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I'm casting 2" thick concrete countertops using ¾" melamine forms/molds. The largest piece is about 26" square and the longest is about 55" x 11".

I thought I'd make a large flat work surface for setting these forms for pouring and curing. I purchased this nice flat 7-ply 4' x 8' sheet of ¾" plywood sheathing and some 2" x 4" studs to stiffen it up. It will be set on heavy welded steel saw horses.

I've never cast concrete counters before and I have very tight tolerances for a slide-in style range and a farm sink. As such, my templates are very accurate (within 1/16") and I'd like to cast as uniform 2" thickness as reasonably possible. I plan on a concrete mix that is as stiff as possible without being crumbly. Any pinholes or voids will be filled with slurry when I do the polishing. My point in mentioning this is in regard to any slumping due to leveling issues.

My question is about acceptable tolerances for level and warpage of the work surface as it relates to this project. After stiffening it with the studs, I'm seeing some unevenness on the order of ~¼" or so over the 8-foot length and ~⅛" variance over the 4-foot width. Because of this variance, being flat/level depends on where 48" level is placed. Should I just go find some straighter 2 x 4's? Or am I just making myself nuts? Is this close enough for the concrete to not slump out of level? With all the careful measuring, templating, and mold-making, I don't want to spoil it all with a work surface that's out of whack.

I'm also open to other suggestions. I thought about setting the forms on the garage floor. It's flat, but it's sloping maybe ¼" or so per 4-foot towards the door. I can move to the back where it's not sloping, but it's not perfectly level there either... plus I'm farther away from the door where concrete will be mixed outside.

I've even thought about shimming each mold off the floor or table individually, but don't know if the melamine is strong/stiff enough to not bulge in the middle where it's not laying flat.

EDIT: I'm familiar with woodworking/carpentry. I'm looking for answers from people with some experience casting concrete counters in melamine molds.

EDIT 2: Not sure if it's overkill, but I ended up buying some high quality, super straight, 2" x 6" lumber and built a new framework for my plywood. My table is now level/flat within 1/16". What I built is probably overkill, however, I'm leaving this question open because I'd still like to hear from anyone experienced with making concrete countertops regarding such working tolerances.

  • throw down some sand on your garage floor, level with a level; level and flat. – dandavis Oct 25 '18 at 16:35
  • @dandavis, interesting idea. The problem is that when casting concrete in melamine forms, you're constantly lifting/dropping, tilting, shaking, tapping, and vibrating the molds. – Sparky Oct 25 '18 at 16:50
  • would a sheet of Styrofoam (literal) on top of the sand allow your activity w/o displacing the sand? It's stiff and perfectly flat... – dandavis Oct 25 '18 at 16:53
  • @dandavis, I don't know, which is why I'm hoping people with experience casting concrete countertops will answer. Maybe they would say that my setup is more than good enough to achieve my goal. No idea. Otherwise, I'm going to be spending a lot more time and money on making a flatter work surface. – Sparky Oct 25 '18 at 17:44
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I could suggest that maybe you would want to carefully select your 3/4" plywood first to make sure the sheets are very flat to begin with. Then I would select multiple sheets of this flat material and glue then together to achieve a 1 1/2" or 2 1/4" thickness slab. This will give you the stiffness that you want.

The fact of the matter is that a single sheet of even 3/4" plywood (especially the lower cost sheathing grades) flex more than you think. A higher quality plywood with 8, 10 or 12 layers would be stiffer but even then it may still flex too much if just one sheet was used.

Stiffening with separate lumber like 2x4's may work but 2x6's on edge would be much better. However the stiffening would need to be made into a frame or matrix across the bottom so that the above plywood cannot flex between the stiffening members. Building that is possible but likely takes very straight lumber that is constructed into the frame using tolerances as good or better than you used for the molds.

  • My sheet is 7-ply and it's really as flat as anyone could expect ¾" plywood to be. The problems I described in my post were caused by trying to stiffen it. But I really have to question the idea of stacking and gluing multiple sheets. If I cannot get one sheet perfectly flat, then gluing two together will result in a thicker mess that will take even more effort to flatten out. Your idea of using very straight 2x6 makes a lot more sense. – Sparky Oct 25 '18 at 16:10
  • Your answer describes ways to build a flat surface, but my question is really more about the concrete casting process. If I have to fuss around and replace some structural lumber and be more careful with the shimming, I can eventually achieve somewhat better flatness. My OP is more about how far I need to take that process.... the tolerances required for casting concrete countertops, which you never mention. Like "how flat is good enough?", "how level before slumping occurs?", "will a 2-foot square, 2" deep, ¾" melamine mold bulge if there's an unsupported gap underneath?", etc. – Sparky Oct 25 '18 at 16:19
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As stated in my "Edit #2", I rebuilt my table to be flatter and more rigid by replacing the 2 x 4 studs with straighter 2 x 6 lumber.

Ambient pouring conditions: 50° F with about 80% humidity inside an unheated garage.

Quikrete 5000 recommends 6 to 10 pints of water for one 80-lb. bag. I used exactly 7.5 pints for each, which gave me a very dry/stiff mix, like slightly damp sand, that held its shape until vibrated.

Now that I've poured my concrete, I can answer my own question...

My question is about acceptable tolerances for level and warpage... I'm seeing some unevenness on the order of ~¼" or so over the 8-foot length and ~⅛" variance over the 4-foot width. Is this close enough for the concrete to not slump out of level?

After filling and vibrating the mold, a large amount of water/cream settled on top to reveal the slightest slope of the table. So the answer is if the mold tolerances are within 1/16", then the flatness of the table needs to be within that same tolerance or better. I was able to throw a couple shims under the sawhorses to get closer to perfect level using the still wet water/cream layer on the top of the mold as an indicator.

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