Cement Board Screws & Tape, Hazard?

[Hardly Handy]

I'm installing a free standing Dutchwest Wood Burner with legs. Has a bottom heat shield. I'm building the Hearth on an old 3/4" wood floor. So far, I've laid two 1/2" cement boards (Lowe's) with 24 ga sheet metal sandwiched between. For reasons I won't trouble with here, I was strongly advised to screw the boards to the floor with cement board screws, which I did. Before I lay the 1/2" thick terra cotta tiles (as close together as possible) I'm wondering:

The screws are 1-5/8" long, which means they're about 1/2" into the wood floor. Is it possible that the screws can carry the heat to the wood, thus a hazard?

Also, is there a concern with the the paper-like binding on the edges of the cement board? although I torched it all a good bit, much didn't burn off and remains in a strip that would be directly under the stove.

My wife prefers not to have the hearth any higher than what we come to with this plan.

 

[swestall]

Just get some extra long screws and set them in corners and accross the girth of it to keep stress from pulling the others: long screws will minimize stress.

 

[begreen]

I sincerely doubt that there would be enough heat conducted by the screws to provide appreciable risk. However, the bigger question is whether this setup meets the manufacturer's R value requirement for the hearth. Which Dutchwest stove is this? Some require R 1.1. Durock is rated at R.26 per 1/2".

 

[Hardly Handy]

It's the Dutchwest Model 2479 Non-cat 55,000 btu. Any input is appreciated. I have my concerns. It's 1/2" Cement Board, 24 ga sheet metal, then another 1/2" cement board, then I'll have 1/2" terra cotta tiles. the local "experts" are saying overkill, especially with the heat shield, and I'm skeptical. I had looked for mycore 300 and couldn't find, other than ready made panels that my wife doesn't care for and doesn't want the hearth higher if possible. But if it comes to safety she'll bend for sure.

 

[begreen]

The Dutchwest Large NC has a minimum R 1.19 requirement - with the bottom heat shield installed. This stove gets hot on the bottom and back.

Usually air is the easiest and best insulator. 1" air gap gives you an R 1 value. But as an alternative you could add another 2 layers of 1/2" durock and it should meet the requirements.

 

[bebopin]

Here is a web site that you can order the mycore 300 from. (broken link removed to http://www.mantelsdirect.com/MC300-Micore-Board?sc=18&category=304)

 

[Hardly Handy]

Thank you bebopin and BeGreen, great help. I decided to order micore to keep the height down. This leads me to another question, which I've noticed has been a topic of some debate:

I'd like to place the micore on top of the cement board, then the terra cotta tiles. One advantage would be that the screws will be protected (although this is probably minor, as BeGreen suggests).

Would the micore weaken under the weight of the stove, thus mandatory that it be under the cement board? I've already screwed down thru 2 thicknesses of boards and the sheet metal, would have to remove them all, replace the micore, etc. I can think of several scenarios to then follow, but it seems to me that the weight of the stove on the tiles wouldn't be so concentrated to weaken the micore. The other concern is whether or not thin set will hold micore to cement board and to tile.

 

[begreen]

Your concerns are correct. Micore is not strong, nor is it a good bonding surface. You want the durock to be on top as the substrate for the tiles.