Laying stone floor questions

[poppy]

Have started working on joist placement and need to plan for a section of floor under the Franklin fireplace.  Am planning on using natural stone.



The fireplace is to be on the far side in the above pic., just off center to the right so that the stove pipe will go straight up.




Here are some of the stones.  They are mostly sandstone with some limestone thrown in.  They're all found stones on the farm or along the state highway.  They vary in thickness 1" to 1.5".

Most of the flooring will be 1" cherry over a subfloor of 1" oak planks.

Am planning on notching the joists 1" where the stone section will go which covers about 5.5' x 4'.

So that will give me 3" of working room for the stone floor section.

Will the following work OK or do I need to consider something different? 

Plan A:  1" oak plank subfloor, 1/2" backer/cement board, stone layer bedded in; which gives me a 3" height and will match the top of the wood floor.

I'm not concerned about joist strength with using 2x10 full size, plus will be adding some extra joists for good measure.  Largest joist spacing will be about 15".

So will the planks work OK or do I need to add a layer of plywood between them and the cement board?

Any easy alternatives to the 1/2" cement board?

[Redoverfarm]

Poppy one of the biggest problem is making the transition from hardwood to the stone.  The irregular thickness of the stone to the uniform heigth of the hardwood.  I would probably allow at least the thickness of 3/4" ply and 1/2" cement board. This is not going to help on the defelction but might firm the area.

Deflection of the joist is another problem which are greater than what you would imagine.  Here is a link to help determine what your deflection might be. Not so much with holding the additional weight but to keep the grout and stone from cracking.  The plywood or any other substrate will not stiffen the deflection.  Only additional joist or joist support.

With my cabin only spanning 16' the deflection was too great and an additional beam was placed to reduce the span to 8' on 2X10. In addition to the 3/4" T&G subfloor I also laid 1/2" ply and then used Ditra in place of cement board.  

http://www.johnbridge.com/vbulletin/deflecto.pl

[Don_P]

Nice calc John,
Poppy, if you need l/720, that floor system cannot provide it. There is deflection in the joist direction and in the girder direction.

[poppy]

Thanks for the responses guys.  I was aware that deflection was the biggest concern, but was not aware of the L/720.

That is a nice handy defl. calc. sheet, however I'm sure it does not account for the fact that I have cantilevered joists and beams.

Thanks Don, for reminding me that the beam/girder also deflects, so it is a combination calc.

I think now that the fireplace will be located between the beams which will actually help with the deflection, since my earlier calcs. without the fireplace showed that the center of the joists would actually deflect up. 

John, thanks for the recommendations. I had kind of figured that plywood would need to be used for a nice smooth underlayment for the cement board.

I think my system will work for I was planning to add more joists than the pics. show now.  Will keep you posted.

[poppy]

Did some playing around with joist layout for the stone floor section.  This is based on an area 4' x 4' 9.5" with the 4' starting at the center of the cabin and extending towards the north side.  So the stone is supported by the joists between the beams.  What follows is an E-W cross-section looking South.



The 2x10 on the left is at the center line of the cabin, and the joists marked "Extra" are the ones added to the layout in the earlier pic.  They only span the beams so don't connect to the side sill timbers.  The 2x10's under the stone are notched 1 1/2" to allow for 1" subfloor planks, 1/2" plywood, and 1/2" cement board under the 1 1/2" stone.

On either side of the stone is 1" subfloor planks and 1" finish floor.  The std. 2x8 between the 2x10 & 2x8 on the right is at the edge of the bumb out on the south side. I haven't done all the calcs. yet, but it is anticipated that the deflection will be minimal.