What can I use for a flexible hearth pad?

williaty · Dec 27, 2015

I’ve just picked out a stove and placed a deposit on it. Soon, we’ll have a new Woodstock Ideal Steel living here. However, that means I now have to think about a hearth (the stove we thought we were going with until recently needed ember protection only). The problem I’ve got is that the floor where the stove will be installed is VERY flexible! It’s far, far too flexible to put down any kind of brick, ceramic, or other masonry without it quickly cracking. I’ve looked at some commercially available hearth pads that don’t involve stone/ceramic on the internet and they are all god-awful ugly.

The requirement for the Ideal Steel is a R0.41 pad. What can I use that will put up with the floor being so flexible and doesn’t look like something made from cheap vinyl tile?

begreen · Dec 27, 2015

Putting a heavy stove in that area is not going to help matters. Can you put some support under that area to stiffen the floor? If not I would build a hearth pad with a 1 1/8 plywood base and then appropriate protection per stove requirements.

williaty · Dec 27, 2015

Basically the house was built before code was enforced in this county so it never had to pass inspection. The floor joists are 12" chord engineered I-joists under a 3/4" plywood subfloor. The joists are 22" on center. This just baaaaaaaarely meets guideline minimums for the loading of the I-joists. The result is that the floor flexes a lot because the 3/4" ply is spanning the 22" in each bay (which it doesn't really like) and the I-joists themselves are carrying just about all the load they can. The outside ends of the I-joists are connected to the rim joist using joist hangers and all that bears on marine plywood-faced SIPs, so the foundation itself isn't as rigid as in most construction. In doing research about what types of flooring I could use here when I first moved in, I found out that this is way too flexy for any sort of masonry. In fact, if you jump and deliberately land hard, you can actually feel it's slightly bouncy.

I already planned to add additional 2x12 traditional wood joists in the middle of each bay that the stove will set over plus one bay to each side in order to cut the span down to 10-ish inches per bay rather than the existing 22" just to carry the load of the stove itself with a little more safety margin. I still don't expect this to make the floor rigid enough to avoid cracking grout, tile, or stone though.

Nick Mystic · Dec 27, 2015

I'm with begreen on this one. If it was me I would get under that section of the house and see what I could do about shoring up the floor before I put a stove on it. Most stoves are going to weigh at least several hundred pounds and then there is the weight of whatever hearth you go with. Now add another 40 - 50 lbs. for a full load of wood in the stove and your weight while you're next to the stove loading it. You could easily have a total of 600 - 700 lbs. on that section of flexing flooring! If the span isn't too long perhaps you can add some more floor joists in between your 22" I-joists. If that's not possible for some reason you might be able to put a post one or two under that area of flooring. If you go this route you will need to figure out how to stub in some short floor joists under these posts since you indicate your current set up is already carrying about as much weight as it can handle. Good luck.

DanCorcoran · Dec 27, 2015

williaty said:
Basically the house was built before code was enforced in this county so it never had to pass inspection. The floor joists are 12" chord engineered I-joists under a 3/4" plywood subfloor. The joists are 22" on center. This just baaaaaaaarely meets guideline minimums for the loading of the I-joists. The result is that the floor flexes a lot because the 3/4" ply is spanning the 22" in each bay (which it doesn't really like) and the I-joists themselves are carrying just about all the load they can. The outside ends of the I-joists are connected to the rim joist using joist hangers and all that bears on marine plywood-faced SIPs, so the foundation itself isn't as rigid as in most construction. In doing research about what types of flooring I could use here when I first moved in, I found out that this is way too flexy for any sort of masonry. In fact, if you jump and deliberately land hard, you can actually feel it's slightly bouncy.

I already planned to add additional 2x12 traditional wood joists in the middle of each bay that the stove will set over plus one bay to each side in order to cut the span down to 10-ish inches per bay rather than the existing 22" just to carry the load of the stove itself with a little more safety margin. I still don't expect this to make the floor rigid enough to avoid cracking grout, tile, or stone though.

Or perhaps a few of these: http://www.amazon.com/Vestil-FJB-10...249219&sr=8-4&keywords=adjustable+floor+jacks

They have a capacity ranging from 5,600# to 8,500#, depending on the height to which you extend them.

williaty · Dec 27, 2015

I can't install any vertical supports under the floor. They'd go right through my wife's car (garage below this room).

DanCorcoran · Dec 28, 2015

williaty said:
I can't install any vertical supports under the floor. They'd go right through my wife's car (garage below this room).

Perhaps put a jack on either side of the car, with a beam extending across the top, beneath the ceiling. (I like simple solutions...)

