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Old 02-08-2018, 03:39 PM   #1
kristinmcall
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Natural Stone Over 2x6 T&G car decking

I would appreciate some input on an enclosed 16'x8' timber frame entry porch I'm designing for my home. I'm trying to determine if my proposed floor structure will support 1/2" Pennsylvania bluestone tile.

The interior spans will be 14'8" and 7'2-1/2".

I intend to use three 6"x8" #2 eastern white pine beams spaced 45.5" on center. Alternatively, if I find the time, I will use mixed hardwoods milled on my property to the same dimensions.

The first layer of subfloor will 2x6 EWP T&G decking. (16' lengths if I can find them).

The second layer of subfloor will be Advantech.

Thinset, Ditra, thinset, and stone will be the final layers.

Following are some of the specific details that are unclear to me:

1. White pine is supposed to have superb dimensional stability. Do I need to put anything between the decking and the Advantech to decouple them?

2. Since this is the main entry to the house, will the tile survive an occasional heavy load, e.g., a 1000 lb safe riding on a hand truck? Should I bump up my live load design parameter?

3. What panel thickness do I need? The beam calculator at Forestry Forum tells me that I exceed l/720 by a long shot with a single layer of 3/4" even when I bump my live load up to 100 psf. Did I unduly oversimplify the calculation by using a thickness of 2.25" for the combined "beam" height of the car decking and the Advantech. I noticed a disclaimer on the beam calculator about its being valid only for dimensions beyond 5"x5".

4. Should I consider "industrial" grade decking?

The purpose of the beams and the decking is to provide an aesthetically appealing view in the basement office below. I really don't want to exceed three beams in that area.


Thanks,

Ben (I had to use my wife's account because I can't seem to unlock my old account).
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Old 02-08-2018, 04:23 PM   #2
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I may be wrong, but it's my understanding that that type of construction was evaluated and found sufficient for ceramic installations, but not for natural stone.

You do, absolutely need something on top of the dimensional wood. 3/4" Advantec should be adequate for ceramic.
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Old 02-08-2018, 05:01 PM   #3
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I have no idea what you could do, if anything, to bring that joist structure up to spec for stone, other than adding a lot more joists. I would look into a porcelain tile that you could use in place of the natural stone. It would require much less work for you on the joist structure.
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Old 02-08-2018, 05:12 PM   #4
kristinmcall
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I think I might be able to achieve l/720 in part because the beams are not the limiting factor, e.g., mine span a drastically shorter distance, they are 2.5 inches wider, .75 taller, and they're spaced 2.5 inches closer. As far as the subfloor is concerned, I'm using 3/4 Advantech instead of 1/2 ply, the spans between beams/girders are nine inches smaller, the decking spans are continuous, and I'll have full edge support of the panels on the short dimension. If necessary, I'll go to 1-1/8 Advantech, or 2-ply 3/4 before I use more beams or alternative tile. I'm curious where I might have gone wrong with the calculator. I was getting under .03 inches of deflection between the joists.
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Old 02-08-2018, 06:57 PM   #5
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Quote:
Originally Posted by Kristin
Did I unduly oversimplify the calculation by using a thickness of 2.25" for the combined "beam" height of the car decking and the Advantech. I noticed a disclaimer on the beam calculator about its being valid only for dimensions beyond 5"x5".
First, you should probably believe the disclaimer.

Second, you can't just add the heights like that. You could if advantech was solidly glued to the car decking (although there's an incompatibility in expansion due to humidity that would probably make that unviable in the long run), then the two layers would act as a single 2.25" unit. But with periodic mechanical fasteners, a better approximation is simply that the two layers are lying on top of each and free to slip relative to each other.

In which case you should calculate the deflections for each layer separately and then add their denominators. Eg if the car decking is L/400 and the advantech is L/150, the combined deflection would be L/550.

However, the L/720 criterion for natural stone is only for deflection along the joists. The deflection between joists is specified via a minimum known good assembly, such as 3/4" plywood plus 1/2" plywood spanning 16" o.c. joists. So it's not as simple as calculating the deflection of your combined subfloor between your 6x8s. You might be better off using flush upright 2x4s at 16" o.c. between your 6x8s.

Cheers, Wayne
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Old 02-09-2018, 12:58 AM   #6
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I appreciate the idea, but I think the 2x4s between the beams might look a little odd.

