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 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
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.
We have a 2×4 expansion joint in the middle of a garage with a finished game room under it (the house is on a hill). When snowy cars are parked in the garage the melted snow leaks through the cracks into the ceiling of the room with obvious damage to the sheetrock. Right now it has silicon on it from our failed attempt to waterproof it last winter. Any suggestions how to waterproof it? Should the other small cracks in the cement floor be patched as well?
Hello Sandy. Silicone is not a good sealant for concrete. Also, the water can potentially be coming from a variety of places including the cracks. We recommend removing it and fill the joint with a polyurethane self-leveling sealant such as SikaFlex. After that you will want to repair the other cracks in the slab and then seal the concrete with a penetrating sealer. Siliconate penetrating sealer would be a good choice for this.
Thanks! The silicone is primarily on the wood so once it is removed would the polyurethane work on the wood expansion joint as well? How should we prep the concrete for the siliconate sealer?
It’s important that the wood in the joint be at least a 1/4″ lower than the floor surface Sandy. The SikaFlex (or equivalent) sealant needs to cover both the wood and uppers sides of the joint in order to seal it properly. In terms of the concrete, it just needs to be cleaned real well before application of a siliconate sealer.
Question: Grind/Etch before after filling joints with ElastiProxy before Painting floor.
If I am going to fill my expansion joints with ElastiProxy joint Sealant after I have prepared the surface with an etch and de-greaser do I need this if I am going to grind afterwards anyway? Seems to make more sense to fill the joints and cracks and grind the entire surface and then paint the floor
Seems odd t etch and de-grease first if i have to grind the joints anyway..
Any help would be much appreciated.
Assumption – the ElastiPoxy joint sealer will work well with Colorado and cold weather.
It’s always easiest to do all your crack repairs and joint filling first before anything else Darren. The ElastiPoxy will work well for cold weather. However, remember that many joint fillers as well as epoxy coatings have minimum temperatures in which they can first be applied. Make sure you are not below those temps before you start your project. Once the cracks and joints are filled, you can then grind the joints and crack repairs flush. After that you can etch the concrete or continue grinding the rest of the floor. If you grind the entire floor you do not need to etch.
Hi. I recently sealed a crack with a self leveling type of material. It is soft to the touch. How do I remove that before I seal it properly in prep for a new epoxy coating?
Hello Billy. The easiest way to remove polyurethane self-leveling sealant is with a crack chasing blade. Once you do that, fill the crack with the appropriate crack repair product before applying an epoxy coating. This one from Amazon is inexpensive and works very well. Home improvement centers charge $45 or more if they carry them.
Interested in finding a “metallic looking” joint sealeant for use on a high traffic (pedestrian only) building.
Could you suggest a source?
Hello Jose. We are not aware of anyone who offers such a product. You may be able to create your own, however, by purchasing metallic pigment separately to apply to the right sealant. Torginol for example, is one of many companies that sells metallic pigments. The issue will be trying to finding the right sealant. It needs to be 100% epoxy, it needs to be clear, and depending on the joints, it will need to have a high elongation rate (flexible). Epoxy in general is not flexible and most epoxy joint sealants that are flexible have a more cloudy look. We suggest contacting Torginol an tell them what you are trying to accomplish. If you search “metallic epoxy pigments” you will find a variety of other vendors you can contact as well to see if you can find a solution.
Legacy industrial has a product called xtreme set 100, that is advertised for filling cracks and joints. They say it stays flexible and is very quick to cure – 20 min, however not much tech data is available to make a comparison with the gel crack filler that you mentioned in your very informative article. Do you have any experience with this product, and if so would you recommend it for filling contraction joints before epoxy? Or is the gel crack filler better? Thank you.
Here is the product I’m referring to:
http://www.legacyindustrial.net/products/polyaspartic-polyurea-coatings/xtreme-set-100-crack-joint-filler.html
Hello Levi. The Extreme Set 100 is flexible (relatively speaking) with an elongation rate of about half that of the gel crack filler. Legacy Industrial uses it to fill contraction joints with their commercial operations (garage floor coatings) since it cures so quickly. If time is not an issue, then the gel crack filler would be better. If you don’t have much movement in your slab or very few cracks within the bottom of the joint, then the Extreme Set would work fine. Just know that regardless what product you use, if you get extreme movement the cracks will still telegraph through the coating causing it to crack.
