Floor Patch and Low Spots Before Flooring

Find the low area, mark its actual edges, confirm the patching product suits the substrate and the depth, prepare and prime the area as the product requires, fill it, feather the edges out to the surrounding surface, let it cure and check it again with a straightedge.

The two failure modes are under-filling, which leaves the dip you started with, and leaving a ridge at the perimeter of the patch, which creates a new high spot where there was none.

Lay a straightedge across the suspect area in several directions. Daylight under the tool is the low area; where the straightedge touches on either side is the plane you are filling up to. Marking the real extent matters, because a dip almost always reaches further than it looks.

• Work on a clean, exposed substrate — the check is only as good as the surface under the tool.
• Cross the area in at least two directions so you find its true shape, not one slice through it.
• Mark the outer edge where the straightedge first loses contact, all the way around.
• Note the deepest point, which tells you whether the depth is inside the product's range.
• Decide whether this is one localized area or part of a broader condition across the room.

This is a starting point for a decision on site, not a diagnosis of your floor. The difference between the two add-material methods is covered in self-leveling compound.

Adding material to every low area is not always the only approach, and often not the best one. If one hump is why everything around it reads low, grinding or removing that high point can reduce the amount of filling needed across the rest of the floor — and keeps the finished height down.

Which approach is right depends on:

• The substrate — what can be ground, sanded or scraped, and what cannot.
• The floor's condition overall, including whether the high point has a cause worth investigating.
• The flooring product's requirements for the surface it will sit on.
• Finished height, and what it does to doors, appliances, cabinets and transitions.
• The preparation product's own requirements for thickness, substrate and feathering.

1. Confirm the product is compatible with this substrate — wood panels, concrete and existing adhered surfaces are not treated the same by every patching compound.
2. Clean the area thoroughly and remove residue, loose material and anything protruding above the surface.
3. Prime where the product requires it, and as it requires it.
4. Mix only what can be placed within the product's working time, using its stated water or liquid proportion.
5. Fill the low area, working up to the plane established by the straightedge rather than simply overfilling.
6. Feather the edges out to the surrounding surface where the product allows feathering — some are formulated for it and some are not.
7. Screed or trowel across the area with the straightedge to check as you work.
8. Let it cure or dry as the product specifies before touching it further.
9. Check the area again with the straightedge in more than one direction.
10. Remove any ridge or high edge the patch left behind as the product permits, then clean up the dust.

Patching compounds differ in how thin they can be feathered, how thick they can be placed in one application, whether they can be built up in layers, what primer they need, what substrates they accept, and how long they must dry before flooring goes over them. FloorGauge does not publish a maximum patch thickness or a drying time, because those belong to the specific product in your bucket — and to the flooring manufacturer, whose instructions also govern what preparation materials are acceptable beneath their floor.

For the substrate requirements that sit behind all of this, see how to prepare a subfloor and LVP subfloor requirements.