How to Grind Highs and Patch Lows Before a New Floor

Why this matters

Thin glue-down flooring shows everything under it. A ridge that a straightedge barely rocks on reads as a raised line across a vinyl tile floor in low light; a patch left with a trowel ridge telegraphs as a stripe; a dip filled without primer releases from the slab and sounds hollow under a chair leg. Spot prep is where most of the labor of a flat floor goes, and it is also where a crew can make a floor worse: grinding a dish into a ridge, patching over dust, or patching a low that was only low because the straightedge was resting on a high.

This card covers the spot fixes: grinding high spots and filling low spots with a feather-edge patch, on a concrete slab. Finding and mapping them is 'How to Check a Floor for Flatness and Map It'; tolerance numbers are 'Floor Flatness Tolerances by Floor Type'. When the lows cover too much of the floor for spot work, the fix is a whole-floor pour, owned by 'Self-Leveling Underlayment Application'.

Before you start

• The map and plan. Every high and low marked with its measured value, a target from the flooring maker, and a fix assigned to each.
• Moisture first. The slab moisture test is done and passed for the flooring and adhesive, or the moisture plan is decided. Patch over a slab that fails the test traps the problem under a fresh surface. See 'How to Run an In-Situ RH Test on a Slab'.
• A sound, clean surface. Patch bonds to clean, sound concrete. Paint, curing compound, sealer, oil and old adhesive residue are removed mechanically where the patch goes. Any black adhesive or old resilient floor of unknown age is presumed to contain asbestos and is not ground, scraped or sanded until a test clears it; see 'Asbestos in Old Resilient Floors and Adhesive'.
• Equipment. A handheld grinder with a diamond cup wheel, a dust shroud and a HEPA dust extractor that meets the Table 1 airflow for the wheel size, a HEPA vacuum, a 10 ft and a 2 ft straightedge, the patch maker's primer, a cement-based feather-edge patch, a mixing paddle, a flat steel trowel, and the patch and primer data sheets.

The section, before and after

BEFORE  (section, not to scale)
 ======================================== 10 ft edge
          ___                             rocks on H
 ________/   \_______________       _____
          H: ridge +1/4 in   \_____/
                             L: dip, 5/8 in read
                             bearing on H

AFTER
 ======================================== 10 ft edge
 ____________________________:::::::_____
          H: ground to plane \:::::/
                             L: patch in 2 lifts,
                             feathered to 0 at edge

Step 1: Grind the highs before anything else

Grinding goes first for two reasons. A ridge holds the straightedge up, so the lows next to it read deeper than they are until it is gone. And grinding throws dust that ruins a primed or freshly patched surface, so nothing wet goes down until the grinding is finished and vacuumed.

Set up before the wheel touches concrete. Table 1 of 29 CFR 1926.1153 lets a shrouded grinder run without exposure monitoring only when the dust collector provides at least 25 cfm per inch of wheel diameter, has a filter of 99 percent or better efficiency, and has a cyclonic pre-separator or filter-cleaning mechanism. That ties the wheel size to the extractor: a 5 inch wheel needs 25 x 5 = 125 cfm, a 7 inch wheel needs 25 x 7 = 175 cfm. An extractor rated at 150 cfm can run the 5 inch wheel under the row and cannot run the 7 inch. Indoors with the shroud, the row requires no respirator for up to 4 hours of Table 1 work in the shift and an APF 10 respirator, worn under a written 1910.134 program (applied to construction by 29 CFR 1926.103), beyond that; the shift total is the rule in 'Silica Dust in Flooring Work'.

Wear eye and face protection and hearing protection, keep both hands on the grinder with the guard on, and keep the cord behind you. Grind the crown of the ridge, not a line beside it, in overlapping passes, and taper the grind out on both sides so the ground area blends into the slab. Check with the short straightedge every few passes and stop when the ridge is within the target with margin. What breaks if you skip the checks: a crew grinding by feel takes the crown down and keeps going, and a ridge becomes a trough that now needs patch too.

A ridge along a sawcut control joint is curled slab edge. Grind the edges down to the plane on both sides; the joint itself stays a joint and is handled under the flooring maker's rules for slab joints, not filled solid.

Step 2: Vacuum, then re-measure the lows

HEPA-vacuum the whole area; no dry sweeping and no compressed air, which put the finest dust back in the air and leave a film the primer will not bond through. Then scrape off any crayon marks inside the areas to be patched, because wax is a bond breaker.

Re-run the straightedge over every low on the plan, now that nothing high is holding it up, and write the new reading beside the old one. What breaks if you skip it: you patch to a depth that no longer exists, and the patch crowns above the floor around it.

Step 3: Prime as the patch maker requires

Read the patch data sheet for the primer and the surface condition it requires. On porous concrete, primer keeps the slab from sucking water out of the patch before it cures, which causes pinholes, weak patch and edge release. Some patches require a mechanically profiled surface instead of, or in addition to, primer. Apply the primer the patch maker names, at its dilution, and let it dry to the stage its data sheet requires before patching. What breaks if you skip it: the patch looks fine for a day, then releases and sounds hollow, and the floor fails over it.

