I have floating shelves in my kitchen that carry a lot more weight than a microwave (like dozens and dozens of hardback cookbooks). My shelves are so strong I could probably do chinups on them. They are supported by brackets that were bolted to the studs before the sheetrock was installed. What pokes through the sheetrock at intervals are steel rods about 1/2" in diameter and 10" long that are welded to the hidden brackets. The floating shelves are carefully drilled on the back edge with 5/8" holes that match the spacing of the rods, and the shelves just slide on until the back sides butt into the wall. There are small, unnoticeable screws that are on the underside of the shelves -- one per rod -- that are tightened against the rods to keep everything securely in place.

shannanoz,
Thats one of my inspiration photos too! Thanks for the tread you attached. I had searched gardenweb but did not find that thread. I will check out the different ideas.

kudzu9, unfortunately our drywall is done and painted. We should have planned better. I didn't realize it would be such an issue finding shelves to hold the weight.

debrak-
If you have trouble finding an alternate solution, you could consider cutting in to the sheetrock to mount a pair of brackets and then patching and repainting. It would just be two small patches, and I don't think it would be a lot of work to make it look good as new.

One other option is what I did in my study, where I have a different type of floating shelf. I wanted something sturdy and clean-looking for bookshelves, so I had some fabricated out of 9 gauge steel, which is a bit more than 1/8" thick. The back edge is a right angle that is about 3" high, and is drilled so I could screw 1/4" diameter lag bolts into the studs. The fronts of the shelves were bent so that they have an apparent thickness of about 1". Then all the shelves were spray-painted. If you did something like this, the metal is so heavy gauge that there would be no flexing with just a microwave sitting on it. Here are a couple of pictures:

kudzu9, I really like those bookshelves. I will check into that option.

I'm not sure HOW ours are attached but we have a couple 60" shelves currently holding hardback cookbooks - it's only been a few weeks though! Our project manager, whom I trust completely, said they were monsters that weren't going anywhere. He is not at all affiliated with the cabinet people so it doesn't seem he has any vested interest in saying that. And he personally built our bookshelves in another room - miles of them - so I assume he knows what he is talking about there, too.

They were put in after our sheetrock was done, outlets hooked up, and beadboard applied to the wall.

Oh - just found a picture of the back of one of our monster shelves. Don't know if that tells you anything!

It looks to me like the shelf is laminated together from several thicknesses of plywood (which makes for a strong shelf), and that the rectangular area just above the double-headed arrow has been rabbeted out to accommodate a bracket that is bolted to the wall and then hidden when that shelf is slid over it. I'm sure there are at least two of these rabbets per shelf.

The bracket is probably a small steel plate with a support arm welded to the center and holes for bolts on either side. That rabbet is a couple of inches wide because that's how wide the plate needs to be to accommodate the arm and two holes. That means that, during construction, the contractor would have doubled or tripled up the studs in each of the bracket areas to accommodate bracket plates that are a couple of inches wide. When dealing with single studs, the edge is 1-1/2" or less, so you can't securely get two lag bolts into it side-by-side; a plate with only one bolt would be iffy.

It's a nice installation, but one that needs to be planned for before the sheetrock has gone up.