FYI, we have had a chance to work with the Extreme Set 100 and will be doing an article on it soon. It is really easy to work with and does cure fast which means you can easily do all crack repairs and grinding in the same day.
I have a question regarding the edge where my concrete floor meets the wood frame of my pole barn. Is there a product to seal that joint or seam to stop water from seeping in the garage through that seam?
Hello Tommy. MasterSeal NP-1 is a great product for that. We’ve used it before and it bonds extremely well to both concrete and wood. You can get more info on it here.
I have applied Rock solid on my garage floor. I brushed it into the expansion joints. Is it necessary to rough the surface of the joint before I fill with Sica joint filler?
If you are referring to the SikaFlex joint filler, Joe, you will be fine without sanding the inside of the joint.
Hello,
I am planning to do epoxy coating on my 3 door garage floor and how to prevent the paint from flowing into to the contraction or expansion joints? I prefer to seal the contraction joints with self leveling sealer after epoxy coating as you have mentioned in your writeup. There are cracks in the contraction joints and not sure if there would be any issue if epoxy flows into it.
Thanks,
Suresh
There is no issue if the epoxy flows into the joints Suresh. Just have a paint brush with you to push out any heavy amounts of epoxy that may flow into the joints.
Hello Shea,
Thanks for the very informative website. About two months ago, we got a new one-car garage built with a new concrete pad and driveway. In the garage, there are cracks that follow the entire contraction joint, which is expected as I understand it. The average width of the cracks is about a width of a quarter. I’ll take care of them as you have outlined.
I’m more concerned about the cracks in the driveway along the contraction joint (also about the width of a quarter). We have harsh winters and I’m afraid that water will enter the cracks, freeze, and do more damage. How would you recommend taking care of that?
One last question: do you think this is something that our contractor should be responsible for? We still have not paid them the final installment and they are still finishing up some details on the garage. It seems to me that they should take care of it but I could be wrong.
Thank you for the advice!
Cheers,
Eric
Hi Eric. Concrete cracks – period. The question is not if it will crack, but where it will crack. If you are just getting the cracking along the control joints of your driveway that is perfectly normal and expected. It an excellent sign the contractor did a good job. If you had multiple cracks forming across the surface, then that is the time to talk to your contractor about the quality of their work. Hopefully rebar was used. The rebar ties in the control joints and prevents individual sections of the driveway from shifting up or down next to each other.
You have a couple of choices for sealing the joints to prevent water intrusion. The first would be to fill the control joints with self-leveling polyurethane sealant such as SikaFlex. It’s the least expensive method, but some people don’t care for the look. Another option would be to use a 2-part epoxy or polyurea sealer that allows for some flexing. These are much thinner and can be used to fill the actual crack and not the joint. This makes for an almost invisible crack seal that blends right in. The key to using these is to fill just the crack (not the joint) with clean sand and then soak the crack with sealer. We just did a review of Xtreme Set 100 by Legacy Industrial which would work real well for this purpose.
After you make your repairs, we would highly suggest that you seal the entire driveway as well. This will help to prevent any freeze thaw damage and spalling that can occur from harsh winter climates. A siliconate sealer like this is and excellent choice and inexpensive. Choose one for rough or broom finished concrete, not smooth concrete.
A painter suggested that i could use the NP1 elastomatic polyureathane filler to fill the expansion gaps an expoxy over it. Would you please let me know if that will work?
No, you can’t use NP-1 for that Premnath. It’s too flexible and is not recommended by the manufacture to paint over or apply a coating over as well. You can find that information in the technical data sheets for Np-1. You do not want to use flexible sealant for filling joints under a coating anyways. The reason is that weight applied over the joint will cause the coating to crack as the sealant flexes.
Thank you.. So it better to expoxy first and fill the contract joint withe NP1 or sikaflex or vulkem? also i have cracks along the garage floor where the floor meets the wall.. do i need to fill them before expoxying?
Yes, if you are going to use NP-1 or SikaFlex you need to epoxy the garage floor first. The area where the floor meets the wall is actually a control joint as well. The slab and wall are two separate sections. Fill the open area between the slab and wall after you epoxy.
Thanks for the help. do you have the link for 2-Part epoxy polyurethane hybrid by EpoxyMaster?
