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Can I Fill My Contraction Joints?

contraction joints garage floorOne of the most oft asked questions by homeowners when it comes to prepping their garage for an epoxy coating is the subject of filling contraction joints. 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 cement floors may have saw cut joints at a minimum of 3/16” wide and 1” deep.  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.  When the time comes to paint or epoxy coat the garage floor, many people want to make these joints go away in order to have a seamless looking floor.

So, is it advisable to do?  Well, the answer depends on why you want to fill them.  Concrete expands and contracts with temperature and also moves as the earth under it settles.  This is what causes it to crack.  Contraction joints play an integral part in keeping cracks from running across your slab.  They create a weakened line in the concrete that encourages cracks to run within the joints.

Filling the joint is not an issue if you are covering your garage floor with vinyl composite tile and isn’t even necessary for garage mats or interlocking tiles.  However, if you plan on painting your garage floor or doing an epoxy coated floor, you can potentially create some cosmetic problems with that seamless looking floor later down the road.

seamless epoxy garage floor

Seamless epoxy coated garage floor

The reason for this is because when one part of the slab moves and the other doesn’t, the movement is within the contraction joint.  If the joint is filled in and epoxied over, a crack can appear right above the joint in the epoxy coat or paint.  It does this because you have a solid seamless coating (epoxy coat) that is bonded to a surface (concrete) that moves in places.  As a result, you can create a break in the surface of the epoxy.  This is why professional contractors will not warranty an epoxy coating that the owner has asked to have the joints filled.  Most contractors will fill the joints if asked but will likely advise against it.

One thing to keep in mind is that the older your garage floor, the less likely you will encounter this problem.  Most cracking occurs within the first few years of a floor as it slowly cures and the earth settles underneath.

If you want to take the chance of filling your joints anyway, make sure you use the proper material to fill those joints with.  Contrary to what some people recommend, do not use latex caulk.  Latex caulk is too soft and will slowly shrink as it dries.  As a result you will end up with slight depressions that outline where your contraction joints are.  If you have rolling toolboxes, jacks, or other heavy objects the move across the joint, you can cause the coating to crack.

The best material to use is a 100% solids epoxy filler or polyurea with an elongation factor of 50% or more.  It cures to a solid that will flex slightly underneath your epoxy coating as the concrete expands and contracts, thus helping the paint or epoxy coat above it to stay in one piece and not crack.

Many epoxy manufacturers offer such products and they can also be found in some home improvement centers as well.  Be sure to check the Technical Data Sheets for the elongation rate or contact the manufacturer directly.  This material will allow you to grind the joint smooth for a seamless look.

The grinding of the joint is important because many garages with the deep V groove actually are lifted at the edges.  This happens when the surface of the slab cures quicker than the rest of the concrete and as a result pulls up the edge.  If you apply the filler but don’t grind it smooth, your joints will telegraph right through the surface.  The lifting of the edges is generally not an issue with saw cut joints.

If your contraction joints are deep, you may want to fill it first with a backer rod so you don’t use as much filler. Make sure it’s approximately 1.25x the width of the joint.  If your joints have large cracks in them the backer rod will also prevent the epoxy from slowly sinking into the cracks as it cures.  Grind the joint smooth after it has cured and you will have a nice seamless surface to paint or epoxy over.


  1. 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?

    • 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.

        • 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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