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.
Hi. Thank you for this great article. I do have a quick question. Any product available at home depot or lowes that can be used for “V” shaped contraction joints in my 2 garage car before I apply Rust-Oleum RockSolid Polycuramine? I would like to work on this project next weekend instead of waiting weeks for shipment on the products you recommended.
Also “V” shaped contractions have a crack right in the center. Will the product you mention take care of that as well when applied?
Hi Daniel. If you can find PC-Xtreme at Home Depot, then that will work. It has the right elongation numbers to work for a contraction joint. If you have a crack at the bottom of joint, then the joint is doing exactly as it should – encourage cracks in the joint and not across the slab. The joint filler will cover the crack without issue.
Is it safe to use degreaser cleaner or etch after PC-Xtreme has cured? Thank You!
Yes, it is Daniel. Degreasers and acid etch solutions have no impact. Make sure you grind the joints flush after application though. If you don’t, the filler will telegraph right through the coating. If you don’t grind it, you need to rough up the surface of the joint real good with 120 grit sandpaper or the coating will not stick to it.
Hi, I filled half of the the contraction joint with concrete (joint already had a crack) and now plan to level it up with DAP Flex concrete. Will this cause major issues in future? It is my Garage floor. I plan to paint the floor when it is all done.
Hi Andy. Contraction joints are supposed to have cracks in them. That is their purpose. They encourage cracking in the joint instead of across the floor. Did you use actual cement in the joint? If so, it will delaminate at some point and come loose. Regular cement does not adhere well and makes a poor repair product. If it was a polymer-modified cement patching compound, then that is OK.
Most of the DAP Flex products are an acrylic latex of some type. The problem with this is that latex shrinks over time. If your goal is to prevent water intrusion, then it is OK to use. However, if you are looking for a structural repair or something that can be ground smooth to provide a seamless surface, then it will not work. Paint can be applied to DAP, but not a true concrete coating such as epoxy, polyurea, and etc.
I have aggregate slab flooring with a 2×4 trench separating the slabs. We have dug out all the rotted wood separators and need to fill the trenches. What do you recommend?
What are you planning to do with the floor, Niki?
It is a floor under a screen porch.
We are going to lay down some carpet over ir
OK, if you are just going to install carpet over it then you can use a self-leveling floor underlayment. Your local home improvement stores carry it. There are many videos available on how it works. It’s actually very easy. You mix up the product and pour it into the joint. Just make sure that the ends of the joint are properly dammed up so the material does not flow out.
Hi. Should I use the same product to fill in cracks and the expansion joint? Or a separate product? I plan on grinding after the repairs? Thank you, Shawn
Hi Shawn. Do you want to fill expansion joints or contraction joints? There is a difference in the type of material to use. Also, are your intentions to apply a coating or something else?
I’m sealing the concrete around my pool and it has several expansion joints. Should I tape off the joints before applying the Concrete sealant. This seems to be what your Saying but I want to be sure before my “do it yourselfer” endeavor. Thanks for your advice.
Hi Sandra. If the joints have filler in them then we recommend taping them off so that you don’t take the chance of the sealer discoloring the filler. However, if you need to refill those joints after application of the sealer, then there is no need to tape them off.
The expansion joints are in good shape so I’ll do tape off before I begin. Thank you for sharing your expertise v
Great article. Plan on filling the contraction joints, grinding, then epoxy sealer.
Questions: Do you recommend using a small amount of sand at the bottom of the contraction/expansion joints to minimize bleed through of filler at the bottom of the joint?
What product do you recommend to complete a nice coving at the right angles of the garage floor?
Hi Greg. Yes, a little bit of sand at the bottom of the joint is OK to use. It will also help to reduce the amount of material used. For the perimeter expansion joints around the slab, we recommend using a self-leveling polyurethane sealer such as SikaFlex or similar. You want to apply that after the coating is applied and cured.
Is it ok to leave the contraction joints unfilled and then use the polyurea coating right over it? Im assuming i’ll need to use a paint brush to get into the joints, but is there any other tips when leaving the contraction joints unfilled?
Hello Ryan. You have the right idea. It’s actually more common not to fill them. Run the roller just over the joints and use a chip brush to run the excess across the bottom of the joints. You will get the hang of it real quick. As long as they get coated like the rest of the floor, they will be fine.
Hello, it says in the Rock Solid instructions not to go over the joints. Do they just mean not to fill them flush or with the excess product? If you use a chip brush to remove the excess, cracking shouldn’t be an issue, correct? Thanks for your time and great advice.
Hi Andrew. What they mean is that you can’t fill the joints full/flush with RockSolid. You would need to fill the joints with the proper filler first if you wanted to create a seamless look. You can run RockSolid down into the joints though and it’s recommended. Using a chip brush works great. If they are the troweled “V” shaped joints, random color flakes will fall in them as well. The cracks that typically run down the center of contraction joints are not an issue.
Very informative article, thanks for the share. I plan on filling my expansion joint at the entrance of my driveway, I’ve got my backer rods on hand, along w/ the polyur caulk. I’ve seen a few videos of people applying sand over the caulking while it’s wet and sticky to hide any imperfections and to leave a more attractive finish. If only applied on top for cosmetics, will this prevent the expansion of the slabs? What type of sand should be used for this: fine sand, joint sand, or a playground sand? Also, should you caulk the joint all the way to the top or leave some space?
Hi Deniz. No, the sand sprinkled on top will not affect the expansion or contraction of the joints. You should use clean silica sand or play sand. You always want to fill the joint to the top if your goal is to prevent it from being a dirt trap.
