Advice on designing hearth flush to floor

[Moorboy3]

I was wondering if anyone can advise me on constructing a hearth that will be flush - or as near as possible - to our hardwood floor. I'm new to wood stoves, and my contractor and architect also have only limited experience in this area.

I've found this site invaluable! Thank you so much.

After many weeks of trawling the building code and multiple redesigns we finally have installed flue and stove pipe for a Jotul F3. Our engineer has said that we can remove the floor and subfloor as the floor joists have been reinforced. We will bridge the spaces between the joists with plywood so this will be flush with the top of the joists.

That leaves 1.5" to the surface of the hardwood floor. Within this thickness we need to get an R>1 and some kind of stone (slate? Basalt?) on top. I don't know how thick the stone has to be so it doesn't crack under the stove legs.

We were thinking of using this Hy-C stove board to give the R value as one layer. Does that sound suitable? Can stone or tile go directly on it or is it too soft?

Any thoughts or links/references gratefully received.

Here's a rough sketch...

 

[begreen]

You should be able to do this with a 1/2" layer of Micore (300 or SB), 1/2" cement board, latex modified thinset and then a 3/8" stone or tile top.

 

[begreen]

If you want premade, Yoder Hearth Classics are 1 1/4" thick.
(broken link removed to http://www.hearthclassics.com/yoder_original_sizes.php)

 

[GENECOP]

Ok First off if you are committed to a flush condition the basic plan is fine with the exception of the single 3/4" between the joists.....double that up....either use 2x material or double 3/4" which would be better...glue and screw the whole system together stability will be key to your top stone not cracking....over the whole system install 1/4" hardy board...this will leave you 1-1/4" to FF.....now you will need a 1" stone....+ 1/4" thinset...That would work.....however being a decorative concrete guy I would Pour directly over the hardy ...1-1/4" of solid concrete with diamond lath embedded....you need someone who can finish concrete well...you can add color to match any stone you want...you can embed or stamp...your options are endless...let me know if you need more specifics...or options...

 

[begreen]

If they need an R=1.1 rated hearth, 1/4" of Hardibacker is not going to accomplish that.

 

[M1sterM]

I'll let others chime in on the insulation board.

I'd second the doubling up the plywood that's fitting between joists.

Also, if you have access from below, maybe use plywood on the bottom of the joists, as well, as I'm not too confident that recessed ply between the joists would tie the joists together as positively as if it was a continuous sheet over the top. Putting ply on the bottom would help create a giant box beam that would really ensure no flexing (which will not do good things for tile or stone). At a minimum, bridge the joists in a couple locations under the hearth (with smaller material than the joists, to allow the recessed ply).

 

[GENECOP]

If they need an R=1.1 rated hearth, 1/4" of Hardibacker is not going to accomplish that.

Maybe not but 1-1/2" of cementious material under a stove will never be a problem...I don't care what might drop out the front door.....of the stove...

 

[begreen]

The issue is not embers, it's heat transfer. Cement mortar has an R value of 0.2/inch.

 

[GENECOP]

I'll let others chime in on the insulation board.

I'd second the doubling up the plywood that's fitting between joists.

Also, if you have access from below, maybe use plywood on the bottom of the joists, as well, as I'm not too confident that recessed ply between the joists would tie the joists together as positively as if it was a continuous sheet over the top. Putting ply on the bottom would help create a giant box beam that would really ensure no flexing (which will not do good things for tile or stone). At a minimum, bridge the joists in a couple locations under the hearth (with smaller material than the joists, to allow the recessed ply).

If you look at his drawing he has kleets running along the length of the joists ....the plywood should be glued and screwed to that...essentially that will form a solid box....

 

[GENECOP]

The issue is not embers, it's heat transfer. Cement mortar has an R value of .2/inch.

I would be even less concerned with a new stove.......how hot would the BOTTOM of the stove have to get to ignite Doug fir 1-1/2" below concrete? I don't think the bottom of the stove would be remotely close for ignition to occur....

 

[begreen]

It has nothing to do with whether the stove is new or not. Every stove varies. Some only require ember protection, some require exceptional protection. The hearth must meet the manufacturer's minimum spec.

 

[GENECOP]

It has nothing to do with whether the stove is new or not. Every stove varies. Some only require ember protection, some require exceptional protection. The hearth must meet the manufacturer's minimum spec.

