First time posting. I was caulking the 2in wide x 16ft long gap between my driveway and garage yesterday and I screwed it up. I cleared most debris out then put play sand as a base. Then I put foam backer in there but I suppose it wasn't deep enough. It was taking a lot of bottles of caulk to fill the gap. Now the foam backer has risen to the top of most of it. Its basically a covered caulk rope.

Should I just rip out the foam backer and place a new one in there lower down as a I can? Then put new caulk on top. Will I even be able to do that at this point? I was trying to get away with not using as much caulk but it looks like that backfired.

Any advice is welcomed. Thanks!

  • 4
    Hello! Any chance of posting some pictures?
    – gnicko
    Oct 7, 2019 at 14:27
  • I second that, pictures would really help. Also, how deep is the gap? And what caulk are you trying to fill it with? I wonder if caulk may be the wrong approach here.
    – beswald
    Oct 7, 2019 at 14:28
  • I am not at home right now so I don't have a picture. But the gap is now filled with sand and the foam backer. It was about ~3in or little more in some places. I am worried that caulk may not be enough here either. What would you think is a better solution?
    – user107839
    Oct 7, 2019 at 14:32
  • For the foam backer rod, just take metal weed fabric stakes and drive it over the backer rod. It'll hold it down for the caulk to successfully setup. Oct 7, 2019 at 17:33

3 Answers 3


I've had that happen too -- I pressed foam backer rod down into an expansion joint between concrete slabs and applied Sika self-leveling sealant over the top. In some places the backer rod came to the surface within 5-10 minutes. I pushed it back down, it floated back up -- ultimately I lost that battle.

The problem is that self-leveling sealant is liquid and foam backer rod floats. If the rod doesn't fit the gap snugly then sealant leaks down under the rod and the rod rises.

Your gap is rather wide to fill with self-leveling sealant and the answer from isherwood gives some good alternatives to consider.

If you do want to go forward with the sealant, and if you want to use foam backer rod filler, it'll have to be anchored down somehow. Landscape staples like those used for securing weed barrier fabric or drip irrigation tubing could work. It might also work to spread a layer of sealant in the bottom of the joint, lay backer rod on top of the sealant, and wait for it to cure. Then apply more sealant over the top. The rod would be prevented from floating because it's stuck in the base layer of sealant.

You could also just skip the backer rod. It's simply a convenient filler for the right size gap. You could use exclusively sand as the filler -- just bring the sand up to 1/2" to 3/8" of the finished level and pour in the sealant.

  • I'm dealing with the same issue... Wouldn't something like sand just wash out way too easily?
    – JPhi1618
    Oct 7, 2019 at 17:17
  • Hmmm. You're right that water infiltration could erode the sand away over time and leave the sealant bridge unsupported. Maybe it's better to fill with larger aggregate: pea gravel or 3/4 minus gravel. It'll be difficult to make the sealant stay on top of something so coarse long enough to cure, though. Gravel plus sand? Maybe a layer of sill gasket fitted so the sealant won't leak around and float it up? Maybe this all just lends support to isherwood's suggestion that sealant isn't the ideal fix here.
    – Greg Hill
    Oct 7, 2019 at 17:45
  • i was going to suggest pipe insulation to get it big enough to stuff in the gap. 1" or 1 1/2" pipe insulation should do it, one or the other ought to have an outer diameter of 2 1/2" or better. That will keep it in place. Also, self leveling caulk will seep into small gaps. If dry sand will run through a place, so will the caulk. Use regular caulk to fill these places so the good stuff stays where it's needed.
    – Jack
    Oct 8, 2019 at 2:10

I'm not sure caulk is the right approach in the first place for a gap of two inches. There are commercial products that do a good job (seen in cases such as pre-cast building walls), but you'd need the appropriate applicator tool and large quantity of material. Typical caulks are only suitable for gaps of up to half an inch or an inch, depending on the product. I'd be surprised if any claim usefulness to two inches.

I think I'd use asphalt patch mix. Clean out the channel to 3" of depth, rinse and compact with water, and install using a suitable tamping tool (a sledge hammer and 2x4 on end to set, flat to finish).

If both of your slabs are concrete, consider renting a diamond saw, cutting the channel to 12" or so, and pouring a proper slab segment with a few bags of ready-mix.


I used a skinny pool needle for some of my larger gaps, and it worked great


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