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I'm currently remodeling my kitchen, and I'm going to have a 9'x40" granite island in the new kitchen. There will be only two 33" wide base cabinets under the granite top, one on each end. The two base cabinets definitely will not have enough strength to hold the heavy weight of the granite top.

My contractor is going to build a support structure to support the weight of the 108"x40" granite top. He laid a cement board on top of the subfloor in the kitchen, then tiled (porcelain) the entire kitchen. He is planning to build the support structure on top of the tiles, but I'm not sure this is the right method.

Is building the support structure for the granite on top of the tiles the proper way to do the job, or should the support structure be built on top of the subfloor, or the cement board?

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Is there going to be free space under the granite and between the two base cabinets? –  auujay Aug 24 '12 at 18:10
Is it going to be 36" or 42" tall? Is he planning to attach it to the floor, or leave it loose? –  dbracey Aug 24 '12 at 18:27
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2 Answers

The tile beneath the pony wall should be removed.

One of the things that a thinset and tile underlayment do is decouple the tile from the subfloor beneath it. By contrast, you explicitly want the pony wall supporting the slab to be coupled to the subfloor.

Then the pony wall should be secured both to the subfloor and the cabinets against it.

The sub-counter (I recommend using MDF) should then be a single piece attached directly to the cabinets and the pony wall.

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Is the OP referring to a pony wall? My impression was that it was not a half wall but was simply a long island that does not have cabinets running under all of it. I think the builder is just going to create some structure between the two base cabs to support the weight of the granite. How would this be any different from placing cabinets onto tile and then installing a granite counter on the base cabinets? –  auujay Aug 24 '12 at 18:19
Either the contractor will be adding arm brackets to the sides of the cabinets, or some sort or arch system to support the weight. In either case it should be attached to the floor. –  Matthew Aug 24 '12 at 20:49
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It's not unheard of to just leave the island loose and sitting on top of finished floor like a piece of furniture. That gives you flexibilty to change things later, as long as you aren't having electricity or water coming into it. Since he already tiled the floor, I'm assuming that's not the case.

However, given that the short dimension of the footprint is 33", it would be too tippy if it was a 42" bar-height island. Even a 36" height is kinda questionable, given the granite top. I would attach it to the floor - this can still be done through the tile, but you'd want some base shoe or something around the bottom to make it look OK. I guess I would attach some cleats to the floor, then put the island over them, then attach it to the cleats.

Structurally, the best way would have been to attach it to the subfloor - but then the flooring guy has to be pretty good to make the tile go around the island and meet up properly on the other side. It's done all the time, but it ain't easy (I'm told...).

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