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One of the more common questions about garage floors is how to fill contraction and expansion joints. In particular, many want to know if you can fill the joints in the garage floor before applying an epoxy coating. Others want to know if you can fill the joints before or after a sealer is applied. What about sealing the expansion joint between the garage floor and the driveway?
We will answer all these questions and more, as well as discuss how to fill expansion and contraction joints in your garage floor. But first, you need to understand the difference between these two joints because it can determine which type of joint filling materials you use and why.
Difference between expansion and contraction joints
In a typical 2-car garage, contraction joints usually look like a big plus sign that divides your garage floor into what appears to be four separate slabs. These joints (sometimes called control joints) are the deep V shaped grooves that run from one end of your garage floor to the other. Some concrete floors may have saw cuts as a contraction joint instead. Saw cuts are usually a minimum of 3/16” wide and 1” deep.
Because concrete slabs will crack with expansion and contraction, these joints create a weakened line in the concrete that encourage cracks to follow the line within the joint. This helps to discourage cracks from following a more resistant line across the surface of your garage floor.
Expansion joints (sometimes called isolation joints) are joints that separate one slab of concrete from another and can be at a minimum of 1/2” in width.
They are commonly filled with a compressible fiber board material. The joint between your garage floor and driveway is a good example of this.
Many times the garage floor is poured independent of the home foundation. When this is done, there is an expansion joint that will run at the perimeter of where the garage floor meets the house foundation.
In simple terms, expansion joints help prevent adjoining slabs from damaging each other when they expand and contract.
Filling contraction joints before epoxy coatings
When the time comes to paint or epoxy coat the garage floor, many people want to fill their contraction joints in order to have a seamless looking floor. So, is this advisable to do? Well the answer depends on what material you use.
When your garage floor expands and contracts with temperature changes, it can create movement (very slight) within the contraction joint. This is why there are cracks within the joint in the first place. If the joint is filled with a solid material that does not flex and then is covered with epoxy or paint, you have just created the potential for the crack to telegraph up through the coating.
The reason for this is because you have a solid seamless coating which does not flex (epoxy) that is bonded to both sides of a jointed surface (concrete) that can move. As a result, you can create a break in the surface of the epoxy right above these joints if they are filled.
Though the chance of this happening is not great, many professional contractors will not warranty an epoxy coating for cracks where the owner has asked to have the joints filled. Most contractors will fill the joints if asked, but they will warn you about the possible problems.
If you want to fill your contraction joints, the key is to use the proper joint filler.
The best material to use is a 100% solids epoxy filler or polyurea filler with an elongation rate (flex %) much higher than that of the coating you are using. It will cure to a hard solid that is sandable and will flex slightly (relative to the concrete) underneath your epoxy coating if the joint moves in order to prevent cracking.
This flexibility also prevents the filler from pulling away from the concrete as the floor expands and contracts. These qualities help tremendously to keep the paint or epoxy coating in one piece and not crack.
Contraction joints filled with Legacy 2-Part epoxy gel and ground flush with floor
product recommendations
Most of these special joint filling compounds can be found online from your favorite concrete floor coating vendors. They are not cheap and home improvement centers do not carry them.
There a few different products we can recommend. The first is a 2-Part 100% solids epoxy gel by Legacy Industrial. You mix the two components together and then fill in the joints. They also have a 2-part polyurea crack filler that works extremely well. It works great as a crack filler also. We did an article on it here.
Another great source is Garage Flooring LLC. They have a few different types of joint fillers depending on what the job is. You can find them here. We recommend that you talk with them first to learn which is best for your project.
ElastiPoxy is yet another product that works very well for filling joints. You can find it here at Amazon
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Application of these joint fillers is not hard and requires little preparation of the joint other than making sure it is clean.
If you have saw cut contraction joints, we recommend that you fill them first with a foam backer rod or silica sand so you don’t use as much filler.
You want to leave approximately 3/8″-1/2″ from the surface. If you have “V” shaped contraction joints, the foam backer rod will not work well. Silica sand is a better choice. Both the silica sand and foam backer rod can be purchased from your local home improvement center.
