The surface must be in good condition and show no signs of excessive moisture conditions. If the existing floor is warped or damaged, or shows signs of mold, this could indicate moisture is coming up from the sub-floor, and installing Modin on top can lead to this moisture getting trapped and warping the existing floor, or causing the existing floor to mold and rot further. Remove the existing floor and test the sub-floor moisture levels.
Do not install flooring over heavily cushioned floors or over tile installed over concrete below grade level. The grout joints in ceramic tile and marble must be leveled so they are flush with the tile surface. Additionally, the tile may require several skim coats to achieve a flat surface.
Thinking of installing over existing wood or laminate floors? Wood floors and laminate floors are very sensitive to moisture. By installing Modin Rigid on top of these floors, you risk moisture from the sub-floor getting trapped and accumulating. This can cause the wood and laminate floors to warp underneath your new flooring. This is less of a concern if your existing floor is on a second story or over a crawl space, but if the existing floor is installed directly over a slab it is always safer to remove the floor and go directly down on the slab.
Please note these guidelines only apply to Modin Rigid Click. Glue down installations have more specific sub-floor requirements.