I'm looking at a uniform load span table from Huber that shows the 1-1/8 product alone will achieve l/720 with 41 PSF at 48-inch centers and 63 PSF at 40-inch centers (assuming 1.5-inch perpendicular supports). My clear span with a 6" support corresponds to 41-inch centers with a 1.5" support. Wouldn't the 2x6 t&g push the allowable load from about 60 PSF to over 90 PSF, especially since it bridges four spans?
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Old 02-09-2018, 12:57 PM   #7
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OK, let me reiterate that the L/720 deflection criterion for natural stone installations is to my understanding only for checking the joist deflection. Industry standard call for controlling subfloor deflection between joists by using one of several minimum acceptable assemblies, all of which are based on joists at most 24" o.c.

I don't have a TCNA manual handy, but the Schluter Ditra Installation Handbook is readily available. So if you want to use Schluter Ditra, they have two assemblies for natural stone that would be relevant to your design, both for joists up to 24" o.c.: D-W-S-18 call for a layer of 3/4" plywood plus a layer of 3/8" plywood and D-SP-TS-18 calls for a layer of 3/4" structural planks plus a layer of 1/2" plywood.

Thus your safest option is to provide joist support at 24" o.c. or better (22.5" clear span), and use one of the above assemblies with Ditra, or use an assembly from the TCNA manual. I take it the underside of this porch floor will be visible and that's why you don't like the 2x4s? You could use 4x6s if you like. [The clear span is what matters, so you could go 26" o.c. with 4x6s, it would still be 22.5" clear span.]

Or if you don't want to have such frequent cross members between your 6x8s, you have a variety of other options: Add a 4th 6x8, then I think your clear span between them is about 24", you'd be cheating a little. Add a 4x6 in between each pair of 6x8s, parallel to the 6x8s; I don't think it could span the 14'8", so you'd want one central crosswise 4x6 between the 6x8s to split the parallel 4x6 into two spans. Or any other design you like where the joist deflection comes out to L/720 or less, and the clear span for the subfloor is 22.5" or less.

Alternatively you can try to extrapolate from the acceptable assemblies an assembly to use for your 44" o.c. condition, but you'd be leaving the realm of industry guidance and be on your own. From first principles, it should be OK to scale up D-SP-TS-18 by a factor of two, so use 1-1/2" thick planking (as you plan) and 1" nominal plywood (which seems like overkill). Of course, that is not a true scaling, since the plywood would not be in 8' by 16' sheets, but it would be close.

If you want to try to use deflection ratings for plywood to extrapolate from the accepted assemblies, you should start by figuring out the deflection rating of the two layers of plywood in D-W-S-18 for 22.5" clear span at some standard load (e.g. 10 psf dead plus 40 psf live). Then find an assembly of 2x6 car decking and plywood that would give you the same deflection rating at that standard load at your clear span. [Edit: alternatively, you can determine the allowable load at some standard deflection to compare assemblies that way, since the plywood tables are often presented that way.]

But again, these extrapolations are not warrantied, and industry guidance is to simply provide supports at 24" o.c.

Cheers, Wayne
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Old 02-09-2018, 01:05 PM   #8
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One last comment that I'm not sure I believe: in theory you could treat your car decking as your joists, and the 6x8s as girders. If that calcs out to less than L/720 combined, then your plywood would effectively be continuously supported by joists. In which case maybe 1/2" plywood (or 3/4" to safe) would be enough?

Don't believe the above unless someone else here endorses it.

Cheers, Wayne
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Old 02-09-2018, 01:56 PM   #9
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Wow, thanks for your extensive reply. I'll have to reread that a few times. I also found and read your extensive dialogue on the PNW installations.

At any rate, my intent was to run the 6x8s across the 7'2" span with the 16' lengths of 2x6 t&g bearing on those beams. I imagine two layers of 1-1/8" on top of the decking would eliminate all doubt, correct? That would give me a subfloor that is 3.75" thick. I guess the room underneath might make a good vault once I'm through. 😀
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Old 02-09-2018, 03:27 PM   #10
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Plywood and/or decking primarily provides support in between the supports...your joists provide the strength along them. You need both of those components to be sturdy enough to ensure your natural stone installation will survive. Previous work by a structural engineer ran the numbers for this type of install and found that it did not provide the necessary strength between the joists for use with natural stone, but was sufficient for ceramic. Now, natural stone is just that, a natural product with sometimes unknown internal defects...it might work fine, it might not. The specs are designed to help ensure it takes out that 'if' factor. ARe you feeling lucky?
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Old 02-09-2018, 03:51 PM   #11
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Quote:
Originally Posted by Jim
Previous work by a structural engineer ran the numbers for this type of install and found that it did not provide the necessary strength between the joists for use with natural stone, but was sufficient for ceramic.
This gets quoted alot here, but what span was used for the car decking in the analysis? I think sometimes this install is done with a 6' span, which would be alot less stiff than a 4' span. At first glance, I would think that if the assembly is good for ceramic tile with a 6' span, it would be good for natural stone with a 4' span, but don't quote me on that.

