break up in a run across the floor.