Designing Kitchens and Bathrooms - Warped 3/4" ply okay to use as support wood to use for granite tile countertop?

I have my Mills Pride (particle board) cabinets in-place and was going to pick-up some 3/4" plywood. However, I noticed that I really can't get a flat sheet from Home Depot/Lowe's - all of them have warping. Will that impact my installation? Should I get something that's flatter like 3/4" MDF?

Thanks.

If you are laying tile, the mdf will provide a flat surface, but you don't lay tile on wood. You will need to install a cbu (concrete backer underlayment) on top of that strata to put your tile down on. Wood will absorb the water from your thinset, and will.......well, warp.

MDF is particle board and should not be used anywhere in a tile installation. When it absorbs any moisture it swells and will kill a tile installation fast. Go to a real lumberyard and get a good exterior grade plywood. The plywood should be at least 3/4". If you can spare the height, two layers of ply is preferred. Then go over the plywood with 1/4" cement board. The cement board gets bedded in thinset using a 1/4" notched trowel, and then screwed into the plywood. If your gonna use tile you need to use materials for your substrate that will resist movement. If not youll have cracking grout and tiles.

Alright, I'll go with two layers of 3/4" exterior plywood. Thanks for the responses.

As Johnny said, make sure you can afford the height difference, as doubling the plywood will make a 1 1/2" + 1/4" + tile height from the top of the cabinet frame. Check receptacle height, and other obstructions to make sure the additional height won't be a problem. MDF isn't "particle board", but plywood is the best choice if you can find flat pieces. Generally with MDF, you don't have to imbed the cbu, since the substrate is almost perfectly flat, eliminating the moisture from the thinset.

I would not install cbu without bedding it in thinset. All manufacturers of cbu require that cbu be bedded in thinset as far as Im aware. Not bedding the cbu will void any warranty the manufacturer may provide. Bedding the cbu in thinset will eliminate any voids that may exist between the plywood and the cbu. Voids between the plywood and cbu can cause movement and tile/grout failure.

You can get flat sheets. You just have to slide all the warped ones off the top and pull from the good stock that's been weighted down from the stuff above. A series of slices through the bottom veneer with a table saw will make it go nice and flat and it will stay that way.

Any how, to emphasize what Johnny said, never use mdf (or any particle board, any 1/4" plywoods or masonite under tile installs) and never dry lay cbu, no matter how smooth the supporting surface may be.

There are also membranes you can use instead of cbu to keep height down.

What have you considered as an edge treatment?

What mortar and grout have you chosen?

Picked up the ply from the bottom of the stack and it still has some warping. What do you mean by slicing through the bottom veneer? The planned edge treatment is just an edged tile. Will be using Flexbond for the thinset and PolyBlend for grout.

Go to a real lumber yard and get 3/4" exterior grade plywood bc grade or better. Its a bit pricey but its the right way to go. Ive never had a problem getting a flat sheet of this stuff at a lumber yard. As Tilebri pointed out you can make a series of kerf cuts on the bowed up side of the ply that will make it sit flat, but I havent found this necessary. For the price youll be paying for bc ply you should be able to get a flat sheet.

I've got a somewhat related follow-up question to this, so figured it would be prudent to post here.

I'm in the process of deciding what support to use for the 15" overhang on one side of the cabinets. If I wanted to use flat steel bars, as shown here (http://www.ikeafans.com/forums/sources/3715-countertop-support-island-seating.html), would it be okay to route a 1/4" hole in the bottom side of the plywood? (So that the steel bar rests on the cabinets) How many would you guys go with?