How To Level Concrete Slabs
Save that slab!
If you have an uneven concrete slab, you could replace it or cover it with leveling compound. But consider grinding it instead. Powerful grinding equipment is available at rental centers that cater to contractors. Here are a few of the most common uses:
- Tapering cracks or heaves to reduce tripping hazards.
- Flattening high spots or ridges before you install flooring.
- Grinding down leftover thin-set or other hard coatings.
- Removing a sealed surface to accept adhesive, thin-set or epoxy.
Different sizes for different jobs
If you have a large floor that needs to be ground down, plan to rent a walk-behind floor grinder. You’ll spend $90 to $150 per day, but a big machine can grind a large space within the rental period. Depending on the store and the machines it carries, a diamond grinding wheel may be included in the rental or you may have to buy diamond grinding inserts for about $150 a set.
For a small job, rent an angle grinder equipped with a diamond grinding wheel ($50 a day or less). It’s the perfect tool for taking down just a few high spots or tough-to-reach areas near walls. Be sure the one you rent comes with a means of attaching a vacuum to suck up the dust. If you plan to grind right up to a wall, make sure you rent a grinder with a convertible dust shroud that will flip up to expose the wheel enough to get into the corner.
Diamond grains do the cutting
The cutting surfaces of these grinders are diamond grains held by “segments,” the raised metal parts of the wheel. An aggressive wheel, which can take down 1/4 in. of concrete or more, grinds away material faster but leaves a rougher surface. A less aggressive wheel removes smaller amounts of concrete and leaves a smoother surface.
Here’s the rule of thumb for choosing a wheel: The more surface area it has, the less aggressive the wheel. Choose an aggressive one to remove high, long ridges or large, rough areas of concrete, and a less aggressive one for tasks like cleaning up tile thin-set residue from a wall or prepping for an epoxy coating.
Wear safety gear!
Plan to deck yourself out in safety gear: hearing protection, safety glasses and a respirator. The high-speed machines are loud and can send concrete rubble flying from the grinding wheel. Grinding concrete also releases fine silica dust, which can cause health problems. Always wear a NIOSH-certified N95-rated respirator. Disposable masks cost as little as $2 each, but for $25 you can get a half-mask respirator that will have a better seal. Check the seal with this easy test: If you can exhale and inhale sharply without air escaping, you’re good to grind. If you have facial hair, even a bit of stubble will ruin the seal. Go ahead and shave—it will grow back!
Keep the machine in motion
While you’re grinding, keep the cutter head moving, especially with a large floor grinder. Pivot the machine on its wheels, moving the cutter head in an arcing motion. If the cutting surface is riding on only one edge, work the machine in small circles to flatten out the surface.
Choosing dust control
There are two great ways to keep the dust from going wild. You can use a hand-pump sprayer to soak the floor with water as you grind to keep the dust down. But you’ll have a messy slurry that will need to be hosed away or sucked up with a wet vacuum.
Or you can rent a heavy-duty vacuum (regular shop vacuums won’t be able to keep up) to hook up to the grinder. It will also do a very good job of dust removal, but it will cost $75 and you’ll need to put up walls of plastic sheeting and cover nearby vents. The choice is yours, but remember to wear your respirator either way!
Nibble at a heave
For a heaved sidewalk or cracked slab, tilt the grinder slightly and remove the meat of the concrete and then feather back the high spot. You may never get toe-stubbing heaves completely flat, but you can lessen the hazard. When tilted, the dust shroud can’t catch all the dust, so you’ll need a second hose and a helper.
Grinding to the edge
Never remove a dust shroud to get close to a wall. You’ll increase your exposure to nasty silica and remove a safety barrier. To grind near walls with a convertible dust shroud attached, push the open end of the dust shroud against the wall and make steady, shallow passes. Be sure you don’t dig the edge of the wheel into the concrete as you move it back and forth.
Mark where to grind
1. Trace the high spots (above)
Outline the space that needs to be ground. Using a level or other straightedge, find where the high spot meets the flat surface. Balance the straightedge on the high spot and mark where the ends of the level are equidistant from the ground and trace around the high spot. This can be done by eye; it won’t be perfect, but it will be a good start.
2. Check your progress (below)
Start grinding on the crown of the high spot and work your way toward your marks. When you feel you’ve made progress, use the straightedge again to check for flatness. Retrace the high spots and grind them down. Repeat this grind-and-check process until the high spot is gone.