Hearth Weight / Support / Reinforcement

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trevarthan

trevarthan

Member
Jul 27, 2011
59
Chattanooga TN
www.createthis.com
I bought a beautiful used Jotul Oslo F500 this year to replace the Pacific Energy Vista Insert.

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The Vista just didn't put out enough heat for my old 1700 sq ft 1920s house.

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Also, I suspect a lot of the heat it did produce went into heating the masonry chimney rather than the room, even with the blower attachment. Wood had to be cut small. Yada yada.

I've been building a custom hearth pad for the past two or three months in my spare time.
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Construction is black slate and white iridescent glass tile over durock and two layers of select grade 8x10x10 wood planks (both photos were taken before I added the second layer of wood, so there's that). The Jotul will go in the center. It's doors are mirrored in mosaic tile. The pad weighs (eyeballing it) between 150 lbs and 250 lbs. I think the stove weighs ~500lbs. I'm concerned that the weight may be too much, for the existing masonry hearth underneath the new hearth pad, to support. This is a view of the masonry chimney + hearth from the basement:

11brmdk.jpg


As you can see, it is basically a wedge shaped ledge of concrete extending from a fieldstone masonry chimney. I can't tell if it is attached to the surrounding wood subfloor, but my guess is that it is not.

Sure, I could slap some 4x4s on it and call it a day. However, the basement gets wet, and the house has already settled quite a bit. I don't want to do something that will end up *causing* a crack due to temperature expansion, moisture expansion, settling, etc.

What is the best way to brace this thing?
 
I'm not a carpenter so I'll let others help here. ButI have to say that project is going to end up with a very good looking hearth. Looks like you are going from a microwave oven to serious Gothic.
 
Jotul says the stove weighs 445 lbs. Your hearth pad looks like it might approach ~48 ft² or so, and the way you're building it, it should spread load out pretty nicely in both directions. Even if the hearth pad ends up weighing more than 500 lbs, the pad and stove together will be about a 20lb/ft² dead load. There won't be a lot of live load traffic on the hearth pad. I'd think that even your old house can handle this without modification. But it's never a bad idea to bring in an engineer to eyeball it personally and in detail and give you a recommendation on what, if anything, needs to be done. There's just too much that can't be seen by viewing a pic or two on the Internet. Rick
 
Well, there isn't really anything much that any of us can do over the Internet. I'm an engineer, but without being there I'm not about to go beyond what I've already said. I think that to put your mind at rest you're going to have to hire someone local to come in and give you some first-hand on-site advice. Rick
 
Really? There isn't some standard tried and true method for bracing a masonry ledge from below? That's surprising. I was under the impression that concrete has been around for thousands of years and we're pretty good at it these days, architecturally.

Brain storming possible braces makes me think... metal bar with masonry bolts to form a triangle with the existing chimney? Solid stone brace? (i.e. limestone block - seems excessively expensive), 4x4 wood cut so it forms a triangle with the rest of the structure and bolted to the masonry? 4x4 bolted to the floor (concrete pad) and the ledge to form a rectangle? Hang a wood frame from the subfloor to support the concrete ledge? etc...

I'm not looking for someone to tell me if it needs support. I've already decided I want to add more support, if only just so I can sleep at night. I'm just curious what the best construction technique might be given the situation. This whole hearth thing has me reading a lot about material properties, and I know concrete has a low tensile strength. I also know the house has moved a lot in the past due to settling. I'm trying to play the "best solution given the variables" game here.

Wood is the easiest bracing material to work with. I'm just worried it will expand or contract at a rate different from the stone and cause problems. I can always have the local metal fabricators whip up one of those concrete filled metal posts for me or something. I dunno.

I've had an engineer out to the house before for $100 about a different issue. I can do it again. I'm just going to be really annoyed if he tells me to slap a 4x4 on it and call it a day. I could have just done that and saved the $100, you know?
 
trevarthan said:
Really? There isn't some standard tried and true method for bracing a masonry ledge from below? That's surprising. I was under the impression that concrete has been around for thousands of years and we're pretty good at it these days, architecturally.

Brain storming possible braces makes me think... metal bar with masonry bolts to form a triangle with the existing chimney? Solid stone brace? (i.e. limestone block - seems excessively expensive), 4x4 wood cut so it forms a triangle with the rest of the structure and bolted to the masonry? 4x4 bolted to the floor (concrete pad) and the ledge to form a rectangle? Hang a wood frame from the subfloor to support the concrete ledge? etc...