You’re welcome. You can find the link here.
Hi – I just purchased some Sika Self leveling Caulk, a few bags of play sand, and some backer rod (3/4ths inch thick) – as I am wanting to replace the old concrete expansion board (cardboard type that heaves). I have not started the project yet – but I am very concerned – I believe I read in your post that the backer rod will float if the crack is not narrower than the backer rod. Our expansion crack where the driveway meets the garage is approximately 1.5 inches (the backer rod is only 3/4ths of an inch) – so it would not be a tight fit and I am afraid that it would float. I thought about just filling the joint with the play sand, and then skipping the backer rod, and putting the Sika Self Leveling Caulk over just those – would that work ? I think I also read that you should not just put the caulk over just sand. I am totally at a loss – could you please help ? I don’t want to just put the old style (cardboard like) expansion boards back in with new ones. I have also seen foam expansion boards – so thought of using those instead. How would you recommend repairing these expansion joints ? Thank you for your help and great website ! Jerry
Hello Jerry. Yes, backer rod will float if it’s not wedged in. Best Materials sells backer rod in the sizes that you would need. Remember to go one size larger than the width of your gap. Also, remember that you need the depth in the joint to accommodate the depth of the rod, including leaving enough room to fill with the SikaFlex. You can place the SikaFlex over sand if you need to. Just keep in mind that if you live in a climate where you experience large changes in temperature, sand in a joint that large can interfere with how well it will allow for concrete expansion.
Hi Shea
Thank you for the information and your help ! I have one more question. Would it work to put down some sand, then put in the backer rod, and then pour sand on top of the backer rod, and then put down the self leveling caulk ? (IE. sandwich the backer rod i the middle between a bottom and top layer of sand). We are in Omaha NE, and we do get a lot of fluctuation in our temps (25 below zero windchills in the winter and up to 105 in the summer) which is why I believe the concrete expansion joints move so much. I just am concerned with the backer rod eventually heaving up. Thanks again for all your help !
No Jerry. If you have the right size backer rod, then the ideal solution is to wedge the backer rod so that the top is 3/8″-1/2″ below the surface and then apply the self-leveling sealant. The backer rod does not move since it is wedged in and it prevents the sealant from leaking below while it cures. The main strength comes from the sealant bonding to the sides of the concrete (make sure it is clean). When you get expansion and contraction of the slabs, the sealant will flex (elongate or compress) as the slabs move while keeping all moisture from above out of the joint.
Great website! Large DIY project for 4 car garage so want to be sure to get it right the 1st time. What I’m not understanding is even if you use the proper 100% epoxy filler for contraction joints and it still allows some degree of flexibility, if the epoxy floor coat on top does not flex, how the does the epoxy floor coating not crack when the material below flexes?
I don’t plan to entirely fill the V contraction joints for that seemless look but do want to fill the existing cracks on the joints. Is it best to fill the cracks only with the 100% epoxy method before applying the epoxy floor coat or use the 100% polyurethane method after? The desired end result is no more cracks to collect debris and water.
Hi Gary. Because the gap between the actual cracks within the joint and the bottom of the epoxy floor is filled with a hard material that will allow some flexion, slight movement between the two sides will not immediately telegraph to the epoxy and cause it to crack like it would if epoxy was laid directly atop a crack on the surface that moves. The proper joint material does not guarantee it won’t crack, but it does greatly reduce the chances with slight movement (which is normal).
In your case, filling cracks at the bottom of a contraction joint that you do not intend to fill flush is not necessary unless the cracks are quite large. The color flakes and epoxy effectively cover or camouflage the cracks that are deep within the joint. If you are doing a solid color floor without flakes and you have wide contraction joints, then we could see the need to fill the cracks at the bottom. If that is the case, then the proper flexible joint filler would be the correct choice.
how much movement will be possible after filling of expansion joint?
The movement possible is determined by the elongation rate of the filler you are using and the width of the joint. For example, if you use a filler with a 20% elongation rate, it will allow a 5 millimeter joint to expand or contract 1 millimeter.
I have a 1/8″ gap running along the length of my garage where the concrete floor meets the stem wall. If I am planning to apply an epoxy coat, can I use an epoxy gel crack filler to fill this gap? Or should I use a patching compound like HD-110 instead? Thanks!
