How flat does the subfloor need to be for a proper installation?

No subfloor is perfect, but there are tolerances that should not be exceeded. Urban Surfaces flooring has the following requirements:

Deflection (unevenness, low spots and/or high spots) in a subfloor cannot be greater than a 3/16” over a 10ft span, or 1/8” over a 6ft span (or otherwise appropriate ratios of this requirement). Subfloors should not slope more than 1/2” per 6ft. (25mm per 1.8m) and must be structurally sound.

To bring a concrete substrate to level, use a portland cement based floor patch.

To bring a plywood substrate to level, use a portland cement or gypsum based floor patch.

When installing over existing ceramic tile, use a portland cement based floor patch to fill the grout joints and raise any low spots to level.

Glue Down LVT Flooring
Subfloor flatness is especially important when installing Glue Down LVT which is only 2-3mm thick. This flooring type is thin and flexible and will telegraph any subfloor imperfections that have not been remedied prior to installation.

Click/Floating Floors
Rigid vinyl planks, like Sound-Tec and SurfaceGuard, will float over minor subfloor imperfections and mask some floor unevenness, greatly reducing telegraphing. However, due to the precisely milled clip system a flat substrate is required for support.