It currently looks like this. The previous owner did a lame job and now I have to rebuild this. What do I use to seal that gap? He did not put any rubber backer rod there or anything like this.

enter image description here the dime coin shown in this picture is 18mm in diameter enter image description here

Here is what it looks after two weeks enter image description here

  • 4
    There is your problem right there: a Canadian penny. Nov 2, 2014 at 17:47
  • Are you getting water infiltration?
    – Mazura
    Nov 2, 2014 at 18:59
  • Nope no water infiltration. There is a roof right above the platforms at the end of the step.
    – MiniMe
    Nov 3, 2014 at 1:56

2 Answers 2


Are those two concrete slabs just laying on dirt or are they attached to anything?

If they are just laying there, which is what it looks like, putting goo between them is not going to work because they will move around due to changes in temperature and work loose.

Assuming the blocks are free standing, the natural thing to do is just leave them as is.

If you are determined to get a perfectly sealed fit, the blocks need to be attached both to each other and to the stoop. One way to do this:

  1. Remove the two blocks and set them aside and check out the foundation of the stoop.

2a. If the stoop is just laying on dirt, like the blocks (doh), then dig under the stoop all along its length until you have at least 6 inches of overhang, then put down an inch of gravel, then two inches of concrete, and finally a thin mortar bed. Before the mortar bed sets, put the two blocks back on top and the mortar bed will hold them firmly to the stoop and both will be joined to the concrete mini-foundation.

2b. If the stoop has a good foundation (going below the frost line 4 feet down into the earth), then you are going to need some kind of bolt system at the very minimum. The idea here is the same as 2a except that the mini-foundation will be poured over bolts or rods that are driven into the stoop, thus joining the two together.

  1. Either way make sure that the new mini-foundation is drained. You need to have gravel underneath and a pipe of some sort leading from the gravel to wherever. By the way, water flows DOWNHILL, so that pipe needs to be sloped down and it needs to keep going down.

Obviously, as you can see, all of this is a lot of work. The bottom line is: no foundation, no seal. This is because without a foundation stuff will move around. If you just throw pavers on dirt, they are not going to be sealable in a climate which has below freezing temperatures.

  • So what happens when you seal the space between concrete slabs that sit on sand and gravel? Those have no foundation yet it is a common practice to seal that space How about sand? Can polymeric sand be used in this case?
    – MiniMe
    Nov 2, 2014 at 18:56
  • This sounds like a lot of work for the exact reason 'stop-GAP' products were invented.
    – Mazura
    Nov 2, 2014 at 18:58
  • @user2059078 that's actually a 'foundation' (sand and gravel) as the gravel is typically (at least, should be) properly compacted to act as a foundation.
    – DA01
    Jan 30, 2015 at 1:34

They look pretty stuck-in-there to me; use the good stuff:

enter image description here

Vulkem 116: (standard surface prep applies; thinned with xylene, clean-up with same or mineral spirits)

Basic Uses

  • Designed for use on poured and precast concrete, masonry work, window and door perimeters, and similar types of construction joints.

  • For use on exterior applications only. Do not use this product inside an occupied building even if there are no occupants present during use

WARNING! Contains chemicals known to the State of California to cause cancer, birth defects and/or other reproductive harm. FOR INDUSTRIAL USE ONLY! For use only by trained and experienced professional applicators.

So, according to the state of California, this stuff must actually work. I've used it before; it really does stay elastic. And it's expensive too, so its got that going for it.

  • Too late, I already went to HomeDepot and bought this quikrete.com/ProductLines/PolySelfLevelingSealant.asp and a backer rod. I squeezed the backer rod in there and after that I put the sealant. I will probably post a picture tomorrow if this thread sees more interest and it reminds me about it
    – MiniMe
    Nov 3, 2014 at 2:02
  • I can not say exactly what is under the slabs. The far end of the step rests on concrete. The close end ..possibly but all I can see there is asphalt from the driveway. If I see cracks in that sealant I will have to think of a better way to do it. BTW the sealant that I have used does not look like it is going to become very hard. The remainings of what was there before looked pretty solid ..not sure what was used before..see the picture above
    – MiniMe
    Nov 3, 2014 at 2:12

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