How flat must the subfloor be for a proper flooring installation?

No subfloor is perfectly level, but any signs of unevenness and high or low spots must be remedied. Please follow these requirements:
  • Subfloor unevenness cannot be greater than 3/16 of an inch over a 10 foot span or 1/8 inch over a 6 foot span
Subfloors must:
  • Not slope more than ½ inch per 6 feet (25 mm per 1.8 m)
  • Be structurally sound
Making a substrate level
  • Concrete substrate: use a portland cement or gypsum based floor patch
  • Plywood substrate: use a portland cement based floor patch
Before installing flooring over existing ceramic tile
Use a portland cement based floor patch to fill the grout joints and raise any low spots to level.

Is a flat subfloor important with the types of flooring installation listed below?

GlueDown LVT flooring
Yes. It measures only 2-3 mm thick. Thin and flexible, it telegraphs imperfections in the subfloor not remedied before installation. Click/Floating floors
No. Rigid vinyl planks like Sound-Tec and SurfaceGuard float over minor subfloor imperfections. They mask subfloor unevenness. However, because of the precisely milled clip system, a flat substrate is recommended for support.