• How should I fill this V shaped, shallow control joint in my garage most effectively?
  • Joint rough dimensions:
    • 5/8 in. wide at bottom
    • 1 in. wide at top
    • 3/8 in. deep
  • Do I need backer material or can I just caulk it?
  • The joint has been painted over with an epoxy but it is an uneven surface
    • Should I rough up the sides of the joint prior to applying?
    • Will self-leveling sealant adhere to the roughly painted joint?

Details & Photos:

I need advice on how to caulk the control joints in my garage. I'd like to fill in the joints to make it easier to clean and make the floor a little smoother. That will help with my rolling hydraulic jack, etc.

I've done a bit of research so far and it seems like I need to get (something like) Sikaflex self leveling sealant for the control joints.

My primary issue is this: the control joints are not very deep. Do I need to use a backer rod when I caulk this or should I simply fill it the way it is? I have also heard of "silica sand" being suggested.

The joints appear to be about 5/8 in. wide at the bottom and 1 in. wide at the top. They are approximately 5/16 in. deep (maybe 3/8 in).

Additionally, the floor and joints have been epoxied and I am wondering if this will have an impact on the caulk's ability to adhere to the surface.

Here are some pictures:

Width of the joint from the top (perspective slightly skews measurement): Joint Width From Above

Width of the joint from the bottom: Joint Width From Bottom

Depth of the joint: Joint Depth

View down the joint: Control Joint Macro

Any pointers are greatly appreciated!

Update: Project Complete

Thanks for the help! Sorry I can't award more than one answer!

I finished caulking the floor almost a week ago and it turned out all right! I went a little overboard in some spots on the joints and should have used less sealant, but it will get the job done functionally. The sharpest looking areas have caulk just below the line of the joint.

Joints with tape around them after caulking: Joints with tape

Completed joints: Completed Joints

My garage is rather small - roughly 19.5' deep x 16.5' wide. I ended up using about 3-1/4 tubes of the 29 oz. Sikaflex Grey self leveling sealant. The joint is mostly 3/8 in. deep but I noticed some rough spots in the joint where it was closer to 3/4 in. This wasn't widespread, so I didn't address it and thus far everything has held out just fine.

I also used some extra painter's tape on the outward facing end of the center control joint to create a dam for the caulk. This is the end of the joint closest to the garage door that doesn't abut a wall; it's just open to the outside.

The caulk spilled out of the painter's tape "dam" a little bit, but I cut the spilled caulk off the tape when it was partially cured about 9 hours later. When the caulk is a little green it's pretty easy to cut with a utility knife. I've left the dam in place and plan on removing it tonight or tomorrow as I wanted the caulk to cure all the way through and not spill out the bottom.

Finally pulled a car into the garage on Sunday, so about 3-1/2 days after I caulked it. The caulk does seem to attract tire marks, but that's all right. The garage floor is intended for use.

2 Answers 2


The purpose of backer rod is simply filler so that less caulk is required. This is desirable for (at least) two reasons: Caulk isn't free. Also, it cures because of exposure to the atmosphere; if applied too thick the caulk at the bottom of a joint may not be able to cure. Your joints look shallow enough that backer rod won't be useful.

The self-leveling sealants from Sikaflex are great. However, there's something to beware of: after the material is applied it remains flowable, albeit slowly, for some time. This can result in unpleasant time-delayed surprises for the beginner.

One is related to foam backer rod -- it floats. In case you use any foam fillers, ensure they're stapled, glued, or otherwise well-attached. Otherwise you may find that after some 10-20 minutes the foam floats up to the surface of the sealant. You poke it back down; it slowly surfaces again.. eventually the sealant sets with at least some of the foam visible.

Another surprise is that the sealant will drain, slowly, into large cracks. It looks like you do have a few of these. One way to deal with this is to plan on multiple applications. Apply sealant directly into/onto the crack, just enough to cover it, and wait for that to cure. Some may drain into the crack, but it'll coat both sides of the crack and the gap will be smaller. After one or more repetitions of this the gap will shrink to the point that there's no more appreciable drainage into the crack. Go ahead and fill the entire joint at this point.

After you've been working for 5-10 minutes go back and look at the area where you began. If the self-leveled surface is lower than you like then you can add a little more material. It'll settle and blend in. If you wait too long (the first application has skinned over) then the second application would sit on top of the first rather than blending in.

  • Thank you! I'll check to see where the cracks are most prevalent and run a bead over those points first. How long should I be waiting to apply the final layer after filling those cracks? You say "wait for it to cure" - does that mean the full 24 hours or can I give it just a couple hours and come back? Would hate to have a whole tube of sealant set up while I'm waiting to apply another layer :)
    – Shrout1
    Jan 27, 2020 at 20:01
  • 1
    I should have said "wait for sealant to set" when pre-filling cracks -- no need to wait for full cure. Time depends on temperature and humidity but probably 30-60 minutes would suffice. When it has formed enough of a skin that it's only minimally tacky then you can apply more. The only risk of applying more too soon is that you lose more sealant into the abyss, I mean crack, and the job consumes more sealant overall. As usual, plugging the caulk tube nozzle with a right-sized nail or covering it with carefully smoothed foil duct tape etc can extend the life of the unused portion.
    – Greg Hill
    Jan 27, 2020 at 21:03
  • Excellent! Thank you! I've only caulked a few things and never a floor joint like this. Will update with results when finished :)
    – Shrout1
    Jan 27, 2020 at 21:44

I am not sure if it will hold well with the epoxy paint (I would have filled prior to painting) but it really won’t matter much in the long run as when things poke the soft sealer the epoxy will crack. I would rough up the edges below the surface but that may not be required, you have existing material in there so no backer rod is needed

I would prep the surface rough up and clean, then fill with the sikaflex it works great on a flat slab, not so good on a sloped sidewalk (oops). I think you will be happy with the results , make sure to add enough with cracks it dose a good job of filling them and looks like it shrunk if you don’t put enough down.

This may slightly help your jack but remember it is not hard like concrete if you leave the jack in a depression it will sink in but after the jack is removed it rebounds , so rolling over you still feel a bump but not as bad depending on how full the joint is filled.

  • Awesome! I will sand and clean the joint before I caulk then. I painted first intentionally as I did not want the floor coating to crack with wear / expansion of the garage floor. Appreciate the pointer about the floor jack too; I definitely won't be leaving the jack in there, just rolling it over. I'll update with my results when I'm done :)
    – Shrout1
    Jan 27, 2020 at 17:09

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