Step 4: Mix and fill the lows in lifts

Mix the patch per its data sheet, powder into the measured water, at the stated paddle speed. Bag dumping and mixing release silica, and wet cement-based patch is strongly alkaline on skin: mix with the bag low to the bucket and exhaust pulling away from the house, wear waterproof gloves and eye protection, and wash skin that gets mix on it with clean water. If patch gets in an eye, flush it with clean water for at least 15 minutes and get medical attention if it still hurts.

Fill each low no deeper per application than the data sheet allows. A thick lift of a patch made to go on thin cracks and shrinks, and it does not reach full strength. Where the low is deeper than one lift, fill the deepest part first, let it set to the stage the data sheet requires, then fill to grade.

Step 5: Screed and feather

Use the long straightedge as a screed: rest it on sound slab on both sides of the low and drag it across the patch, so the patch is struck off to the plane of the floor around it rather than to your eye. Feather the edges to zero with the flat trowel held nearly flat, so there is no step at the perimeter of the patch. What breaks if you skip it: a patch troweled freehand ends up a hump or a dish, and the perimeter step is exactly what telegraphs through thin vinyl.

Knock down any trowel ridge while the patch is still green with the trowel edge or a scraper. Sanding cured patch makes fine silica dust; if a ridge has to be sanded after cure, do it with a sander on a HEPA extractor and HEPA-vacuum after, not by hand with the dust left on the floor.

Step 6: Recheck, tap, and let it cure

Once the patch has cured per its data sheet, run both straightedges over every ground high and every patch, and record the readings next to the plan's originals. Tap each patch with a screwdriver handle or a coin; a hollow sound means it has released and comes out. Do not install flooring over the patch until the data sheet's cure time before flooring has passed, and where the patch maker or the flooring maker requires it, test the patched areas for moisture along with the slab.

Worked example: a dining room slab for glue-down vinyl tile

A 12 ft by 18 ft dining room on a slab, getting glue-down luxury vinyl tile. The flooring maker's instructions, read for this product, set 3/16 in over 10 ft and require a smooth surface free of ridges. The map shows two marks: H, a ridge +1/4 in high and about 4 ft long along a cold joint; and L, a dip about 4 ft by 5 ft, read at 5/8 in deep with the 10 ft straightedge bearing on H at one end.

Grinding H. The crew's extractor is rated at 150 cfm with a 99 percent filter and a self-cleaning filter, so they run a 5 inch cup wheel (needs 125 cfm) rather than the 7 inch (needs 175 cfm). Grinding takes about 40 minutes, under the 4-hour line, so no respirator is required for that task by its row. The ridge is ground until the 2 ft straightedge shows under 1/32 in, inside the 3/16 in target with margin, because thin vinyl telegraphs well before a flatness tolerance is broken.

Re-measuring L. With H gone, the 10 ft straightedge bears on flat slab and reads L at 1/2 in, not 5/8 in. 1/8 in of the original reading was H's height carried into the dip. 1/2 in is still over 3/16 in by 5/16 in, so L gets patched.

Lifts. The patch data sheet, read for this product, allows up to 3/8 in per application in an area this size. 1/2 in is more than 3/8 in, so L takes two lifts: the first fills the center of the dip about 1/4 in deep, and once it has set to the stage the data sheet requires, the second brings it to grade and feathers to zero at the edge. Each lift is within the 3/8 in limit, and 1/4 + 1/4 = 1/2 in.

Primer. The slab is porous (a drop of water darkens and soaks in within a minute), so it gets the patch maker's primer at its stated dilution, dried before the first lift.

Recheck. After cure, the 10 ft straightedge shows under 1/16 in across L and across the ground ridge; the tap test is solid across the patch. The plan gets both new readings beside the originals: H +1/4 in before, under 1/32 in after; L 5/8 in as first read, 1/2 in after grinding, under 1/16 in after patching.

What would have happened in the other order. Patching first to the 5/8 in reading would have left the patch 1/8 in proud of the floor once H was ground, and grinding H afterward would have thrown dust onto fresh patch.

How to verify it is ready for the floor

• Every high and low on the plan has an after reading within the maker's target, written next to its before reading.
• No ridge or perimeter step can be felt with a flat hand or seen with a light low across the floor.
• Every patch sounds solid to a tap, and has passed the data sheet's cure time before flooring.
• The floor is HEPA-vacuumed, with no crayon left in patched or glued areas.

References

• 29 CFR 1926.1153, Table 1, handheld grinders for uses other than mortar removal.
• See related: How to Check a Floor for Flatness and Map It
• See related: Self-Leveling Underlayment Application
• See related: Silica Dust in Flooring Work
• Patch and primer makers' data sheets for lift thickness, primer, cure time and moisture limits.