I don't entirely disagree with meeting the manufacturers specs.....I will say, and I am sure the By The Book guys will have a field day...that some of the specs are so mired in liability concerns that they are way over board in what they suggest.....certain times when building, the client requests a detail....sometimes to satisfy, margins must be pushed...not ever pushed to there limits,,, I would never suggest that....just pushed a little beyond the specs....Commercial airlines built by Boeing are designed to not exceed 40,000 ft....they can fly much higher....most things have built in margins...this guy is looking to pull something off in 1-1/2"....I believe it can safely be done...even if the I's are not dotted and the T's not crossed....just my opinion after years of building things....no disrespect intended...

 

[begreen]

In the field you may be right, sometimes and sometimes not. Some stoves get the hearth very hot. Heating with a hot fire, in a home should have a generous margin of safety. It has to satisfy the insurer and inspecting authority which will be guided by the manufacturer's tested specs.

FWIW, my wife's childhood home nearly burned down due to heat transfer through a 6" stone and mortar hearth to the wood supporting it. It took 40 year for it to happen, but pyrolysis eventually lowered the ignition point to the point where timbers caught fire. Fortunately for all it was caught and put out early.

 

[GENECOP]

Maybe something like this sandwiched between the Hardy and Slab


http://www.csiaccessories.com/25070.html

 

[bholler]

I am totally with begreen on this one go with the specs of the stove at a minimum. If it requires insulation there is a very good reason and you really don't want to push your luck. If you don't know what stove you will be using yet either figure it out before you start or build to the most stringent requirements you can find that might be the way to go anyway so if you decide to change stoves later you will have no problems. And i have to say that when you are dealing with a fire in a metal box in the middle of a house you really should be a "by the book guy".

 

[elmoleaf]

I'm confused. An architect is a licensed professional who should be able to research the codes and design this for you. What services are they providing? Or is this person a designer without a professional stamp?

 

[GENECOP]

I always enjoy figuring this stuff out.....if you want to go by the book forget the Hardie....Go with Astro foil, it exceeds the R value needed....5/6" thick, pour over.....here is the link

(broken link removed to http://www.videcomp.com/reflectech/astrofoil.html#Features)

 

[GENECOP]

The minimal compression of the Astro should be consistent and minimal, the concrete would hold IMO

 

[bholler]

I am sorry genecop I don't see how could work it is not rigid and not classified as non combustible it is just class a fire rated. Something like Micore needs to be used

 

[begreen]

plastic air pockets doesn't sound like a base for anything hearth related.

 

[bholler]

plastic air pockets doesn't sound like a base for anything hearth related.

agreed

 

[Highbeam]

Only a fool would build or recommend building something not by the book when the results can so easily be deadly. None of us know how much extra margin for safety any of the minimum specifications has as extra. I cheat on plenty of stuff, but only where the results of failure wont kill anybody.

 

[elmoleaf]

Well, here go my 2 cents:
1. Strip floor and subfloor to floor joists as proposed.
2. Add perpendicular solid blocking between joists at 12" on center (ideally, locate blocking at stove leg locations).
3. Add 1/8" continuous plate steel over top of floor framing, drill and screw 16" or 12" on center each direction.
4. Adhere 1/2" layer of micore 160 (for r value)
5. Adhere 1/2" layer of cement board.
6. Thinset and finish with 3/8" tile. There are many color/finish options for tile, some look like stone...there is even tile now that looks like wood.
This should put you close to flush or slightly proud of existing floor. Hold back tile and cement board at perimeter to allow installation of beveled wood transition trim.

If you want to omit the steel in favor of your recessed 3/4" plywood, still add perpendicular solid blocking, but you can hold it 3/4" below tops of existing floor framing. Make sure your 2x cleats are screwed and glued to the existing joists. Keep in mind top of existing floor framing is not likely to be perfectly level across the tops of all the members.

Good luck.

 

[bholler]

I agree with everything but the steel plate I don't see any reason for it.

 

[begreen]

Yes, the steel is not needed and makes it a lot harder to screw through. I prefer micore 300 for greater density. But I don't agree with the adhering, it's unnecessary. Screw down the sandwich of Durock and micore every 8" as recommended and it is totally captive and going nowhere. The adhesive is dubious for hearth use and adds nothing.