How to Repair Cracked Concrete Patio Slabs

What do you need for patio crack repair?

A UV-stable crack kit, dry silica sand and a few hand tools. A patio sits in the sun and rain all year, so it needs a filler that flexes and holds up to sunlight.

The sand isn’t in the kit, so buy a bag of dry silica sand. The urethane underneath isn’t UV-stable on its own, so the polyurea top layer is what protects it outdoors.

How do you fix cracks in a concrete patio?

Open up thin cracks, clean and dry them, pack them with sand soaked in urethane, then top them with a polyurea filler and scrape it flush. That’s the method in the UV slab kit’s instructions, and it suits cracks up to 1/4 inch with flush edges.

Here’s how to fix cracks in concrete patio slabs with that kit:

1. Check the edges

   Lay a straightedge across the crack. If one side is higher, or the crack is wider than 1/4 inch, read the section on patios that need more than filler first.

2. Open up thin cracks

   For cracks 1/8 inch wide or less, the kit’s instructions say to grind the crack out to 1/4 inch wide and 1/4 inch deep so the filler has room to grip. Wider cracks skip this step.

3. Clean and dry the crack

   Vacuum out the dust, dirt, weeds and loose concrete. The instructions say it’s critical that the crack be dry, so work after a few dry days.

4. Prime with urethane

   Wet the inside of the crack with the kit’s urethane. Just wet it, don’t fill it.

5. Pack in sand

   Pour dry silica sand into the crack until it’s 1/4 inch below the surface.

6. Soak the sand

   Saturate the sand with more urethane and let it set for about 30 minutes.

7. Top it off

   Fill the last 1/4 inch with the polyurea filler. It’s thin and levels itself.

8. Scrape it flush

   Scrape any overflow flush with a razor scraper.

If the patio is new, wait before you start. The urethane’s data sheet says the concrete needs at least 28 days of curing first.

Is it a crack or a control joint?

A straight, even groove across the patio is a control joint, and a crack at the bottom of it is normal. The NRMCA calls joints simply pre-planned cracks (opens in a new tab), cut so the slab cracks in a straight line instead of at random.

Don’t fill control joints with crack filler. If you want them sealed against water and weeds, the joints guide covers joint fillers, and the crack or joint guide shows how to tell the two apart.

What about the gap between the patio and the house?

That gap is an isolation joint, and it gets a flexible joint filler, not crack filler. The NRMCA says isolation joints separate driveways and patios from sidewalks, garage slabs, stairs (opens in a new tab) and other fixed points so each can move on its own.

When the old filler in that joint rots or splits, rain runs down the gap along your foundation. Cut out the old filler, set backer rod, and refill it with a flexible filler such as a polyurea joint filler kit. The urethane’s data sheet says the crack kit isn’t meant for expansion joints.

The expansion joint repair guide walks through it, and the backer rod guide covers sizing.

How long before you can use the patio?

About 30 minutes for foot traffic in mild weather. The polyurea top takes light traffic in 30 minutes and full traffic in 60 minutes at 70°F, according to its data sheet.

Cold slows every cure. Keep furniture and the grill off until the full traffic time has passed.

Will the patio crack repair show?

Yes. The polyurea top cures gray, so the repair shows as a thin line against the patio. The NRMCA says repair material will not match the color and characteristics (opens in a new tab) of the original concrete.

You can keep the line thin. Grind thin cracks only to the 1/4 inch width the instructions call for, keep the filler inside the crack, and scrape the overflow flush before it sets.

What mistakes make patio crack repairs fail?

Moisture, a crack that was never opened up, and filling a slab that’s still moving cause most failures. Each one has a simple fix.

• Filling a damp crack. The kit’s instructions call a dry crack critical. Wait for a dry spell, not just a dry-looking surface.
• Skipping the grinding on a thin crack. Filler can’t get into a crack 1/8 inch or narrower, so it bridges over the top and peels off. Open it up to 1/4 by 1/4 inch.
• Leaving dirt or roots in the crack. Filler bonds to concrete, not soil. Vacuum the crack down to clean concrete.
• Using crack filler in the joint at the house. That gap needs a flexible joint filler.
• Working in the cold. The polyurea data sheet calls for concrete at 50°F or warmer.
• Filling a patio that has sunk. The crack will open again. Fix the movement first.

When does a patio need more than crack filler?

When it has moved. A patio that has sunk or tilted, or a crack with one edge higher than the other, means the soil underneath changed, and filling the crack won’t stop it.

Thin cracks are usually nothing to worry about. InterNACHI says some cracks in patio slabs, walkways and driveways (opens in a new tab) are not necessarily a cause for alarm, and treats cracks under 1/4 inch with no displacement as not structurally threatening.

• Sunk or tilted patio. It can often be lifted back into place. The sinking slab guide compares the options.
• Patio now slopes toward the house. Water runs to the foundation. Lift or replace the slab rather than filling cracks.
• Cracks wider than 1/4 inch. The large crack guide covers the method, after you find out why it opened.
• Cracks everywhere. The repair or replace guide helps you decide.

If a patio crack lines up with a crack in the foundation wall, have a structural engineer look at the house first.

Why do concrete patios crack?

Mostly from shrinkage, too few joints and a poorly prepared base. The NRMCA says the majority of cracks come from issues like omitted joints, improper subgrade preparation and excess water (opens in a new tab) in the mix.

• Joints too far apart. The NRMCA says joint spacing should be 24 to 36 times the slab thickness, about 10 feet for a 4-inch slab. Patios poured with fewer joints crack between them.
• A soft or loose base. Topsoil and soft spots should be removed and the base compacted before the pour.
• Freeze and thaw. Water expands as it freezes, and the NRMCA says that causes stresses in the concrete.

The why concrete cracks guide goes deeper, and the concrete cracks hub covers driveways, steps and pool decks.

How do you keep a patio from cracking again?

Keep water out of the slab and away from its base. Seal the patio once the repair cures, keep the joints filled, and point downspouts away from the slab.

The NRMCA recommends a silane or siloxane-based breathable concrete sealer (opens in a new tab) against freeze-thaw damage, and says late summer after a few dry days is an ideal time to apply it. It also warns against de-icers on new concrete in its first winter.

The sealing after crack repair guide covers which sealer to use over a repair and how long to wait.