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Old 02-21-2012, 09:40 AM   #1
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Second story weight limits


I am about to start working on a grueling kitchen remodel. My dream kitchen would have a stained concrete floor, but after speaking to a local contractor, I don't think it's possible, because my kitchen sits on top of a basement. He informed me that because it's not on a slab, the floor would flex and crack the concrete, and the weight of the concrete would be too much for the beams.

My second choice in Vermont slate flagging, typically used on outdoor patios, etc. (I'm really sick and tired of standard tile!) I have seen it installed indoors, but I wasn't paying attention to whether it was on a first story or not. The average thickness for flagging is around 1", and when you figure in the backer, mortar, etc the flooring would end up around 2" thick, and substantially heavier than tile.

My house has 2x10 beams, standard 16" distance. I am concerned obviously about the weight, but now the flexing as well. Will I be able to install something this heavy in my second story kitchen, or will I have to brace the floor or something of that nature?

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Old 02-21-2012, 10:31 AM   #2
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Second story weight limits


Depends on the span of the 2x10 joists (length they are running without intermediate support) Realistically though I doubt this install would work well over time. Too much load will cause movement and possible joist cracking.
There are a lot of nice flooring options with lots of materials. Using a product that may not work is not something you should need to do.

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Old 02-21-2012, 10:50 AM   #3
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Second story weight limits


The room is 13x18, with the joists running with the 13' side. One side is the side of the house, and the other side there is a basement support wall, with steel(?) braces/columns. (sorry if my terminology sucks...)
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Old 02-21-2012, 03:22 PM   #4
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Second story weight limits


Ok, so I did some research and found out some floor loading info, but I still don't feel 100% yet.

For the flagging I want to use, plus all the building materials with it (mortar, etc) it looks like its going to be around 30 psf. (12 for 1" mortar bed, 15 for 1" slate, 3 for backerboard)

Most houses are built to handle up to 40 psf. How do my cabinets, countertops, and all that factor in? The basement is not finished, and probably won't be, as long as I'm there, so there's not a significant bottom load on the joists....
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Old 02-21-2012, 04:18 PM   #5
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Second story weight limits


Sorry, your plan is not a good one. The numbers aren't right either.

First of all, the "basic" residential load code for non-sleeping rooms is usually 40/10. 40 lbs per sq. ft. "live" and 10 lbs. per sq. ft. "dead" load. This 40/10 is in fact the "worst" it can be built and is not a good base for a tile floor, not to mention natural stone which requires a much stiffer floor.

Your plywood subfloor sheet (s), mortar base, thin set mortar, tile, grout, cabinets, etc. is the dead load. As you can now see, your floor is not close to being strong/stiff enough for what you'd like to do.

You may be able to modify the framing and subfloor to install the slate, but I would not recommend that particular type of stone even with the needed modifications.

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Old 02-21-2012, 04:26 PM   #6
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To figure the load from concrete, use 150 pounds per cubic foot. This translates to approximately 12.5 psf for each inch thickness of concrete. This is considered dead load, since it does not move. For stone floor tile, figure 165 pounds per cubic foot (this varies a bit by rock type, but this is a good start). This translates to approximately 14 psf per inch thickness of floor tile. For mortar, use the same unit weight as concrete.

So for two inches thickness total, your estimate of 30 psf is pretty good. Now here comes the problem. That is all dead load. You also need to add in at least 5 psf for framing, possibly 10 psf if you have heavy framing and double thickness of plywood. This gets you close to 40 psf dead load. Code live load for a kitchen should be checked with your building inspector, but is likely to be 40 psf live load plus dead load.

The live load refers to the stuff you can move around, which includes cabinets, appliances, and people. This is modern code, a lot of older houses were designed for 30 psf live load, occasionally even less.

So let's look at the flexure rating of your floor under the load in question. You have 2x10 solid wood joists 16 inches oc, with a span of 13 feet. We will use a total of 40 psf dead load, and assume code requires an additional 40 psf live load. Total is 80 psf. Your flexure rating (L/D) for the joists would be about 340, using a generous 1.5 million psi for the modulus of elasticity of the wood. You are also just barely above allowable factor of safety for bending, assuming 1500 psi modulus of rupture for the wood, which is high.

For natural stone, the recommended flexure rating is 720, so you are half the recommended rating. This suggests that there is high probability of cracking the stone. For such an installation, you should consider doubling the joists, or go with a small format (say 6x6 inch) porcelain tile on thinset.
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Old 02-21-2012, 05:30 PM   #7
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Thanks for the responses, guys. I like to hear the scientific/math explanation behind things, it helps me out a lot. (I'm a science geek)

Ok, I called a local quarry, and he said they can plane the flagging down to 3/8 to 1/2" thick, which changes the game a bit. I originally estimated 1" thick slate, 1" thick mortar (which is a high-ball estimate). With 1/2" thick pieces, I'm estimating the psf at around 8 for the slate pieces. The mortar isn't going to be 1" (again, I high-balled it for a conservative estimate) I've done half a dozen tile floors, and I think 1/2" was the thickest mortar bed I've done, and that was with a radiant floor heat (I wanted a little more 'squish' room for that).