When using foam backer rod, make sure it’s approximately 1/8” wider than the joint. If your joints have cracks in the bottom of them, the backer rod or silica sand will also prevent the filler from slowly sinking into the cracks and creating low spots as it cures.
Once the joint is prepped, follow the instructions of your product of choice and fill the joints up. Make sure you don’t have any low spots or you will have to go over them again. It helps to use a putty knife to force out air pockets and to remove excess material.
Once the joint filler has cured, you will need to grind the joint flush to create a seamless surface to paint or epoxy over.
The grinding of the joint is important because some garage floors with the deep V groove can be lifted at the edges of the joint. This happens when the surface of the slab cures and contracts quicker than the rest of the concrete and pulls up the edges. If you apply the filler but don’t grind it smooth, your joints will telegraph right through to the surface of your coating and become visible.
Contrary to what some people recommend, do not use latex caulk or water based crack fillers.
Latex caulk and water based crack fillers are too soft and will slowly shrink. As a result, you will end up with slight depressions that outline where your contraction joints are. Additionally, the coating will crack if you have rolling toolboxes, jacks, creepers, or other heavy objects that move across the joint. This happens because these fillers are too soft.
Filling contraction and expansion joints after epoxy coatings or sealers
Sometimes the contraction joints in a garage floor and the expansion joints around the perimeter of the floor can be very wide and collect a lot of dirt and debris. This can create an eyesore for some people plus make it more difficult to clean them out. The same can be said for the expansion joint between the driveway and garage floor.
You can always fill these joints in your garage floor after an epoxy coating is applied. It can also be done after clear sealers, stains, or other floor treatments are applied as well. Not before.
The easiest solution is to fill these joints with a self-leveling polyurethane joint sealer/filler. These are a 100% solids polyurethane which means that it will not shrink as it cures. They are gray in color and applied with a calking gun.
These type of joint fillers are inexpensive and can last for years. They also do an excellent job at sealing the joint from moisture. The material is flexible and somewhat spongy feeling after it cures, yet strong enough to drive vehicles over. Most are not paintable.
We personally like SikaFlex SL for these type of projects. You can find it at your local home improvement centers for less than $10 for a 10 oz. tube. The best deal is the 29 oz. tube, but you will need to purchase the larger caulking gun to use it. If a home improvement center isn’t nearby, you can also find SikaFlex here from Amazon.
This is a good example of how to fill and expansion joint when using foam backer rod
Prep of the joint is the same as before, however, do not use sand to fill up an expansion joint. Expansion joints need the material withing the joint to be compressible and sand is not. It can be used though if you are just applying a thin layer over old fiber board to prevent sealant from leaking through.
It is also extremely important that the concrete is bone dry before application or it will not adhere well. For expansion joints it’s important to remove any old filler using a utility knife or grinding wheel. If the joint is filled with fiber board or felt, be sure to remove any of the material in the joint that is protruding past the surface of the concrete.
If the fiber board in an expansion joint is missing or deteriorated, you will want to clean out the joint and use backer rod within the joint before you apply the joint sealer. Do not place it more than than 1/2″ deep.
Because these joint fillers are self-leveling, the viscosity is similar to honey. As a result, they will seek out any low spots along the joint. So be prepared ahead of time for any areas where the sealer/filler may want to run out of the joint.
Once applied, it skins over in an hour or two and completely cures within 3 to 5 days. You need to wait at least 24 hours before driving over a freshly filled joint.
Final points
Sealing and filling your contraction and expansion joints for a garage floor is not difficult. The key is to use the proper sealer or filler depending on the intended purpose and what type of joint it is.
Just remember that expansion joints should always be sealed and filled with a flexible joint sealer and never be epoxied or coated over. Contraction joints can be filled in the same manner after a coating or sealer is applied.
Nevertheless, if you want a seamless looking epoxy coating, contraction joints must be filled with a specialized epoxy or polyurea joint filler that cures hard, but has a high elongation rate for flexibility in order to prevent the garage floor coating from cracking at the joints.
