We found a huge gap under our concrete slab due to erosion after stripping the deck away during our backyard renovation. Not wanting to find ourselves with dangerous, broken concrete, we had to figure out a solution ASAP. So in this blog post I take you through how to fix erosion under concrete slab with poly foam and CPVC pipes.
After stripping the deck away during our backyard renovation, we found a huge gap under our concrete slab due to erosion. We had to figure out a solution quickly before we were left with dangerous, broken concrete in our backyard. Follow along as I show you how to fix erosion under concrete slab with poly foam and CPVC pipes.
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PATIO MAKEOVER PART 2!
This is the second part of our backyard patio makeover series. We had a terrible, rotting back deck that needed ripping out. From there we discovered new problems that needed fixing (isn’t that what always happens?) and that’s what we’re talking about today. We will be getting to the fun décor part of the patio makeover soon, so stay tuned!
WHAT CAUSES A VOID UNDER A CONCRETE SLAB?
In our case, it was soil erosion. To make sure we understood the source of the problem, we had someone come out and look under the concrete slab. They confirmed it was in fact erosion, probably due to a tree trunk or something large underneath the house when it was built. After 20 years or so, natural elements slowly degrade and land settles, causing erosion and therefore a gap underneath a concrete slab.
You should do the due diligence to find out what the cause of your concrete gap is. If you have a larger issue, like a water leak, you want to fix that first so you’re not dealing with the same problem a few years down the road.
TIPS FOR HOW TO FIX EROSION UNDER CONCRETE SLAB
To find the gap under your concrete, use a tape measure extended out fully. That you can see how far the gap goes and then pull the tape measure back towards you.
Wear lots of protective clothing, including respirator, goggles, and long sleeve shirts and pants. We had an accident where the poly foam sprayed onto my dad’s bare arm, and it was not fun getting it off of him. We used a razor blade ultimately. But you can prevent any kind of spraying by waiting at least 2 minutes before changing each pipe out. There is a lot of pressure built up after each round, so use a towel at the end of the pipe to catch any spray so you can avoid this mistake.
If you can’t access the void with a CPVC pipe, you can also drill a hole in the concrete. The Secure Set website has a tutorial on this as well.
The foam hardens very quickly, so be sure to do a few dry runs first.
Make sure to visit the Secure Set website and read their instructions as well.
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TOOLS USED
1/2” diameter CPVC pipe (we got ours from Ace Hardware)
Work gloves
Respirator
Goggles
Reinforced fiberglass strip packing tape
Chalk
Recommended: long sleeve shirt and pants to cover yourself up!
NOTE: The company who makes the foam to fill voids under concrete also provides helpful videos. Click here to watch these videos before filling your concrete void.
HOW TO FIX SOIL EROSION UNDER CONCRETE
First, observe where the void exists under your concrete slab. Use a tape measure to extend out as far as possible, so you can easily see how far the gap goes.
Make some estimated calculations for the depth and length of the void to determine how much poly foam you need to buy. Use chalk to measure/estimate where the void is and how many CPVC pipes you will need and where to place them.
Purchase Secure Set poly foam from their website.
If you can access the void using CPVC pipe then this is the easiest route to go. The Secure Set website has instructions on how to use Secure Set with CPVC pipe extensions.
If you cannot access the void, you will need to drill holes in the concrete. Secure Set has a tutorial video on this as well.
Get the foam kit set up and ready. Do a couple dry runs. You will not have a lot of time make mistakes once you get going as the foam hardens very, very fast.
Start with one pipe and slowly pull the pipe back while squeezing trigger. The foam will expand so you need to move quick. The method we used was to pull the pipe back about 1 foot every 7 seconds.
Tip: Once you see foam coming through, take caution. The pipe is under pressure so do not just remove the nozzle from the CPVC pipe or you will get blasted with foam. And it will be very difficult to get the foam off your skin and clothing (that happened to us).
Once you finish one pipe, wait at least 2 minutes before disconnecting the pipe. This is very important! We highly recommend also placing a big towel over the nozzle/CPVC pipe connection and put this on the grass. If you don’t do this, the pressure will cause the foam to spray, which did happen to us the first time. From there you can disconnect the pipe from the nozzle while the towel is over it blocking, and spray or blast back. I wish we would have done this the first time.
Continue on with other pipe connections or areas to spray into concrete. The foam will harden very quickly.
