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Unread 08-14-2014, 08:28 PM   #1
sjs
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floor stiffness for woodstove hearth

Hi all,

I want to put a 390lb woodstove on top of a ceramic tile hearth. My floor is Nascor NJH12 engineered I-beams 16" o.c. with 3/4" tongue+groove OSB glued and screwed subfloor on top.

According to the Nascor span table, 'Simple Span: 40 psf LL/10 psf DL (L/480) • 3/4" sheathing glued & nailed' I can span 18' (which is what I have under where I want the woodstove).

So I think that's good. ?
And I can stick the tile right on top of cement board on top of the subfloor if I want to. ?

But then, if I calculate my wood stove's dead weight when it's sitting on it's 18"x18" pedestal to be 390 / (18 + 18 + 18 + 18)" * 1" = 5 psi or 780 psf then my stove is going deflect my floor way past L/480 isn't it?

And that's *bad*!
So I need to add a sheet of plywood on top of the subfloor before I add the cementboard and the tile. True?

But I'm not at all sure I understand the dead load thing. Because, I'm a smallish person at 125 lb. And with smallish feet, I figure my dead load is 333 psf. So my floor wouldn't even be able to handle me without deflecting way past L/480.

So I'm just completly confused!!

Can you help?

Thanks!
Jane
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Unread 08-14-2014, 08:47 PM   #2
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Jane,

Not to bust your bubble, but I'm gonna guess you're gonna add a stove pipe and some wood or coal to that stove at some point, which will add weight to the stove....

Just sayin......
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Unread 08-14-2014, 09:32 PM   #3
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Welcome back, Jane.

I know y'all Canucks like to be different from the Yanks, but I was not aware y'all had even changed the laws of basic math and geometry on us.

Here, for example, we would determine the square area of the footprint of your stove by multiplying the length of one side by the length of the adjacent side rather than adding all the sides together. maybe that's the problem with the currency exchange rate sometimes, eh?

So, your stove's base would cover 2.25 square feet. And at 390 pounds (without Paul's wood) would have a footprint of some 173 pounds per square foot. Once in place on your floor, that would usually be considered a dead load. But while you were getting it set in place you're looking at substantially more live load on the dolly wheels and overall with the couple of 14-stone men in the vicinity moving it and a little 9-stone supervisor such as yourownself (you, by the way, can never be dead load).

For your overall floor, keep in mind that the design deflection calculations done by the engineered joist manufacturer is based upon every square foot of the floor having weighed 10 pounds before any live load, i.e, furniture, people, dogs, gorillas wheeling wood stoves, etc, was introduced. It is also based then upon the introduction of 40 pounds of live load to every square foot of the floor. That's what your building code is based upon and requires (likely) a deflection of no more than L/360, but your joist manufacturer wants the deflection to be fully a third less than that on longer spans (yours) to make the floor "feel" better to the human part of the live load.

Bottom line: If you added CBU and ceramic tile to what you have and could magically levitate that stove in there and set it such that the edges were each sitting directly on a joist, I think it'd work. If you were to install a second layer of 1/2" plywood over your existing subfloor, which would further spread the point load of the stove, and levitate the stove in there it would be even safer.

In either case, if you haven't the capability to levitate things, I would recommend creating a runway of 3/4" plywood for the stove to be dollied over while installing. And I'd still try to have it set such that it's sitting on two joists. And that all presumes, of course, your stove is technically allowed by its manufacturer and your local fire code to sit upon a ceramic tile installation over nothing more than plywood and a CBU.

If that will not work, consider that you could never invite two friends who weigh in at 200 pounds each without specifically instructing them that they may never get closer together than about four feet, eh?

My opinion; worth price charged.
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Unread 08-14-2014, 10:03 PM   #4
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Big Grin.

I know you Americans are generous types and us Canucks, especially us Canucks with anscestors from Scotland, are tightfisted and generally try to save money where we can. (Not sure if that can explain the exchange rate.)