I recently had my floor done and the joints were not filled. There are now large chunks of paint/epoxy still in the cracks. Any suggestions on how to get this fixed? It doesn’t look good …
Hi KP. A typical installation allows for the epoxy and optional color flakes and clear coating to flow over the edges of the joint and into the bottom of it. How neatly this is done is dependent upon the actual joint and the installer. If you don’t like the way it looks, you can always fill the joint with a self-leveling polyurethane joint sealer such as Sikaflex SL.
I’m trying to level a garage floor with expansion joints and control joints in it. If I go through this process of filling the joints with a sealant, can I then pour a self leveling compound over the whole floor? Meaning these joints would then be covered up. Or do I need to somehow continue those joints up through the leveling compound? Thank you for any insight!
Hi Will. You need to honor the joints when using a self-leveling polymer-modified concrete. If you cover them up with it, cracks will form over the joints at some point. If applying a coating afterwards, the joints can be filled with the correct epoxy or polyurea fillers that provides some elongation as we describe in the article. Self-leveling concrete does not provide any elongation or flex.
Do you etch the concrete first then apply the sealant to a contracting joint or apply sealant then etch ready for paint
Hi Callum. Do the etching first.
Hello Shea. I want to fill the expansion joint on my driveway with Sikaflex, the part that touches the street with the bottom piece on a slight angle. Two questions. 1) Some parts of the bottom slanted slab that touch the expansion joint are broken. How do you prevent the Sikaflex from dripping down from overflowing and dripping down from these spots? 2) There are other perpendicular joints that connect to the joint I want to fill, also on a downward angle. How do I prevent the Sika from getting in there? I have pics but don’t see an option to share.
Hello Deniz. For any applications that do involve a horizontal surface (slant), we recommend using SikaFlex 1A. It does not self-level and will require that you trowel or tool the filler into the joint. It will not sag or run. We suggest using tape on the edges to create a clean line and to avoid smearing it on the surface.
Thanks for the reply. I can’t imagine doing this job and tooling it myself… it’s such a long strip. I do have a bag of polymeric sand on hand. Would it cause any harm to fill the joint with backer rods and then just sand it to the top? This approach will save lots of time.
The sand will just wash out, Deniz.
Yeah I can imagine play sand or silica washing out, but you think poly sand will wash out as well? It hardens with water and the one I have is said to hold strong for up to 10 years. They say it expands and contracts without breaking its form. Also, does it make a difference if I fill that joint with anything in the first place? Will there be any increased damage to the concrete with that joint empty? This used to be filled with some black stuff that came apart over time. It already cracked a bit, and that was with that black stuff in it.
Hi Deniz. I read that too quick and assumed you were speaking of silica sand. Polymeric sand will work, but not for as long as you may think due to the location. The tires from vehicle traffic in conjunction with the sloped angle is going to cause the sand to wear due to repeated impacts. It’s essentially acting as a ramp. It’s one thing for it to be a flat surface, but it’s another when you are asking it to absorb repeated impact from tires. Proper sealants have the right amount of flex to stand up to such impacts. Joints such as that are filled for a few reasons. The first and primary is to prevent soil erosion in the joints which could possibly lead to bigger problems if left unchecked. Aesthetics and ease of cleaning is popular reason as well. The third is to protect the concrete from spalling due to freeze thaw (if your winters are cold enough). It prevents water from seeping into the sides of the concretes so that it can’t freeze and cause chunks to pop of the edges or crack.
I’m hoping to epoxy the floor and the stem wall. Is the gap/crack between the concrete pad and the concrete stem wall an expansion joint, contraction joint, or neither? Can/should I fill it with specialty epoxy before doing the floors? I’m hoping to have a seamless/clean transition from floor to wall. Thanks!
Hi Brett. The joint between the stem walls and the slab is an expansion joint. The garage slab is always placed separate from the house slab. The recommended method is to coat your floor and stem walls first. After it has cured, then you can go back and fill the expansion joints with a self-leveling polyurethane sealant. SikaFlex is an example. There are companies that offer a variety of colors as well if you want something that would be close to matching the color of the coating.
I am wanting to paint the concrete floor of a small guest room in my basement and cover most of it with a decorative area rug. I am thinking I might be able to get away with not filling in the joints at all. Does this sound advisable?
Hi Ellen. Filling in contraction joints is for aesthetics only. You don’t have to fill the joints if you don’t want to.