Cheers, Wayne
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Old 02-09-2018, 04:09 PM   #12
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Quote:
Originally Posted by Kristin
At any rate, my intent was to run the 6x8s across the 7'2" span with the 16' lengths of 2x6 t&g bearing on those beams.
Ah, I thought your count of (3) 6x8s included the boundary elements, but you have some other supports at the boundary and just counted the intermediate elements.

You could, of course, double the number of beams (and optionally reduce them to 3x8 or 4x8) to get the decking clear span down to under 23". And then you could reduce the decking to 1x T&G material if desired. That would certainly be the conventional approach, but I take it the framing will be exposed and you have a certain look you are going for?

Cheers, Wayne
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Old 02-10-2018, 11:09 AM   #13
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Welcome, Kristin.

Couple comments:
Quote:
Originally Posted by Kristin
Alternatively, if I find the time, I will use mixed hardwoods milled on my property to the same dimensions.
Keep in mind that your site-milled beams will not be kiln dried and will not, therefore, be as stable as KD material. Your hardwoods will also not likely be as rigid as your pine beams.

I do not see any indication of the spacing of the supports for your beam/joists. That would be a significant consideration in achieving your desired (and necessary) L/720 deflection criterion.
Quote:
Originally Posted by Wayne
This gets quoted alot here, but what span was used for the car decking in the analysis?
That calculation was made for a car decking span over nominal 4" wide beams at 48 inches on center, Wayne.

My opinion; worth price charged.
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Old 02-10-2018, 11:56 AM   #14
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Quote:
Originally Posted by CX
That calculation was made for a car decking span over nominal 4" wide beams at 48 inches on center, Wayne.
Thanks, CX. The calculation came out that the deflection would be between L/360 and L/720, so adequate for ceramic tile but not for stone?

Suppose we calculated the exact deflection for the OP's situation and found that the deflection of the beams and the car decking together was less than L/720, would you say that would mean it would be sufficient to use just 1/2" nominal plywood over the car decking to provide a stable base for a tiling substrate?

Cheers, Wayne
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Old 02-10-2018, 01:04 PM   #15
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I thought I'd check the deflection characteristics of the framing system described in the OP. I'm going to assume all wood is #2 grade and the 2x6 T&G decking is full length (no butt joints). I'll use 50 psf live load as that is what the Deflectolator uses, and to represent the standard 40 psf live load plus a 10 psf allowance for creep.

The AWC Span Calculator states that for #2 grade EWP 2x8s at 12" o.c. with 10 psf dead/50 psf live and L/720 deflection the allowable span is 8' 4". Note that 5.5" width of wood every 44" is same as 1.5" width of wood every 12", hence the choice of 12" o.c.

So the beams check. With a deflection of L/720 at 100" span, the actual span of 74.5" gives a deflection of L/1740 via the deflection equation. Namely that D/L varies as w * L^3 / E * I, where D is the absolute deflection, w is the uniform load, L is the span, E is the modulus of elasticity and I is a second moment of inertia, a property of the cross-sectional shape of the member.

As to the deflection of the 2x6 decking, I've run across this document from the AWC, "Tongue and Groove Roof Decking". It states in Table 2, page 9, that for 1-1/2" thick decking, supports 6' on center, a 2-span continuous layup (the best case, each deck board has at least 3 supports, and butt joints are staggered), and wood with a modulus of elasticity of 1.1E6 (EWP), the allowable load for a deflection of L/180 is 118 psf.

That number is not directly useful for us, but we can adjust it using the deflection equation. [I spot verified the tabulated numbers in the AWC reference obey this relation as expected.] So a 2-span continuous layup with 4' on center supports, a deflection of L/720, and E = 1.1E6 gives an allowable load of 100 psf.

That suggests for the 2 span continuous layup condition, the deflection of the car decking with a 50 psf load is L/1440. While it is not clear how to properly add this to the L/1740 deflection of the beams, as they are in perpendicular directions and have different values of L, I would suggest that as they are both less than or equal to half the allowable deflection of L/720, the combined deflection is less than L/720.

Also, please note that per the AWC document referenced, the deflection of the 2x6 decking varies considerably according to how the deck boards are laid out. The 2 span continuous layout is the best case; the worst case is a simple span, where each deck board has only 2 supports with butt joints over every joist. The allowable load for the simple span condition is only 43% of the allowable load for the 2 span continuous layout, which would mean that for a generic worst case analysis, the car decking on 4' supports would be adequate for ceramic tile but not for natural stone.

Cheers, Wayne
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