How to Raise a Sunken Concrete Sidewalk: 3 DIY Methods I Tested

The problem: a sunken sidewalk

The sidewalk around my pool is about 30 years old, and it had sunk 3 to 4 inches in places. A drop like that is a tripping hazard and a drainage problem, and it looks bad.

Here are the three ways I tried to raise it, in the order I did them.

How to raise a sunken sidewalk with bottle jacks

This is the method that worked. You need:

• A shovel
• Several bottle jacks (about $20 each on Amazon, or borrow them)
• Scrap wood to set under the jacks
• A level and a string line
• Something to fill the gap: spray foam, plus rocks or bricks if you have them

The more jacks you have, the more lifting points you have on the concrete, and that helps prevent the slab from cracking. I borrowed mine, so if you don’t own any, ask around before you buy.

How to lift the slab

1. Dig holes under the edge of the slab, big enough to fit a bottle jack.
2. Put wood under each jack so it doesn’t sink into the dirt. I used leftover cutoffs from my gazebo roof.
3. Pump the jacks until you feel the slab lift, and keep checking with a level as you go.
4. Work gradually. I got the best results by pumping a few times, moving to the next jack, pumping some more, and repeating.
5. Add height when needed. A few times the jacks topped out, so I lowered the slab, added more wood, and raised it again.

I raised mine 3 to 4 inches in some areas.

Method 1: Froth-Pak foam (the first half)

Once the slab is at the right height, you have to fill the empty space under it. With a lift of 3 to 4 inches, that’s a lot of filling. If you have rocks or bricks lying around, they’ll save you some foam.

I had leftover Froth-Pak tanks from another project, so that’s what I used. I made two mistakes you can avoid:

• Test old tanks first. If your tanks have been sitting in a garage for months, spray into a garbage bag for about 5 seconds and confirm the foam expands the way it should.
• Skip the extension. I tried using a piece of PEX as an extension, and it was a mistake. The Froth-Pak shoots far enough without it, so just use the regular tip.

I sprayed all along the sidewalk, going as deep into the cavity as I could. Ten minutes later I sprayed again onto the foam that had already expanded, which let the whole cavity fill. Then I waited 24 hours before lowering the bottle jacks, because I was nervous about releasing them too soon.

I also trimmed the excess foam around the edges with an exacto knife.

Method 2: Small cans of Great Stuff (the second half)

I only had enough bottle jacks for half of the sidewalk, so I split the project in two. But the second half I did not use the froth pak.

This time I measured before lifting: there was a 3-inch drop over 3 feet. I dug and lifted this section in under two hours, then removed about 4 inches of dirt around the slab.

The next morning I started spraying foam, and my tanks ran out as soon as I started. I switched to small cans of Great Stuff: 1-inch big gap foam and 3-inch super big gap foam. I used five cans for this one little section, and the 3-inch cans made little to no difference compared to the 1-inch ones.

I let everything cure for two days before removing the jacks. But I noticed this stuff was not rock hard like the froth-paks, it was squishy, more bouncy. When I released the pressure, the sidewalk dropped about an 1/4 of an inch. That’s not much, but it isn’t nothing.

The two foams were very different. The foam from the small cans was squishy and spongy when I pressed on it with my finger. The foam from the big tanks was dense and hard. Great Stuff, it turns out, is not so great for this job.

What happened six months later

The side I filled with small cans sank again, about ½ to 1 inch, within roughly six months. The soft foam couldn’t hold the weight, which is what the finger test suggested. So I lifted it a second time and tried something different.

Method 3: Packing concrete under a sunken sidewalk (the cheapest fix)

For the second attempt I skipped foam entirely and filled the gap with concrete. It was the cheapest method I tried, and the results were great. I didn’t make a full video of this fix, but I did take a few photos.

1. Lift the slab again with the bottle jacks, the same way as before.
2. Mix the concrete dry. You want it just damp, enough to turn it darker, but not wet.
3. Pack it in under the slab, and keep packing it tight with a 2x4.
4. Wait a day or two for it to set.
5. Remove the bottle jacks.

What about lifting a sidewalk with an excavator?

If you watched the video, you know I tried this first. My neighbour had brought an excavator over that weekend to remove dirt for my paver patio, so I figured I’d try lifting the sidewalk with it. We lifted one side at a time, added shims every couple of feet, and went around all three sides twice.

I thought we’d raised it a lot, but the string line and level showed it was still off by about 3 inches. I’d estimate it raised the sidewalk about an inch, though I can’t say for sure because I didn’t measure beforehand.

I don’t recommend it. An excavator bucket is a single point of contact, so it’s easy to crack the slab, and I was too scared to lift any higher. Bottle jacks give you many lifting points and much more control, and the more points of contact, the better.

Which method works best? A comparison

How I’d repair a sunken sidewalk today

If I had to do it again, I’d use bottle jacks to lift the slab and packed concrete to fill the gap. It costs less than foam, and I don’t have to worry about soft foam compressing under the weight.

If you do use foam, use the dense stuff from the large tanks, not small cans, and expect to buy a lot of it for a big gap.

Whichever method you choose, measure the drop before you start, and check the level constantly while you jack.

Have you raised a sidewalk yourself? Tell me what worked for you in the YouTube comments.