Extending a concrete slab or concrete patio is a great way to add usable space to your yard. Although adding to a concrete slab is a do-able DIY project, working with concrete is strenuous and hard work. As someone who recently completed a concrete slab extension project, I have compiled all of my tips, tricks, and lessons learned within this article.
To extend a concrete slab, start by planning out the concrete slab expansion by marking the area with spray paint or rope lines. Next, excavate the slab extension area and backfill the excavated area with stone. Assemble concrete forms using lumber, install rebar, and pour concrete in the slab extension area. Finish the concrete slab surface with a bull float and hand trowel and allow for the concrete to dry.
This article will outline every single step of the concrete slab extension process and will also outline all of the lessons learned along the way. If you are more of a visual learner, you can watch my “How to Extend a Concrete Slab” Youtube video which is linked at the end of this article.
1. Plan Out Your Concrete Slab Extension Area
The first step in a successful concrete slab extension project is to plan out your slab extension area. Spend some time thinking about how large your concrete slab extension will be and how it will be shaped. Additionally, think about how the concrete slab extension will compliment your existing slab.
Once you have an idea of the general layout of your concrete slab extension area, it is recommended that you mark the area with spray paint. You also have the option to mark the area with stakes and a string line, but this is usually more difficult then simply marking the area with spray paint.
You don’t need to have the layout perfectly dimensioned at this stage. Final dimensions will be established as part of the concrete form construction process – which is discussed later in this article.
After preliminarily planning out your concrete slab extension area, it’s time to begin the excavation process.
2. Excavate the Concrete Slab Extension Area
After establishing your concrete slab extension area, excavate the area to approximately 8″-10″ below grade. Your excavation depth should allow for approximately 4″ of gravel sub-base and 4″-6″ for your concrete slab.
Continue to excavate your entire slab extension area to a depth of 8″-10″. I completed the excavation by hand using a shovel, but depending on the size of your slab extension, you may want to consider renting mechanical excavation equipment.
After excavating your slab area, it is recommended that you compact the excavated area.
I used a hand tamp to accomplish soil compaction, but I recommend that you rent a mechanical compactor for larger slab extension projects.
3. Backfill the Concrete Slab Extension Area with Stone
Once your slab extension area has been excavated and compacted, it is time to backfill the excavation with approximately 4″ of stone. There are a variety of stone types that can be used used for your concrete slab stone base, but No. 57 stone/gravel is typically the most common. Crusher Run gravel and Recycled Concrete Aggregate (RCA) are also sometimes used as a concrete slab stone base.
You can typically have the stone delivered to your home or job site for a small delivery fee. However, you also have the option to pick it your stone from a local concrete or landscape supply company.
Once you have purchased your concrete slab stone base material, apply it to the excavated area until a 4″ stone bed has been placed over the entire excavation area.
Once all of your concrete slab stone base has been applied to the excavation area, you can proceed with compacting the stone sub base.
4. Compact the Concrete Slab Stone Base
Once the stone base has been installed and you have a consistent 4″ stone bed across the entire concrete slab extension area, compact the stone using a hand tamp or a mechanical compaction device.
For larger slab extension areas, it is highly recommended that you rent a mechanical mixer for the compaction of both your soil and the stone sub base. This will ensure that your concrete slab stone sub base is well compacted and will minimize the amount of settling that occurs beneath the slab.
5. Construct Concrete Forms
At this stage of the concrete slab extension process, you will need to construct your concrete forms. Concrete forms are used as the perimeter of the concrete slab and are used to give the slab shape during the pouring process.
Depending on the thickness of your concrete slab, use appropriate lumber for your concrete forms. For my slab, I used 2″x6″ lumber to form my rectangular slab extension area as shown below:
As you construct your concrete forms, spend some time thinking about drainage. You want to ensure that your concrete slab has a slight slope (away from your home) to facilitate drainage. As shown above, you can use a piece of lumber and a level to check your slope to ensure that the slab will drain water away from your home.
Once you have constructed your concrete forms with lumber and established your slope for drainage, secure the forms in place with stakes.
Securing the forms with stakes will prevent the forms from shifting during the concrete pouring process. Additionally, be sure to screw your concrete form lumber together at any corners.
6. Reinforce the Concrete Slab by Installing Rebar or Steel Mesh
Once your concrete forms have been constructed and positioned with stakes, you will want to install rebar or steel mesh to your slab extension area.
Although concrete has very high compressive strength, it cracks very easily when subjected to tension or torsional forces. As a result, steel rebar or steel mesh is typically added to concrete slabs to increase the slab strength and minimize the potential for large cracks.
For my slab extension, I chose to use steel mesh as reinforcement. I cut the steel mesh to size using bolt cutters and positioned the steel mesh in place within the slab extension area. Keep in mind that you want to have a minimum of 1″ between your reinforcement bar or mesh and the edge of the slab.
Once you have cut your steel reinforcement and positioned it within you slab, you can proceed with the next step in the Concrete Slab Extension process.
7. Tie the Existing Slab Into the Concrete Slab Extension
One of the most critical steps in any concrete slab extension project is anchoring the new slab to the existing slab. If you do not tie the new slab to the existing slab, you can almost guarantee that a crack will form between the two slabs due to differential settlement.
To tie the new concrete slab into the existing slab, it is highly recommended that you connect the slabs together with rebar. To accomplish this, I used a 5/8″ masonry bit (carbide tipped) and drilled approximately 6″-8″ into the existing slab.
