Start with a Smooth Subfloor
Laminate floor systems don't function well on uneven surfaces. Before starting any flooring work, inspect the subfloor. Crawl around with a straightedge to find any areas that are more than 1/8 in. high or low. Subfloor seams are the usual suspects.
A belt sander sporting a coarse-grit belt will knock down seams pretty fast, but you may have to rent a commercial floor sander to grind down severe peaks.
A dip in the floor will cause a soft, spongy section in the laminate floor. Most dips can be taken care of with a trowel and vinyl floor patch. Buy a fast-drying variety if you want to start laying the floor the same day.
“Avoid self-leveling floor compounds, especially on older homes,” says Jay, our flooring pro. “An out-of-level floor could take a whole truckload of self-leveling
compound to flatten out the floor. And oh, yeah—this is the time to screw the plywood to the joists anywhere there's a squeak.”