I appreciate the quick feedback. Ok, so I’ll use the play sand as it comes a lot cheaper than silica. Should I wait to apply the sand or do it immediately after caulking? Not sure how long the caulk will be wet and sticky. Also, how should it be applied? Should I pour on a nice layer and have it sit there for a few hours before blowing away the extras?
I also have a question about the v joints on my driveway. They have some cracks in them as they should. I’m noticing ants traveling back and forth in them and I’d like to seal those cracks. Will just a thin layer of a polyurethane caulk do the trick on this one? I wouldn’t want to fill all the way to the top. And what are your thoughts on Trim-a-slab for filling joints? I’ve heard mixed reviews.
As soon as you fill the joint, go back and apply the sand before the surface of the sealer can skin over. Let the sand sit on for a day before sweeping it off and don’t drive over it for 48 hours. I wouldn’t worry about ants. A sealer won’t stop them and they will still use the bottom of the joint or some other path to travel. Not the biggest fan of Trim-A-Slab, but it can work fairly well for some expansion joints. It won’t well for troweled “V” joints though. That type of joint is a contraction joint and not and expansion joint which separates two slabs or concrete structures.
What product would be used to fill the cracks but will stand up to welding slag?
Hi Richard. There isn’t a product that will not get burned if hot slag settles on it. It won’t affect the integrity of the material, just the cosmetics.
I have a local builder concreting me a very large and very smooth floor fo my garage in 2 weeks time. I don’t think he is planning on have any expansion joints as this will make getting such a large slab really smooth(with a helicopter) really difficult.
Could we cut expansion joints in the next day and fill them after a couple of weeks or so with an appropriate rubber fill? The floor will be 14m by 8m (45’11” x 26’2″) and another with underfloor heating pipes done the same day will be 8m by 8m ( 26’x26′). Both will have steel mesh reinforcement and be 15cm (6″) thick.
Hi Steven. Ideally you want to get the cuts done as soon as you can get the saw cutter on the slab. Many times the next day is too late and you will already have cracks starting to develop. Once the cuts are done, you can fill them with the appropriate material after the slab has cured.
Would a cut of 1/4″ wide be sufficient? How often should I put cuts?
Hello Steven. Saw cuts should be 1/8″ wide and 25% the depth of the slab. Joint spacing can range from 10-18 feet. Cuts are typically started at the center of column lines, but not always. This is when it is important to get with your concrete contractor to discuss beforehand. Inside corners typically have cuts that run to the corners as these areas are more prone to surface cracking during the cure. This link here has excellent information on new slabs. It’s from a professional with decades of concrete contracting experience. Scroll down to the section on “Crack Control” and you can get some additional information.
Fab, thanks mate,
I just want to fill the joint that goes under the garage door to keep bugs and water out what should I use for that? Would a Self-Leveling Concrete Sealant work, it would only be about 6-8 inches long
Hello Jay. Are you referring to the expansion joint that is between the garage slab and driveway? If so, you can use a self-leveling polyurethane sealant such as SikaFlex to fill the joint.
just the part of the joint in two areas that come from the garage floor goes under the garage door out to driveway. It leaves a small dap under the door, have seen bugs and water come under it at times
Those sound like contraction joints, Jay. You can use the same product for those. You should be able to easily find it at your local home improvement center or hardware store.
Thanks so much
Best article I have read So far and answered all my Questions. Thank You! I spent hours until i found you.
We are glad you found us as well, James. Good luck!
Great site Shea, what a wealth of information. Thank you!
I am in the process of prepping my unsealed 6 year old triple garage. Will be applying 4 x 21/2 car Rocksolid Polycuramine gray kits and then Rocksolid clear coat. I have degreased, washed, filled a few hairline cracks and ground (diamabrush) the floor, applied Drylok to the stemp wall. I am currently filling the contraction (+) joints with PC Xtreme joint filler that I filled with a touch of silica sand prior. Unfortunately, I used up my 4 -9oz tubes and have 2 more on order to finish up (lesson learned). I have noticed that the filler in a few spots sagged ‘slightly below the concrete level line and I am wondering if I can re-fill more on top of the pc xtreme product on a second pass or should I leave it? I still have to lightly sand with 120grit before I apply the rocksolid over top of it all. After all this is done, I will be filling the Expantion joints with Sikaflex sl.
Hi John. Yes, you can apply more PC Xtreme if it’s been less than 24 hours since applied. If it’s been longer than that, just sand the top with some 120 grit sandpaper first. It’s not uncommon for settling as the filler finds nooks, crannies, and small cracks to fill in. Sometimes it take longer for the sand to absorb it. Just keep in mind that if you don’t grind those joints flush with the concrete, their location will telegraph through the coating because the surface is not flush like the rest of the floor.
It sounds like you have enough RockSolid for two color coats. This is good because the first coat is going to get absorbed by the concrete due to the grinding and it will not look good. It will act like the primer coat. The second coat will bring nice even color and gloss. I suggest you read this tip about ensuring even color when using more than one kit for one coat. It won’t be necessary for the first coat, but we recommend it for the second.
Thanks Shea! I have to wait for the last 2 tubes to arrive on Friday so I will sand it down and do the 2nd coat all at once at the end. I used a good 4″ scraping knife to trim excess but maybe I will sand w/120grit instead to a smooth flush to concrete. Then do a good sweep/rinse again, let dry for 2 days and then epoxy the 3 coats over a couple days.
Yes, I planned on mixing part A & B’s for the 2nd flake coat.
Is it okay if I plan to use 2 kits Rocksolid clear coat ( over total 900 sq ft) and is it okay to add 1 more kit spread over entire floor for a 2nd
and final skim clear coat?
The two kits of clear for one coat is fine, John. However, you can’t do 1 kit for that many square feet. Coatings should never be stretched. It’s imperative to stay withing the coverage rates.