Tips for Filling Contraction and Expansion Joints

how to fill garage floor contraction joints
Filling and sealing a garage floor contraction joint

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 cut joints as a contraction joint.  Saw cuts are usually a minimum of 3/16” wide and 1” deep.

Cracks in a garage floor contraction jointBecause 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 joint between garage floor and drivewayExpansion 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 it advisable to do?  Well the answer depends on what material you use.

Seamless garage floor coating
Seamless garage floor coating

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 in the joint in the first place.  If the joint is filled with a solid material and then covered with epoxy or paint, a crack can appear in the epoxy or paint right above the joint.

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 a high elongation rate of 50% or more.  It cures to a hard solid that is sandable and will flex slightly underneath your epoxy coating 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.

Garage floor contraction joints filled before epoxy coating

Contraction joints filled with Legacy 2-Part epoxy gel and ground flush with floor

Most of these special joint filling compounds can be found online from your favorite 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.  Another product that works very well is a 2-Part epoxy polyurethane hybrid by EpoxyMaster.  This is applied with a special dual cartridge gun which you will need to purchase from them as well.  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.

Foam backer rod for garage floor expansion jointsIf your contraction joints are more than 1/2″ deep you will want to fill them first with a foam backer rod or silica sand so you don’t use as much filler.  If you have “V” shaped contraction joints, the foam backer rod can be harder to wedge in place making silica sand 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 them the backer rod or silica sand will also prevent the filler from slowly sinking into the cracks and creating a low spot as it cures.

Note: Backer rod floats.  It must be wedged in the joint to keep it from floating and to create a barrier that blocks filler from seeping past.

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 the surface.

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.

Contraction joints filled in garage floor after sealer was applied

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 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.

We personally like SikaFlex SL for these type of projects.  You can find it at your local home improvement centers for less than $9 for a 10 oz. tube.  The best deal is the 29 oz. tube for just under $14 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 them 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.

Expansion joint filled with foam backer rod

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.

However, 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.


  1. Jerome says

    The post mentions a 100% epoxy filler with close to a 50% elongation factor. Do you have any commercially available examples of this type of filler?

    • Shea says

      Hello Jerome and thanks for the question. There are many commercially available products for 100% epoxy joint fillers or polyurea joint fillers with higher elongation percentages. RustOleum makes a polyurea joint filler with and elongation of 82% as an example. Wolverine products makes a flexible 100% epoxy joint putty called Integra Flex 1921 available from Alpha Garage with an elongation of 50% as another example. They are out there, you just have to look for them. Legacy Industrial makes a joint filler called HD-821 that they have had much success with but their TDS states an elongation factor of only 14.1%. Remember with all products to check the manufacturer for compatibility with whatever coating you want to use.

      • Craig Metzger says

        Please be careful as many ASTM tests can be misleading. In the case of a slab on grade concrete joints, elongation is nearly meaningless. The ASTM’s that are being quoted are performed in a linear pull test. The concrete joints are simply cuts to induce where the pours will crack. The joint will open in a width fashion rather than an elongated fashion. A rubber band is a prime example. You can easily stretch it over 1000 percent when elongated. But when stretched width wise, you barely achieve a fraction of that. It’s the same with on grade slabs. They are shrinking aay from each other when 1st poured. This will go on for about a year. From then on, the joint movement will depend on many factors. The most influential of which would be temperatures and humidity levels. Please keep in mind there are joint filler and there are joint sealants. Obviously the sealants are the group that are designed move and seal a joint, and the fillers are designed to prevent joint damage from heavy duty loads and to provide a flush contiguous bridge for what ever needs to cross it uninterrupted (high heels, garage floor creepers, forklifts, etc.

        • Shea says

          Hello Craig and thanks for the informative comment. Yes, ASTM tests can be misleading and you bring up some very good points. In this case however, the elongation we are referring to has more to do with the filler being used and not the slab. The reason for using a filler and not a sealant is to prevent stress cracking of the epoxy when rolling heavy objects over the joints as you have alluded to. You want the filler to be able to move well in both directions. Your rubber band explanation is a great example of why we want a filler with a high elongation rate. When the filler is pulled width wise, it needs to stretch much further in relation to its width than the other way around. If the filler has a low elongation rate, it can actually pull away from the edges of the joint because it will not stretch enough.

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