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.
Best Answer
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.