So, you're figuring that the guys making the pedestal are generous folks like you and they took a great big sheet of steel and folded it into a box. So there's a full 18" x 18" bottom to the box spreading the weight of the woodstove nicely over a large area.

Me, on the other hand, I'm figuring that they are cheap like us Scots and they just took a long, narrow sheet of steel and folded it 3 times in succession and then set it on its edge and, voila, it looks like a real box, but there's no bottom to the box. So all that weight of the stove is sitting on 4 thin edges of steel. I did assume that those edges were 1" thick, but still, the area of the edges is just (18+18+18+18) * 1"

But, either way, you worry me -- sounds like my wood stove has a damn good chance of cracking my tiles! Yikes. And, for sure, I'd better make sure that my 14-stone moving men aren't wearing stiletto heels or they'll crack my tiles too.
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Unread 08-14-2014, 10:07 PM   #5
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Oh yeah, Paul, like the situation wasn't tricky enough already without you bringing reality into the question.

Gimme my bubble back!
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Unread 08-14-2014, 10:10 PM   #6
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Jane, I've seen those stoves cause a floor to sag several inches over time. It might be overkill, but if I were to install one, I'd want some support from the ground up directly underneath. And if it was on a second floor, I'd figure exactly where it was going to sit and I'd grossly overdo the joists in that area, and have two layers of plywood to boot.
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Unread 08-14-2014, 10:15 PM   #7
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YIKES!!

Wow, OK, maybe I should get a building engineer involved in this one, eh?!!

And here I thought a wood stove would be a relatively simple thing to install. Silly me.

Thanks for the warning, Kevin.
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Unread 08-15-2014, 06:29 AM   #8
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Ahha, CX is right: the pedestal base is solid. I found an expanded-part diagram. Well that's good at least -- spread the weight. I wonder how the folks who opt for the 4 little legs instead of the pedestal manage with 390 lb on 4 little points....? Maybe they put their stove on a concrete floor in the basement.

No worry about fire: the stove is rated "ember protection only" so I can install it on a combustible floor providing I cover said floor with something that won't burn (tile) just in case of sparks or ash/coal spills.

So: an extra sheet of plywood, CBU, tile, then stove. And underneath, probably another joist... or two. If that won't do it, I'm stymied. Can't put a support post under it; that'd play havoc with the room below.

Thanks guys!
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Unread 08-15-2014, 06:40 AM   #9
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an extra sheet of plywood, thinset, CBU held down with screws, thinset, tile, then stove. And underneath, probably another joist... or two.
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Unread 08-15-2014, 08:34 AM   #10
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Thanks Paul, italics very useful -- I hadn't thought about how to install the CBU on the plywood. Thinset *and* screws. Gotit.

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Unread 08-15-2014, 08:38 AM   #11
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Jane, you wanna read and follow all the installation instructions of the manufacturer of the CBU you elect to use.
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Unread 08-15-2014, 10:19 AM   #12
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CBU manufacturers provide installation instructions? Who knew? I always thought it was a generic sheet good like plywood or OSB. Astonishment.
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Unread 08-15-2014, 11:15 AM   #13
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They even specify which fasteners you use.

We've got plenty of answers.
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Unread 08-15-2014, 12:07 PM   #14
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Plywood an OSB manufacturers provide installation instructions, too, Jane.
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Unread 08-15-2014, 02:58 PM   #15
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What??!!!???

Aint nobody I know of who's read no high fallutin' installation specs for *plywood*. Us lumberjacks up here kin cut up and hammer 'r screw in that stuff 'thout some academic tellin' us what t'do in installation specs.

(Yes, well, OK, except for installing the second sheet of plywood with the edge 1/4 of a span away from the joists and just screwing it to the subfloor, not the joists for a tile job. I admit, I did read that.)

(Oh, well, and about how the plywood should be installed across the joists, not parallel. I read that too. But everyone knows *that*.)
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