Heftiger · Dec 28, 2015

williaty said:
I can't install any vertical supports under the floor. They'd go right through my wife's car (garage below this room).

I wouldn't say "can't". It more so depends on whether or not you like your wife; or if you'd like her to continue liking you. [emoji6]

sportbikerider78 · Dec 28, 2015

I'm with these guys. Figure out how to get the floor rigid. How crappy would it be if your band-aid solution turned out not to work after you got your stove installed? Imagine a sinking stove and pipe separating? Frightening.
You might be able to do something quite localized and not as intrusive as you think.

williaty · Dec 28, 2015

DanCorcoran said:
Perhaps put a jack on either side of the car, with a beam extending across the top, beneath the ceiling. (I like simple solutions...)

There's no room for an additional beam to go below the existing joists. They ceiling height down there is already under 7'. The only reason we can even get cars in down there is because we have sub-compact station wagons. A truck or SUV would never fit. There's also no way to put any sort of support at all along the outside wall due to the garage door track running about 2" off the wall. We've already had a structural engineer look at this space to try to restrain some lateral movement. His verdict was that we couldn't do anything in this area withut basically starting over with the house.

So the floor is getting new joists centered between the old joists and that's it. Is there anyone who can actually offer advice on a hearth based on that limitation?

mellow · Dec 28, 2015

I am not aware of anything flexible that adds R value. As mentioned I would build a large enough hearth pad to distribute that 500+ lbs.

edge-of-the-woods · Dec 28, 2015

williaty said:
I can't install any vertical supports under the floor. They'd go right through my wife's car (garage below this room).

Sounds like the stove will, too, if the floor gives out

How badly do you need to park that car in the garage? We gave up half our garage to storage of other stuff. Now I park in an area we covered with gravel....and got a remote car starter installed. What if you shore up the are with vertical supports, make that part of the garage all wood storage, and park outside? You could even put in a dumb waiter for moving wood up to the stove area.

williaty · Dec 28, 2015

The garage parking is more important than the stove.

begreen · Dec 28, 2015

1 1/8 plywood will not flex, but with a layer of Durock and tile it will weigh perhaps 2-300 pounds in addition to the 620 lb weight of the stove. Can the floor support an additional ~8-900#? If you are not sure about the structural support of the floor then perhaps consider canceling the order and put in a lightweight gas or electric heater.

bholler · Dec 28, 2015

you need another engineer there is always a way to add structure. You just may not like the bill. What about putting in aditional joists between the existing ones? Burying a beam up in the structure to split you span in half ect.

begreen · Dec 28, 2015

Yes, I was even wondering about threaded rod supports to the overhead trusses. That would kind of fit in with the gearhead look of the stove.

Nick Mystic · Dec 28, 2015

It seems like some posters aren't catching that the OP had already said he plans on putting additional floor joists in between the existing 22" on center joists. So long as he has sufficient structural strength where he'll be hanging those joists he should have plenty of structural support for his new hearth and stove.

bholler · Dec 28, 2015

Nick Mystic said:
It seems like some posters aren't catching that the OP had already said he plans on putting additional floor joists in between the existing 22" on center joists. So long as he has sufficient structural strength where he'll be hanging those joists he should have plenty of structural support for his new hearth and stove.

Yes you are right i did miss that. I agree with that done like you said as long as there is enough support at the attachment points most hearths will be fine

williaty · Dec 28, 2015

begreen said:
1 1/8 plywood will not flex, but with a layer of Durock and tile it will weigh perhaps 2-300 pounds in addition to the 620 lb weight of the stove. Can the floor support an additional ~8-900#? If you are not sure about the structural support of the floor then perhaps consider canceling the order and put in a lightweight gas or electric heater.

Overall weight holding was my first concern, which is why I plan on adding, as I've said more than once in this thread, additional joists between each existing joist to cut the joist-to-joist spacing in half. My concern, and the reason I started this thread, is that even that won't make the floor sufficiently stiff to meet the L/720 rule for natural stone so I wanted to know what other options I have.

bholler said:
you need another engineer there is always a way to add structure. You just may not like the bill. What about putting in aditional joists between the existing ones? Burying a beam up in the structure to split you span in half ect.

As stated above, I'm already planning on putting a second set of joists up half way between the existing joists. The joists only span 10', so putting a beam in the middle doesn't make much sense.

begreen said:
Yes, I was even wondering about threaded rod supports to the overhead trusses. That would kind of fit in with the gearhead look of the stove.

There are no overhead trusses. The house is an A-frame built from SIPs. There's no overhead structure capable of bearing any additional load.

What can I use for a flexible hearth pad?

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