What size dip in the floor needs to be filled before carpet is installed?
Levelling compound is your pal. Use it to get the whole floor level. whatever else you do will come back to haunt you. Whether the carpet is free standing or stretched wall-to-wall, it needs to be on a flat, level surface, with no dips deeper than 1/8&quot; over a square foot - and that's pushing it. Underlay may give a little leeway, and indeed, is worth putting down first, but won't solve your problem.