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dbhansen 07-27-2009 08:16 AM

Shims under concrete shelf? Help!
Is "shimming" one edge of a polished concrete countertop a bad idea? And if not, what makes a good shim?

I will be installing said countertop (2" thick, poured off-site) on top of my existing concrete cantilevered hearth (picture of cantilever covered with junk). I was planning on using a construction adhesive to bond the 2 concrete layers together, but if I want a little slope for runoff I'll need to add shims under the back edge. The shelf is about 12" deep, front to back, so I think 3/16" shims would be enough (yes?).

I was thinking maybe a strip of Durock would work nicely, but will the resulting air pocket under the shelf weaken it too much? Would filling that pocket with adhesive solve it?

Or maybe a slope isn't necessary for a small shelf like that? Just trying to get good drainage away from the oven.

- Confused in Wisconsin (aka Daren)

Wiley 07-27-2009 09:44 AM

Re: Shims under concrete shelf? Help!
Daren, Perhaps a strip of something like durock like you suggested and before laying the shelf fill the void triangular x-section (durock, shelf support, bottom of concrete shelf) with dry sand/fireclay/cement or mix thereof and screed that dry mix. Adhesive is pretty permanent should you ever want/need to remove the shelf. Also it gets pretty hot right in front of the entrance and I would have concern for a "What's that funny smell?" situation with heating it a several hundred degrees if you used construction adhesive.

One would think 2 inches would seem to be plenty strong, but filling that void would keep critters out as well as remove potential problems with water and freezing etc.

Hope that helps,

dbhansen 07-27-2009 10:10 AM

Re: Shims under concrete shelf? Help!
I really like that idea, Wiley, and I do have a bucket of fireclay left over. :) Maybe if I put the adhesive around the outside (curved) edge only, that would keep the shelf in place and would probably not get too hot. Thanks!!!

Edit: I just realized the shelf is 14" deep, so I think I'll go with 1/4" shim.

Neil2 07-27-2009 12:04 PM

Re: Shims under concrete shelf? Help!

I would avoid an adhesive type material. The oven will expand and contract through the heating cycle at a different rate than your counter top - the adhesive will give up the ghost. Your design should rely on gravity or mechanical fasteners to hold it together, not any adhesive.

To set my concrete counter top, I first positioned a couple of 1 inch wide wooden shims to get it to the right height. Then, leaving the wooden shims in place, I moved the counter top off enough to apply some ordinary Portland mortar between the shims. Set the counter top back down, kept the mortar moist for a week then pulled out the wooden shims.

I used ordinary portland mortar because my counter top has a one inch gap between it and the hearth bricks as a heat break and does not get that warm. You could also use refractory mortar if you have some left over.

dbhansen 07-27-2009 12:36 PM

Re: Shims under concrete shelf? Help!

Originally Posted by Neil2 (Post 60511)
...the adhesive will give up the ghost.

Do you think that will be true even at the outside edge of the cantilever (14" from the door) and even if the counter touches only 1/4" of firebrick? The 2" countertop will go up against the 2" insulation board mostly, but if I raise the back of the counter with shims it will overlap the firebrick by only 1/4". Although, some decorative arch bricks will be mortared to the top of the counter and also to the firebricks, so some heat will travel that route.

I'll see if gravity might be enough, but I bet the counter won't weigh enough.

Thanks for the input, Neil!

splatgirl 07-27-2009 08:33 PM

Re: Shims under concrete shelf? Help!
I've personally fabricated and have in use about 150 sq ft of concrete countertop in my house. All are 2" thick. All are unsupported from below and bridge as much as 48". I have cantilevered portions in several areas, and many of my slabs carry an additional load above.

If properly constructed (including internal reinforcement) AND CURED, a 2" thick slab will be structural, so shimming it won't make any difference at all. For a 14" cantilevered edge, I would add pencil rod or 3/8" rebar extending from the main portion of the slab out to the cantilevered portion and consider tying these to another bar placed at 90 degrees.

Shim to your hearts content, just make sure you've given that new slab a good few weeks to cure before you dance on it.
All of my slabs are reinforced with carbon fiber mesh (C-Grid) and have rod added in critical areas.

dbhansen 07-28-2009 10:23 AM

Re: Shims under concrete shelf? Help!
1 Attachment(s)
Thanks splat! For what it's worth, here's the way I prepped the surface. I might still decide to remove the fireclay and just use mortar instead, leaving the shims in place. The shims are 3/8", which is a little too much slope for my liking, aesthetically speaking, but I might stick with it.

The "clean edge" in the photo was for an adhesive, but maybe I'll skip that.

I follow directions well, but this "figgerin'-out-on-yer-own" stuff is for the birds!

splatgirl 07-28-2009 10:45 AM

Re: Shims under concrete shelf? Help!
Assuming your countertop slab is perfectly flat, I think you could get away with as little as 1/8" of pitch and still drain water just fine. I'd actually be more concerned about side to side drainage but either way I think you're over-worrying.
FWIW, I would use silicone to attach it to your hearth slab. A little bit provides an incredibly strong bond but is still flexible enough to, in theory, allow for expansion and contraction or accomodate being pushed on by the expansion of your hearth bricks. And your "shim" can be as low-tech as a double thick bead of silicone at that back edge.
One of my kitchen slabs is butted up next to a true commercial 130,000 BTU wok burner and it gets pretty dang hot when that burner has been in use for a bit. Presumably the silicone I used to attach it to the cabinetry underneath accommodates any expansion of the slab vs. mechanical fasteners which I think would just have encouraged it to crack. I realize we're talking about the hearth brick expansion being the issue in this case, but I guess I'm saying that if you do use mechanical fasteners, I would make sure everything can move around a bit by making sure the fastener is of a smaller diameter than whatever hole it ends up going into in the countertop.

dbhansen 07-28-2009 11:36 AM

Re: Shims under concrete shelf? Help!
Thanks again. Yes, I'm definitely over-thinking and over-worrying! :)

Okay, so I think I've decided on 1/4" shims -- 3/8" just looks too angled. Hopefully 1/4" plywood will do the trick.

Any thoughts on filling the void with mortar? I definitely think sealing the edges with a silicone-based sealant would be a good idea. The only disadvantage to a mortar base that I can see would be that it will make the shelf harder to replace if it ever became necessary. Heat transfer shouldn't be much of an issue.

splatgirl 07-28-2009 12:19 PM

Re: Shims under concrete shelf? Help!
in my opinion, running a nice plump bead of silicone 1" or 2" in from the edge under the entire perimeter would be more than adequate as a means of permanent install. If you ever have to pull the slab up (I have, which is how I know just how insanely stuck down silicone will get it), you can run one of those thin flush cut saws or a razor-scraper blade along under there to cut the silicone. I think mortaring it down is major overkill, but my experience is with indoor slabs vs. outdoor so maybe I'm crazy. I do know that if I tried lifting up any of my countertops I'd be pulling up the entire cabinet carcass with it before I'd budge the slabs that are stuck to them with a simple bead of silicone.

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