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Unread 02-08-2005, 11:20 AM   #1
djarchow
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Stone for hearth when deflection not high enough

Hi,

We will be installing a fireplace with a granite surround for a new construction. The finished flooring in the room will be site finished 3/4" hardwood. The floor system (tji) has a worst case deflection of 527. I know the recommended deflection for stone tile is higher than this. Would we be able to get by with laying one row of 12x12" granite tile for the hearth on the subfloor with CBU given our deflection since the hearth won't be walked on?

Thanks for any help.

Regards,

Dennis
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Unread 02-08-2005, 11:41 AM   #2
bbcamp
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Welcome, Dennis!

2 questions spring to mind:

1) How do you know your deflection is L/527? Did the supplier run a calc for you?

2) What makes you think the hearth won't be walked on? Mine does!

Since this is new construction, you have options that will allow you to relax about your hearth:

1) Add an extra joist under the hearth. This has two benefits: a) reduces the overall deflection of the floor, and b) stiffens the subfloor.

2) Sister the I-joists with some plywood. 3/4" plywood attached to the top and bottom flanges on both sides of the I-joist will make for a very stiff joist.

3) Raise the hearth with a boxed section of plywood and sawn lumber. You'd have to make this fairly stiff, but it would sort of float over the floor framing, and you'd have to raise the firebox. I'd do this as a last resort.
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Unread 02-08-2005, 08:31 PM   #3
John Bridge
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Hi Dennis,

It will be walked on. Murphy's Law.
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Unread 02-08-2005, 08:56 PM   #4
djarchow
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Bob, John,

Thanks for your replies. I had my truss supplier check the floor system deflection, and it is a minimum of L527. Where the hearth is it will probably be higher since there is an LVL supporting the TJIs about 3 feet from the fireplace. Since we already have the house dried in, adding another joist probably would give my framers a heart attack.

I figured the 12" granite tiles wouldn't see much traffic since the mantle extends 10 " from the wall but I am sure it can happen. I will look into adding some strongbacks under the fireplace.

Thanks for everyone's help.

Regards,

Dennis
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Unread 02-08-2005, 10:00 PM   #5
cx
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Welcome, Dennis.

I actually think it's less important that your hearth will/will not be walked on than to consider that the rest of the floor surely will. The movement associated with deflection happens to the whole sturcture, not just under the footprints.

Like Injineer Bob said, in new construction this is easy to fix.

My opinion; worth price charged.
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