I'm not looking for someone to tell me if it needs support. I've already decided I want to add more support, if only just so I can sleep at night. I'm just curious what the best construction technique might be given the situation. This whole hearth thing has me reading a lot about material properties, and I know concrete has a low tensile strength. I also know the house has moved a lot in the past due to settling. I'm trying to play the "best solution given the variables" game here.

Wood is the easiest bracing material to work with. I'm just worried it will expand or contract at a rate different from the stone and cause problems. I can always have the local metal fabricators whip up one of those concrete filled metal posts for me or something. I dunno.

I've had an engineer out to the house before for $100 about a different issue. I can do it again. I'm just going to be really annoyed if he tells me to slap a 4x4 on it and call it a day. I could have just done that and saved the $100, you know?
Click to expand...

For old construction being there to look at how it was built is the key to recommend the right solution
If he gives you a stamp for $100, then go for it - the insurance company will listen to his opinion and cover and damages if you follow an engineering plan ...

That said,

It looks like the hearth extension overlaps onto the floor - you could brace the floor and setup the extension to place the load on the floor instead of the hearth (I'm an EE so my opinion means nothing if it doesn't run on electrons)

Aaron
 
Skip the engineer.

I would cut the top of a large wood beam (maybe a 6x6) to match the angle of the concrete. I would then use two steel columns near each end to support it. Pay attention to making sure the beam makes good contact with the concrete and maybe even use some liquid nails between the two. A couple bolts into the concrete wouldn't hurt either to make sure it doesn't creep up the slope, but I highly doubt it would.
 
...forgot to mention the beam should be pressure treated, ideally.

By the way, the hearth looks awesome! I love how it mirrors the stove's arches.
 
Word. There is already a similar arrangement bracing the wooden subfloor in front of the hearth. It's a single 6x4 wood beam supported by a single 6x4 wood column. It's rotting at the bottom, and the gas line is bolted to it. Maybe I can replace that at the same time.

I doubt the existing support column has a proper footer. It looks like it's just resting on the concrete pad. All of the other columns under the house are either solid concrete or steel. The steel columns look like they have proper footers. I don't think they're all filled with concrete though. Some sound a little hollow. At least none of them are the adjustable kind. The joys of home ownership. :roll:
 
They don't have to be filled with concrete - the 3.5" hollow ones that I'm used to are good for 10,000 lbs or more. Steel is better as far as stability but a PT wood column would be fine to, just not as easily adjustable. You'll probably want to dig a couple new footers, ideally 2 foot square by 1 foot deep. Is there a concrete floor under the sloping hearth support?
 
In doing construction in Massachusetts the typical arrangement for the concrete sub hearth is this. The first floor joist is sistered, the box frame is also doubled and 2x hangers are used. The plywood form is angled down into the foundation wall. The masons, prior to pouring this form, will drill and pin the concrete wall and also use wire mesh nailed onto the wooden box. I'm no structural engineer and as Pen is saying it's hard to tell from just your pictures alone. I would say if this form was built in this manner it would accomodate the weight.

btw, I love your design of the hearth!
 
If you would't let three fat guys stand there, then make sure you support it from below.
 
This probably does not apply to your intended stove setup, but technically I believe code says that the concrete hearth extension should not be touching a combustible material, which is why the carpenter always rips the plywood form away afterwards. It depends on the R value of the extension whether this is necessary. Just in case the next owner (or you) removes your stove and goes back to a fireplace, it might then become a concern if you brace with wood.
 
Tony, I think it does apply. The plywood is indeed removed and there is very little (if any) concrete touching the floor. It is floating there. I always wondered why. It makes sense now.

The hearth pad is not drilled, bolted, or otherwise attached to anything. It just rests on the hearth and wood floor with shims. I did this(against my girlfriend's significant protest) in case I sold the house and the realtor hated my hearth, or I wanted to replace the wood floor later on, or just whatever. It made sense to me.

So I guess metal bracing is my only option.
 
No answer for the construction question . . . but the hearth design is freakin' awesome.
 
trevarthan said:
Finished product (well, almost):

Click to expand...


That is pretty damn sweet. Good work.
 
That looks awesome. Nice work.
 
SO.. what did you decide/do about bracing??


and the hearth pad is Sweeeet...!
 
Still loving that hearth . . . one of the most intricate and beautiful ones I have ever seen.
 
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