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)
• Subfloors must be structurally sound
• When making a concrete substrate level, use a portland cement or gypsum-based floor patch.
• When making a plywood substrate level, 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.

This article has some great tips on leveling and evening out a subfloor.

Is A Flat Subfloor Especially Important With The Types Of Flooring Installation Listed Below?

GlueDown LVT Flooring

Yes. It measures only 1.6mm-3mm thick. It is thin and flexible, and will telegraph any imperfections in the subfloor not remedied before installation.

Floating Floors

No. Rigid vinyl planks like Sound-Tec, Studio 12, and Foundations float over minor subfloor imperfections. They mask slight subfloor unevenness. However, because of the precisely milled clip system, a flat substrate is recommended for support.