I'm still a tad fuzzy on the flex, Daniel. I get that the stone has a lot less flex, and that's why I think it's typically installed on slab. If I were to install in-between 2x10 joists, so 8" between, how would that affect the loading on what it was attached to? (cross beams? terminology? sorry....) Or would I also put in new supports, say, at the four corners, and maybe one in the middle of the 18' stretch?

FYI, for anyone reading this, thinking I'm stretching to try to get something that's a royal PITA, you're right, and I do this all the time. I hate being told I can't do something. There's almost always a way to do something, it just might be more $ or labor/time intensive....

Thanks again for the help
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Old 02-21-2012, 06:33 PM   #8
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Second story weight limits


Quote:
Ok, I called a local quarry, and he said they can plane the flagging down to 3/8 to 1/2" thick
That is gauged slate, which is what you'd want anyway for interior use. So this is good and heading in the right direction.

Quote:
1" thick mortar
(Mortar bed aka deck mud.)

If you wanna do it by the book the properly mix/placed mud should be at least 1 1/4". 1" will work most of the time though if everything else is good. There are several methods of installation, I doubt you did a 1/2" "mud" job before. Maybe you're thinking the size of the notched trowel for the thin set mortar? Thin set in not mud. Mud is sand and Portland used to make a base for tiles.

To install gauged slate you'll need to bring both the joists and subfloor up to L720 or less deflection. (higher number). Regardless of what is done to the joists, you'll need another layer of plywood, then choose the installation method. Mud job, backer board (CBU) or membrane such as Ditra.

For the joists I think you'll be fine if you could install a beam at or near the half-way point of the 18" length. So, a 9' span @ 16" o.c. should be good. Do you know the species and grade of the joists? What is your subfloor made of now?

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Last edited by JazMan; 02-21-2012 at 06:35 PM.
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Old 02-21-2012, 07:45 PM   #9
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Second story weight limits


The flex thing....

Stone is less tolerant than porcelain tile about flexure of the underlying floor. Stone tends to have natural weak planes, and if the floor flexes too much, the stone will crack. So will tile, but because tile is a manmade product, it does not have weak planes. The standard way of describing flex is the L/D ratio, which is the length of the joist divided by the maximum deflection (which typically occurs at the center of the joist). So for example, you said your joists were 13 foot span, which is 156 inches. If the deflection at the center were 1 inch, the L/D ratio would be 156. As Jaz has indicated, you need a minimum ratio of 720 for safe installation of stone, so in your case you can tolerate a little less than 1/4 inch deflection at the center of the span. That requires a very stiff joist, or very close spacing, or a shorter span, none of which you have.

I am not going to describe the mathematics of computing the deflection at the center of a joist under uniform loading, it is somewhat complex and not really suitable for a DIY forum. You can look up the deflection in tables, or there may be some deflection calculators on the net (I think there is one on the John Bridge website, or so I am told). I have a spreadsheet I wrote that computes deflection under a wide range of conditions.

The problem you have is that even with minimum mortar and thinner stone, you don't meet the deflection criteria. Not to say you can't put in the stone, but it is likely to crack, and that would be unfortunate after spending serious time and money installing it. In order to improve the L/D ratio, you need to either double the joists, or reduce the span using a beam. Lots of work, maybe not worth it.
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Old 02-21-2012, 08:33 PM   #10
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Second story weight limits


Center a beam under the joists?
That'll stiffen her up.
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Old 02-21-2012, 08:33 PM   #11
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Second story weight limits


If I may go out on a whim this stuff can be used inside install the slate or stone with the morter that's it, it's lite and stronger then any ply.
install it right on the joists.

http://www.tileyourdeck.com/

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Old 02-22-2012, 07:18 AM   #12
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Quote:
Originally Posted by JazMan View Post

If you wanna do it by the book the properly mix/placed mud should be at least 1 1/4". 1" will work most of the time though if everything else is good.

Jaz
So if the slate is planed down thinner, though it still would be thicker than the average tile (1/4?) why could it not be installed as regular tile would be? Say, if I bought a box of 12" slate tile at the store, how would this be different?


Quote:
n order to improve the L/D ratio, you need to either double the joists, or reduce the span using a beam. Lots of work, maybe not worth it.
So I would put a beam running down the length of my floor (18' length) essentially cutting the 13' in half, less flex. I see there are several options for adding support.... As shown here. Headroom is a bit of a concern, my basement isn't necessarily a totally used space, but there's the laundry, utility/work room, etc that would be accessed on a regular basis, So I don't want to lose too much head room.

http://www.finehomebuilding.com/PDF/Free/021184090.pdf

Any experience with these types of support additions? What works better? I like the last option, where you add a perpendicular beam in between the existing joists (though time-consuming...) but I think that particular method is still going to give me more flex than a solid cross beam with an anchor...
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Old 02-22-2012, 07:25 AM   #13
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Second story weight limits


I have worked on projects that have used all the techniques mentioned. You have probably crossed the zone where it might be a good idea to hire a local engineer or architect to actually look at your house, and offer you some hands on suggestions as to the best way to go. Lots of options, lots of variables, cost for each option is going to vary with your location.
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Old 02-22-2012, 07:32 AM   #14
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Second story weight limits


And after all that work you would have one of the most britle and stain prone types of natural there is.
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Old 02-22-2012, 08:32 AM   #15
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Not Vermont slate, it's way harder than the crap you get in a store, shipped in from overseas.....

I do have a structural engineer co worker that I don't think would mind coming over and looking at it....

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