Hello Gary. That gap is actually an expansion joint along the stem wall and not a contraction joint. The slab of your garage floor was poured separate from the rest of the foundation. As a result, you will get more expansion and contraction along that joint than you do with a contraction joint. Most people leave the joint alone and then fill it afterwards with flexible sealant. However, it won’t do any harm to try filling it will a gel crack filler that allows some elongation. It may or may not form a crack depending on how much movement you get. Do not use a product like HD-110. It is designed for floor repairs and will not flex.
Can I use PC-Concrete Epoxy Crack Repair to fill in the expansion joint along the foundation or should I use something else? Also, do you have recommendations on flexible sealants I could use if I decide to fill it after laying down the epoxy coat? Thanks!
Hi Gary. No, you don’t want to use an epoxy filler for the expansion joints. It will not flex nearly enough for the movement you get in those joints. After you have coated or sealed your garage floor, fill the expansion joints with a flexible polyurethane sealant such as SikaFlex or something similar. If the joints are deep, you will want to place foam backing rod about 3/8″-1/2″ deep into the joint first.
Can I use sika flex self leveling in the contraction joints before the epoxy application and then epoxy over the top of the sika?
No, you can’t Jason. There are two reasons for this. The first is that SikaFlex cannot be sanded, painted, or coated over. The second and more important reason, is that SikaFlex is too flexible and will cause the coating to crack if pressure or weight is applied over the joint. Coatings are extremely tough, but they are applied to a solid base. When that base flexes too much, then the coating will crack.
My desire is to have as close as possible to a smooth epoxy garage floor as I will be rolling things around on it(table saw, etc.) and I am afraid the wheels will get stuck in the current control joints if unfilled. The control joints are your typical large tooled in joints. I was wondering would it be appropriate to do a silica sand and epoxy fill and then do a saw cut (i.e. now control joint is only the width of the saw blade ~1/8 in). My contractor is indicating they will not warranty the work if I want the control joints filled and I was wondering if this is a good compromise that may still get me the warranty and allow for “mostly” smooth floor that the wheels will not get stuck in?
Hello John. It’s not uncommon at all for floor coating contractors not to warranty their work for cracks over control joints if they are filled. After all, the joints are there for a reason. However, filling the joints as you stated and then saw cutting all the way down to the concrete (center of joint) will highly reduce any chances of the coating cracking over the joint. Whether or not the contractor will warranty that is something you will need to talk with them about.
Hello Shea.. What about using the Sherwin William Micracle bond polyurea filler? thats the only one available near me
Hello Prem. It can be used as a flexible joint sealer – just as SikaFlex is used. However, it cannot be used to fill joints and then epoxy over. It’s too soft and non-paintable for that.
Its not a polyurethane like Sika flex and the store guy told me its a polyurea filler with 80% elongation which dries hard overnight. So just want to make sure you are not talking about their polyurethane caulk. Its called the CRACKBOND® JF-311.
thanks
OK, this is different than the Miracle Bond you mentioned earlier. We found the data sheets for the CrackBond. This will work for filling, grinding, and then coating over saw cut joints. It’s still a bit soft with a Shore A Hardness rating of 75-80, but it will work fine for saw cuts. If you are filling wide (3/4″) contraction joints then you would want something harder.
Was there a recommendation for Epoxymaster’s flexible joint filler?
Yes there was, DD. They changed some of their links and ours no longer worked. We suggest giving them a call to ask them about their joint filler. Their customer service is great. You can contact them here.
Thank you for all of this great information! … This maybe a bad idea, but figured I would get your thoughts … I am planning on doing my garage floor with a grey epoxy and decorative color chips. As you explained here, after the epoxy I plan to use grey self-leveling polyurethane joint sealer/filler to fill the expansion joint. Would it be possible to apply the same decorative color chips used in the epoxy coat to the joint sealer in order to give the floor a more uniform look?
Hi Brock. Yeah, it’s not the best of ideas…lol. The joint sealant is too thick and viscous for the color flakes to sink into and adhere. Plus, it would look odd because you will still see where the joints are because self-leveling sealant is not going to make the slab blend together smooth. Having a nice clean division between the slabs using the self-leveling sealant looks nicer than you think. It provides a nice contrasting line that many tend to like. As a tip, don’t fill the joint completely to the top. Tires can leave black marks that are hard to clean.