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My house currently has a 3rd car garage that I would like to convert and extend my family room. The biggest question is how to raise and level the garage floor with cement (preferably). I spoke with the city and the fact that there is no vapor barrier is not an issue. However, the thin 1 3/4 inch pour at the shallow-end seems to be an issue. It sounds like my options are:

1) Place rebar in the stem walls (and adjacent foundation) at 16" on center and use standard concrete. From what I read the pour is too shallow already and adding rebar to the shallow part could risk cracking.

2) Use wire mesh instead of rebar to reinforce the concrete (I am not sure if this is acceptable in the CA building code or if the strength is all the matters?)

3) Use some sort of cement/fiberglass mixture. This was a contractor suggestion but I don't have any details.

4) Use a polymer concrete that is designed for this depth. The only problem with this is it will be more expensive and I can't find any mixtures specific to my depth.

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I have done the exact same thing using concrete with pea gravel as the rock and fiberglass. Rebar is too thick and will cause problems. Mesh might work, but in the thin area I think it will still cause cracking. The concrete company can add the chopped fiberglass strands, and these hold overlays together quite well. Even when it cracks, the fiberglass is like micro rebar pieces holding everything together. I also use finer rock—pea gravel or what some call Turkey grit—but not all the way down to the mortar. This is stronger than mortar and won't crack as bad as a standard concrete batch at the thin area. If memory serves the fiberglass cost about $15 per yard. The only negative I have found with the fiberglass is it can end up with some of the fibers sticking up a very slight amount and for bare feet (this was around a pool on a different job) I needed to put down an epoxy coating. Other than that, I love the fiberglass and use it especially on pours with irregular shapes and thin pours.

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  • I recently spoke with another contractor who said that we definitely would need to dowel. I believe this is rebar drilled into the stem walls? Wouldnt this have the same cracking issue as if we put rebar all the way through or would you still dowel with the fiberglass?
    – german129
    Jun 23, 2018 at 0:26
  • When repairing a slab that I have cut through I always dowel using 16 penny galvanized nails as they are cheap and I always have a box. Using small dowels like this is a good idea and won't take two long it will secure the overlay to the original slab so I would agree it is a good idea.
    – Ed Beal
    Jun 23, 2018 at 0:51
  • Ed, thanks for answering yet another of my questions. I really appreciate it. Can you please explain how exactly dowels are to be placed? Do you drill an inch into the current slab and epoxy the nail in there? How many inches between dowels? Also, are these to be placed in the stem walls too?
    – german129
    Jun 25, 2018 at 19:18
  • On a overlay gravity is your friend a couple dozen holes drilled 1-1/2" into the slab then just drop the nails in right before pouring (so you don't step on one, the weight will keep them there. When I cut a hole that I am patching I drill horizonaly. A bit deeper and don't cut the nails in half like I do on a slab overlay. A trick my dad taught me was to purchase a wax ring (think toilet repair) the cheap ones then use a bit of wax in the hole less than the size of a pea. Stick the nail in and it will hold quite well once the patch is cured it is like rebar and won't slip.
    – Ed Beal
    Jun 25, 2018 at 19:28

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