Raising a concrete slab often involves ingenuity and patience. Homeowners with a few tools and know-how can often lift a concrete slab themselves, without expensive machinery or materials.
Today we will discuss the common DIY method for lifting a sunken concrete slab and offer a few tips to perform the project safely and successfully.
Can I Raise a Concrete Slab Myself?
In many situations, yes. Generally speaking, concrete slabs are lifted using jacks placed underneath the slab, gently lifting it. There are other do-it-yourself methods, but most people attempting the project will use the jack method because it is relatively inexpensive and simple to perform. In most cases, the job will only require a few hand tools. However, care should be taken to minimize the risks of injury due to the weight of the slab.
Why Did My Concrete Slab Sink?
More often than not, a sunken concrete slab is the result of erosion of the substrate, which is the material supporting the weight of the concrete. Diverted rainwater or leaking underground pipes are often the cause of this erosion.
If an area underneath a pad becomes unstable, it will often sink under its own weight. Concrete is very heavy, so the substrate supporting it must be able to withstand the weight or the slab will often sink or crack.
When pouring a concrete slab, standard procedure includes installing compacted sand, gravel, or cement as a substrate to ensure the slab has even, consistent support. However, this is not always the case.
In most cases of sinking concrete, the culprit is uncontrolled water getting under the slab and washing away the substrate and the support it provides. When this happens, the concrete endures excessive forces where the substrate no longer provides support and a crack can form, or the slab can sink. Before attempting to lift the slab, it is important to correct the issue that caused the erosion.
A common cause of a sunken concrete slab is often a gutter downspout discharging water in the wrong place. When trying to identify the cause of a sunken slab, the pros usually look for a buried drain pipe first. These are often the black, corrugated pipes often found buried around a home. These pipes were typically installed to direct rainwater from gutter downspouts away from the home’s foundation. However, these pipes commonly become clogged or the pipe pulls away from the downspout, allowing rainwater to collect near the home.
What Is the Best Way to Raise a Concrete Slab Myself?
First, it is important to understand that using the jack method will generally require access to the side of the slab that has sunk. If the low side of the slab cannot be excavated underneath, it’s often best to call the professionals. They have methods and tools not generally available to homeowners that allow the slab to be raised without the use of jacks.
If the area can be accessed, the jack method will usually work. Let’s use a typical sidewalk as an example. In many instances, if the sidewalk can be accessed from the low side, the typical solution is to excavate an area under the sidewalk to allow for the placement of one or more jacks.
You’ll likely need a few digging tools, like a shovel and mattock. This tends to be the most successful method for do-it-yourselfers because it is simple and does not often require expensive tools. However, depending on the size and scope of the project, it might be wise to enlist help, or at least have it available in case you need it.
How to Lift a Concrete Slab From Underneath
Step 1. Excavate the Substrate
The first step is to create a place underneath the slab for the jack(s). This is usually done by hand as the area is often small and shallow. Starting on the low side of the slab, the pros will often locate a midpoint on the slab that is in good condition and free of any cracks. If the slab contains rebar, fiberglass, or both, this step will be easier as those materials will help resist additional cracking.
Most pros use a bottle jack or scissor jack, but the important part of this step is to create enough space for two blocks of wood and the jack. A bottle jack capable of lifting a concrete slab is often less than 12” tall, so an excavated space of 12”-15” under the slab is usually sufficient.
Next, a block of wood is placed between the bottom of the jack and the ground, while the other is placed between the jack and the slab. This prevents the weight of the slab from pushing the jack into the ground while preventing the narrow tip of the jack from causing a crack in the slab.
Step 2. Slowly Raise the Slab
After the jacks are in place, the slab is lifted about ¼” at a time. If more than one jack is necessary, each jack should be raised evenly. The slab should be inspected each time it is lifted to identify any stresses the slab may encounter that could result in a crack.
This is why the process should be done very slowly. Applying too much pressure in one area could cause the slab to break into pieces. This slow raising of the pad continues until the slab has reached the desired height.
Step 3. Inspect the Substrate and Correct As Needed
This step requires diligence and close attention because the supports need to be placed accurately or the problem will likely recur. The first step is to ensure that the substrate under the slab is compacted and firm.
In most cases, the original problem was caused by erosion, so if this is the case, it must be corrected first. The easiest way to confirm that the substrate is firm is to simply dig until undisturbed soil is contacted. If the soil is natural and appears solid, it most likely is.
Step 4. Support the Slab
Many pros and experienced DIYers prefer to use high-density foam blocks for this step. These blocks are specifically designed to support the weight of concrete and will not crush or rot. Using pressure-treated wood blocks is not recommended for this purpose, as they will eventually decompose and the problem will return.
However, stone and masonry products, such as stones, bricks, cement blocks, and other fillers are generally ok, as long as they will support the weight without crushing. The supports should firmly contact the underside of the slab, but not enough to raise it further. Doing so could cause the slab to sit unevenly after it settles.
Step 5. Fill In the Space
This step can be done with sand, gravel, foam, dry cement, or other hard packing material. The important part of this step is to pack the filler as tightly as possible, as this will reduce any settling of the material later.
A technique known as mud jacking is common, which involves drilling holes through the slab and injecting slurry (a thin cement mixture) into the space. However, this method may exceed the skills of a DIYer, so hard-packed clay or small gravel-like chat (a very small, easily compacted stone material) is often used. It’s also a good idea to tamp the material into the space to remove as much air as possible.
Step 6. Return the Slab
The last step is simply to lower the jacks, returning the slab to its intended location. Lowering the slab slowly is important, because as mentioned previously, the slab needs consistent support.
If using more than one jack, it is important to lower them simultaneously, so as not to subject one jack to the entire weight of the pad. If the slab is slightly out of place, sometimes a block of wood can be placed against the slab and gently tapped with a sledge hammer. If the slab is in the correct spot, the jacks can be removed and the fill dirt can be replaced.