One 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.
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.