DIY Doctor UK DIY Forums

Im about to replace the carpet in our hallway but all the floorboards are warped, width ways across the hall. its showing through the exisiting carpet and looks terrible (theres no underlay), i dont really want to buy underlay as its going to be pretty expensive since we have 3 stories to carpet.
Can i just screw ply to the flooring, if so what size, and secondly does anyone knwo why its warping like this?

It may be the boards cupping , or it may be there is not enough expansion gap at the perimiter if so you would need trim back . 6mm ply for overboarding screwed every 150mm , use 20mm screws so as not to penertrate the sub floor.