How to Pour a Concrete Slab Next to Another

Let’s say you want to extend the width of your driveway or create a patio space in your backyard by pouring an additional concrete slab. If you have any reservations, know that this project is manageable and one you can totally do yourself.

To pour a new concrete slab next to an existing one, you will need a rebar system, a frame in which to pour, and finishing tools. Once your concrete is mixed and your area is prepared, you can pour it next to the existing slab.

Pouring concrete is a generally easy concept, but there is a range of techniques that each professional prefers. If this is a project you’re completing on your own, it’s best to know the extent of your project and pick your techniques accordingly.

It’s safe to say that if you are completing a concrete project on your own, it’s a relatively small and manageable job, making the steps even easier.

Framing And Prepping

One of the first steps in pouring concrete is to dig out the depth that you’ll need. Regardless if there is an existing slab of concrete or not, you’ll want your new slab to be level with the rest of the ground in the area. The height of your concrete will depend on where it’s located around your home, but you can always measure the slab if you have doubts.

In your planning, you might have noticed some features that could impede on your concrete, such as protruding pipes or sunken fence posts. If you did not notice these things, don’t worry! The digging step is the perfect time to take care of it.

Pipes or roots could crack your concrete, so you’ll want to solve those issues before you pour your new slab.


A frame is crucial in a concrete project because it acts as a mold for your mix. And because you are pouring the new slab right next to another slab of concrete, your frame will only need to enclose the three other sides of your allotted space.

The frame can be a simple set of 2x4s that are nailed together. Unless you are planning on keeping a small retaining wall for the other side of the slab, you will remove the frame once the slab is dry. Because of this, it does not have to be pretty or expertly done. It just needs to be secure enough to hold all the concrete in while it’s being poured.

You will not need to prime the wood at all. The concrete is not likely to adhere to it, meaning your frame will easily separate from your slab.


Short for “reinforcing bar,” rebar is the metal structure inside of concrete. It acts to strengthen the concrete, supporting it from the inside as to evenly distribute stress and prevent cracks.

The technique you use for your rebar will depend on the size of your project. With a larger concrete slab, you might want to drill holes in the existing slab for your rebar to slide into. This increases the strength of the new slab and helps keep the two slabs aesthetically and physically coherent.

On a smaller job, you can choose either free-floating rebar or wire mesh. If you opt for free-floating rebar, meaning it is not attached to the other slab of concrete, place it in the middle of your frame, extending toward the edges almost entirely. Arrange it evenly so weight is appropriately distributed. If you’d like to increase the stability and security of your rebar, you can also lash the individual rods together with wire ties.

The size of your new slab will affect the amount and frequency of your rebar.

You can also use wire mesh. It acts in the same way as rebar, strengthening and supporting the concrete, but it is not as strong as rebar. They are both made of steel and comparable in price, but rebar is the better option for a larger job. Wire mesh will act sufficiently within the few square feet of your backyard.

Whether you decide on rebar or mesh wire, it is important to layer it properly with your concrete. If you pour straight onto the mesh or free-floating, it will lie at the bottom of your concrete. This will defeat the purpose of using reinforcements because it will do nothing to support the concrete.

To avoid this, pour one layer of concrete across the bottom of your space, then lay the rebar or mesh, and then finish with another layer. Or, if you have an extra set of hands, use them to lift the rebar while you pour the mix. With these techniques, your reinforcement will sit in the middle of your slab, effectively supporting your concrete.


Water is the primary activator in a concrete mixture. Generally speaking, that is the only other ingredient, with the exception of the concrete mix itself.

The amount of water you have will change the composition and result of your slab of concrete. As a typical rule of thumb, you want your concrete to be wet, but not too wet.

A lower water ratio means less shrinkage and, in the end, better concrete. A drier mix is also ideal if you live in a humid area, or a region that receives a lot of rain. The drying process is also less intense when the concrete is not overly wet.


If the job is big enough or in a semi-inaccessible area, you can use a mixer and pump to mix and pour your concrete.

But if your area is small enough and accessible, you can use a wheelbarrow and a shovel to mix and pour concrete into your slab. This is more labor-intensive, but can likewise be a rewarding process.

While you are pouring, you will want to be patting down the mixture. To do this, you can use a simple trowel. The shovel you used to mix is also perfectly reasonable. If you are doing a basic smoothing job as you go while your concrete is wet, you will get a smoother, shinier result.


Not only is the smoothing step crucial to pouring a slab of concrete, but it is also a very satisfying one.

One key goal in smoothing concrete is to eliminate air pockets within the slab and help release the moisture. Not only does this impact your drying process, but it also impacts the integrity and strength of your slab.

Another goal of smoothing concrete is to finish the edges and surface of your concrete. The tools you use will change the finish and final result of your slab.

Scoring is one of the most important features of concrete. A slab that is too big is more susceptible to cracking and succumbing to stress. To avoid this, score your slab at least once, but once again, where and how you score will depend on the size of the slab.

For a slab that is immediately next to another, line it up with the scores in the existing slab. This will give you both a uniform appearance in your concrete and the right frame of reference for how many times you should score your slab.

In addition to scoring your slab, you’ll want to clean the edges of the concrete for that finished, coherent effect.


For just about any concrete project, you’ll need a float, an edger, a joiner, and a screed. Different jobs might require different versions of these tools, but they are the baseline for finishing a freshly poured slab of concrete.

  • Float: this compacts the concrete and compresses the moisture so it rises to the top and dries faster. It helps create a finished look on your concrete.
  • Edger: use an edging tool to create the clean, defined edges around your concrete job. It can also create seams around the edges of the slab so it looks almost like it has a frame.
  • Joiner: a joiner is a tool that you’ll use to score the slab. It creates the lines that help prevent cracking. Make sure to line up your joiner with the lines in the existing slab.
  • Screed: a screed is simply any straight-edge that smoothens the surface of your concrete. This can be an extra piece of 2×4 that spans the width of your new slab.

In addition to a screed, it is recommended to include a level in the process of pouring your concrete. A level will help ensure you get the best possible result from your new slab, while also making sure your new slab is at an equal height with your existing slab.

Screeds are used throughout the pouring process, whereas floats, edgers, and joiners are used in the final stages of the project.

Depending on the needs of your concrete, you can also use brushes on the surface of the slab. They add additional texture to the concrete, which serves both as a stylistic and practical choice. If you are pouring concrete to put under a hot tub or in a place that might be slick with water, the extra texture will be beneficial.

Each of these tools is available at any hardware store, but if you find you need specific tools or guidance on which techniques to use, a local concrete professional will be able to lend their advice and expertise.

Ricky Kesler

With all of the projects I've done over the years, you'd think that I work on my house full-time. But I actually enjoy other things like spending time outdoors and time with my family.

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