We are in the midst of installing hard wood floor in our family room, and also replacing an old fireplace hearth (whch was a brick veeer) with a granite 3/4 inch slab. The plan was to place the slab on the plywood subfloor, and the top surface would be about 1/4 inch raised above the finished floor height. But my builder has now discovered that the town reqiures that there be cement board under the slab, on top of the subfloor - which means the stack up of cement board plus the slab will make it about 3/4 inch higher than the hard wood floor. This is really disappointing. My first question is: I can understand use of cement backer board in a bathroom, or on a wall with tile, but what good does it do under a 3/4 inch thick slab on the floor? And second: is this a pretty common requirement, or is my town nuts?

Are you relaying the same old piece of granite? Why not leave it in place and work around it. Chcek with the city building department, bet this is a fire code.

No - we tore up the old brick veneer hearth and are replacing it with a 23" x 48" granite slab.

I have a call in to the building department, but they haven't called back yet and the bullder wants to lay the cement board tomorrow. If it is a fire code issue, it seems strange to me that 1/2 inch of cement board is considered more fire resistant than 3/4 inch of granite.

They would expect you to lay the granite on top of the cement board. You can't lay stone or tile on top of a dimensional wooden sub floor. What is your current sub made of?

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Originally Posted by ballengerb1

What is your current sub made out of?

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Two layers of plywood. This is in a family room over a full basement.

What is possibility of removing one layer of sub floor. This will maintain your 1/4'' reveal. If u do this make sure that cut out is over floor beams.

Chuck

How thick is each layer of ply? Creahands idea is good but not if you are left with only 1/4" ply.

Does city ordinance specify the thickness of the cement board? Will there be any trim or molding around around the granite. I would think there would be some kind of transition strip.

First layer of ply is usually 1/2 or 3/4 inch. The top layer could be 1/4inch.

Two layers of 1/4 inch would have flex between beams.

There is a reducer strip made that could be used that would give a very nice look.

Chuck

I checked again with the builder and it turns out that there is one layer of 3/4 inch plywood, not two. So there is no way to make this flush short of putting down another 1/2 inch of plywood throughout the room.

Harold - The city building dept has still not returned my call. When they do I'll ask about minimum thickness of the cement board, but my builder is planning on using 1/2 inch. The hardwood floor will be "picture framed" around the slab.

Chuck - what's a reducer strip?

It is a piece of wood trim that is shaped in a wedge form. A square edge to go against the hearth. It would be similar to clam shell casing. The company u bought the flooring from should have what u need to match new floor.

Chuck

Sometimes newer technology and building products need to take a backseat to old fashioned stone and tile trade craftsmanship and carpentry skills. In your situation, you should go with a sunken subfloor and mud set stone. How is this done? Cut out the subfloor where the stone will go and install solid blocking between the joists to support where the plywood subfloor now ends. In the area where the stone will be, install cleats along the joists and blocking between the joists all at a level 3/4" below the tops of the joists. Cut panels of 3/4 ply to sit on the lowered blocking and cleats and now the subfloor in the area of the hearth is now in plane with the top of the joists.

From here you can proceed with tar paper/lath/mud 3/4" deep and mortar troweled over the back of the granite and set into place. It will be dead even with the hardwood, no cement board needed.