In addition to a carbide tipped masonry drill bit, you will need a hammer drill in order to drill into your existing concrete slab.
Once you have drilled approximately 6″-8″ into the existing slab, you can begin preparation for installing your tie-in rebar.
The rebar used to tie-in the existing slab to the new slab should be 1/2″ in diameter (slightly smaller than the hole you drilled in the existing slab) and should be approximately 14″ long.
In order to anchor the rebar into the existing slab, it is recommended that you use a high quality anchoring epoxy.
This epoxy serves as an adhesive and will ensure that the rebar remains embedded within the existing slab.
Inject the anchoring epoxy into the hole that was previously drilled into the existing slab as shown below:
Finally, insert the rebar into the epoxy filled hole and allow the epoxy sufficient time to cure.
After giving the epoxy sufficient time to cure (based on the manufacturer’s instructions), you can begin preparation for your concrete pour.
8. Mix up and Pour Your Concrete into the Slab Extension Area
At this stage, you can begin preparation for your concrete pour. If you have a small concrete slab extension area, you may consider mixing up your concrete by hand. However, if you have a large concrete slab extension area, it is highly recommended that you have your concrete delivered via a concrete truck.
For our project, we rented a concrete mixer and mixed approximately 85, 80 pound bags of concrete. See below:
Concrete Bags 
Rented Concrete Mixer
PRO TIP: Mixing Concrete by Hand is difficult, time consuming, and messy. We mixed approximately 80 bags of concrete for this project and it was a ton of work. Next time, I will have the concrete delivered.
Add water to your concrete mix in accordance with the manufacturer’s recommendations and apply concrete to your slab extension area.
Continue to mix concrete and apply it to the slab extension area until you have added enough concrete to your concrete form area. At a minimum, concrete should be added until it is even with the top of your concrete forms.
Once you have added sufficient concrete to your concrete slab extension area, you can begin the concrete slab finishing process.
9. Finish the Concrete Slab Surface
To finish your extended concrete slab surface, start by “screeding” off any excess concrete using a piece of lumber. To accomplish this, move a piece of flat lumber back and forth along the top of you concrete forms in a sawing motion. This will help to remove any excess concrete and will level out the surface of your concrete slab.
After “screeding” the surface of you concrete slab, it is recommended that you smooth the surface using a bull float.
A bull float can be rented from you local box store for a few bucks and will make the concrete finishing process much easier.
PRO TIP: Concrete slab surface finishing is an art. Achieving a perfect concrete finish is difficult without considerable concrete surface finishing practice. I recommend that you spend some time watching concrete finishing Youtube videos so that you are prepared to take on your concrete finishing job with confidence.
Using a bull float will draw the water to the surface of the concrete and will give you a smooth surface. After the bull floating process, use a hand/finishing trowel to smooth out any rough patches.
Additionally, use an edging trowel to smooth out your concrete slab edges to give them a nice rounded look.
Once you have smoothed out your concrete slab surface with your bull float/finishing trowel and rounded all of the concrete slab edges with an edging trowel, allow your concrete to cure for 2-3 days.
10. Remove Concrete Forms and Perform any Touch Up Work
After allowing your concrete slab 2-3 days to cure, remove the concrete forms by removing any stakes or screws that are holding the forms together. You may need to use a prybar to separate the lumber from the concrete slab.
After removing the concrete forms, inspect the concrete slab extension for any imperfections. If there are any, use a concrete repair mortar to fill in any holes, spalling, or any parts of the concrete that may have been damaged as part of the concrete form removal process.
After repairing any trouble spots on your concrete slab extension, your project is nearly complete.
11. Install Control Joints
The only thing guaranteed about concrete is that it WILL crack. In other words, concrete cracking is usually inevitable. As a result, Control joints are used to decide where your concrete will crack.
Concrete control joints are planned cuts in your slab that create an intentional “weak-point” in the slab. These control joints will then be the first to crack when the concrete slab extension experiences settling.
The general rule of thumb is that control joints should be place no more than 3X the slab thickness (in inches). For example, a 4″ slab should have control joints placed a maximum of 12 feet apart.
Control joints can be added to your concrete slab extension in two ways:
- Create a control joint when the concrete is still wet using a grooving tool
- Create a control joint after the concrete surface has cured using a concrete saw
Both of these options are acceptable and will give you more control over where your concrete will crack. Control joints should be cut to roughly 25% of the slab thickness. For example, a 4″ slab, should have control joints that are approximately 1″ deep.
Since my slab extension was so small, I decided not to include control joints for my concrete slab extension project. Three years later, still no cracks.
At this stage, all steps required for your concrete slab extension project have been completed.
I made a lot of mistakes when tackling my first slab extension project. To avoid making the same mistakes I did, I highly encourage you to watch my Concrete Slab Extension (Lessons Learned) Youtube Video below:
Thanks so much for checking out ATImprovements! If you learned something from this project, you might also like these other DIY Projects:
- How to Install a DIY deck Wire Railing: DIY Cable Railing – How to Install Cable Railing Step-by-Step – (atimprovements.com)
- How to Build a Concrete Paver Patio: DIY PAVER PATIO – Easy DIY Paver Patio Ideas – (atimprovements.com)
Thanks!
DISCLAIMER: This is a reference guide only. Links included in this article might be affiliate links. If you purchase a product or service with the links that I provide I may receive a small commission. There is no additional charge to you! Thank you for supporting ATimprovements so I can continue to provide you with free content each week!