FAQ
How do you tell if there is a void under a concrete slab?
You may notice sloping or settling of the concrete or spots around it if you’re having erosion. For example, in our backyard, we noticed the garden bed was sloped and slanted downward. It was confusing and shocking for a few minutes, trying to figure out why our garden bed was apparently sinking. So my husband used a shovel to dig it out, and then we saw the large gap under the concrete.
The easiest way to see the void is by looking under the concrete. If you can easily dig or remove objects to look underneath, do that. Then, you can use a tape measure to extend as far as possible to see how far the gap goes.
Another option would be a ground penetrating radar system, which can detect gaps without having to look underneath the concrete. Find more info here.
What is the best fill for under concrete?
For filling an erosion gap under concrete like we were, we used the polyurethane foam from Secure Set. While it is not the most environmentally friendly or natural option, it is one of the few options available if you are only looking to fill a small space that you can’t reach.
Any time you’re putting filling under concrete, you just want the material to be non-degradable so it doesn’t erode over time. Crushed rocks or gravel would be a the best natural filling if you were building a patio from scratch and had access to underneath the slab.
How much does it cost to fill a concrete void due to erosion?
If you hire this job out, it will cost at least a couple thousand dollars. We got a quote for someone to do this exact same process for about $3,000 in our area. Purchasing the kit ourselves for $479 allowed us to save a lot of money, but you do need to have the time to do it. And this was not a DIY that we could just pass over. This is the kind of project we wanted fixed as soon as possible!
There is also a process called slab-jacking, which is putting rock and other natural element underneath the concrete to fill the gap and prevent erosion. I would imagine this is even more expensive to have done but we didn’t look into it.
FOLLOW ALONG WITH THE PATIO MAKEOVER SERIES!
How to Replace Siding on your House
Fill a Concrete Void due to Erosion
How to Build a Standalone Patio Roof
DIY Patio Outdoor Couches
PIN IT FOR LATER!
How to Fill Void under Concrete due to Erosion
Learn how to fill your concrete gap or void. This happens because of erosion typically, and can be fixed a couple of different ways. If you can access the gap from the side, you can use CPVC pipes and polyfoam as shown in this tutorial.
Materials
- Secure Set Spray Foam
- Reinforced fiberglass strip packing tape
- Chalk
Tools
- 1/2” diameter CPVC pipe (we got ours from Ace Hardware)
- Hammer drill
- 3/8 concrete drill bit
- Work gloves
- Respirator
- Goggles
Instructions
- First, observe where the void exists under your concrete slab. Use a tape measure to extend out as far as possible, so you can easily see how far the gap goes.
- Make some estimated calculations for the depth and length of the void to determine how much poly foam you need to buy. Use chalk to measure/estimate where the void is and how many CPVC pipes you will need and where to place them.
- If you can access the void using CPVC pipe then this is the easiest route to go. The Secure Set website has instructions on how to use Secure Set with CPVC pipe extensions.
- If you cannot access the void, you will need to drill holes in the concrete. Secure Set has a tutorial video on this as well.
- Get the foam kit set up and ready. Do a couple dry runs. You will not have a lot of time make mistakes once you get going as the foam hardens very, very fast.
- Start with one pipe and slowly pull the pipe back while squeezing trigger. The foam will expand so you need to move quick. The method we used was to pull the pipe back about 1 foot every 7 seconds.
- Tip: Once you see foam coming through, take caution. The pipe is under pressure so do not just remove the nozzle from the CPVC pipe or you will get blasted with foam. And it will be very difficult to get the foam off your skin and clothing (that happened to us).
- Once you finish one pipe, wait at least 2 minutes before disconnecting the pipe. This is very important! We highly recommend also placing a big towel over the nozzle/CPVC pipe connection and put this on the grass. If you don’t do this, the pressure will cause the foam to spray, which did happen to us the first time. From there you can disconnect the pipe from the nozzle while the towel is over it blocking, and spray or blast back. I wish we would have done this the first time.
- Continue on with other pipe connections or areas to spray into concrete. The foam will harden very quickly.
Notes
Recommended: long sleeve shirt and pants to cover yourself up!
NOTE: The company who makes the foam to fill voids under concrete also provides helpful videos. Click here to watch these videos before filling your concrete void.
Excellent guide on fixing erosion under concrete slabs! The blog provides clear, actionable steps and practical solutions for addressing this common issue.