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Old 04-29-2019, 03:12 PM   #1
BasicDIY
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Slate - Deflection - Used Every Resource So Far

Hello,

I have devoured the boards here. I am trying to get this right and to set ourselves up for a successful install.

I cannot find the series for my engineered joists. I have attached the logo on the osb. There is no printing on the flange. Does anybody recognize this? Is it not a TJI?

The joists are almost 12"D, the flange is just shy of 1.5". They are spaced 16oc and the span is 12.5', well that it where the joists rest on the huge support beam that runs through the basement. Otherwise, they are 25'.

I wish to install slate on my first floor, around 600 square feet in the kitchen, bath and living room. 1 layer of the subfloor and 1/2" hardie backer.

I called Weyerhauser already and they need to know the series which I could not find.

I also called 4 structural engineers that didn't answer the phone.

Any help/advice/criticisms would be welcome and greatly appreciated.
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Old 04-29-2019, 03:42 PM   #2
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Hi Catherine,

Those look like Weyerhauser 110 series joists x 11 7/8 height . You can contact their technical support people for deflection rating information for the span and spacing your identified.

I'm expecting hopeful those joists will meet requirements but you’ll need to confirm with Weyerhauser.

You’ll also need another layer of plywood over that subfloor to meet requirements for natural stone.

Last edited by PC7060; 04-29-2019 at 05:56 PM.
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Old 04-29-2019, 05:41 PM   #3
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Here's a couple links to their tech-support page. You can get an idea of the series by the age of your house versus the series dates.

https://www.techsupport.weyerhaeuser...egacy-Products

https://www.techsupport.weyerhaeuser...ure-2003-2008-

Once you have that, the Weyerhaeuser folks will be able to give you more information.

Also, given the number of missed nails in the photo provided, I'd also take some time to set screws from the primary subfloor into the TJI at a spacing of between 12-18 inches.

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Old 04-29-2019, 05:44 PM   #4
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There is no structural benefit from thicker cbu...you only need it if you're trying to match heights. CBU is a compatibility intermediary layer, not a structural one. You absolutely need a second layer of plywood, minimum of 3/8" thick, but it's almost impossible to find flat 3/8" ply in the required grade...most of the stuff is warped like a potato chip. You really should consider at least 1/2" ply.

To minimize the overall build-up, you could use a membrane like Ditra, which is about 1/8" thick installed.

FWIW, unless stone was in the original design specs, most of the time, trusses or I-joists are usually engineered for L/480 or so. That's below the industry recommendations for stone (L/720). You won't know for sure until you can talk to their engineers.
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Old 04-29-2019, 05:54 PM   #5
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Ha, just going to post about the CBU but you got in there before me. Nice detailed post, Jim (as usual).
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Old 04-30-2019, 08:01 AM   #6
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Thank you so much for taking the time to help. We certainly have a lot to consider now.

If we take up the CBU and add another layer of subfloor can we reuse the cbu and screws? Should we scrap it and go with the 1/4"?

We had read that 3/4" subfloor plus 1/2" backer would be fine. I am looking for that resource now.

I will be back on the phone with technical support for the joists as soon as they open.

Thanks so much!

The house was built in the early 90's but no pictures are matching the stamps on these joists.
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Old 04-30-2019, 09:24 AM   #7
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Based on the age of the house it would appear you have the following series of joists; TJI 15 is my best guess. This is also the only time I’ve seen TJI specs broken out by wood species; DF - Douglas Fir or SP - Souther Pine.

We don’t know what region you are in so hard to even guess which of the two type you might have.

https://www.techsupport.weyerhaeuser...bile_site=true.

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Old 04-30-2019, 09:34 AM   #8
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Re taking up the CBU; you will need to pull it up and install the second layer of plywood. Did you screw it down over a layer of thinset?
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Old 04-30-2019, 10:42 AM   #9
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No, we did not apply thinset it is just screwed, lots and lots of screws.

Weyerhaeuser thinks we have TJI 250s from the late 80's, I too thought SP 15. It's the marking on the webbing throwing everyone off. Also LVL.

Even with worst-case scenario, I think we are over the 480 mark, but by how much. The general consensus is that we are fine, I would like a number and the math hurts .

I think we will pull up the backer board this weekend.

Getting to this point was hard, I'll reinforce the joists for this slate.

Also, I'm in Maryland, Eastern Shore area.
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Old 04-30-2019, 11:04 AM   #10
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Remember, 480 doesn't get you there, you need 720! You are probably not there at all.

Since you don't have thinset under the CBU it was going to have to come up, anyway. Is the span 12.5' everywhere? that will help, but I expect you still don't have L/720. But, since you seem to have full access in the basement you can beef things up. I'll leave to the qualified experts to suggest the best solutions!
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Old 04-30-2019, 11:22 AM   #11
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Yes it 12.5 everywhere, the beam runs through the middle of the house.
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Old 04-30-2019, 02:07 PM   #12
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Iíll see if I can find the performer details for those joists tonight. If so I can run the deflections calculations to get you an guesstimate on the floor rating.
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Old 04-30-2019, 02:20 PM   #13
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Catherine, if you don't add that geographic location to your User Profile it will be lost before we leave this page.
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Old 04-30-2019, 03:06 PM   #14
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Every cbu manufacturer requires the panel to be bedded in thinset. That's more to ensure there are no gaps rather than trying to actually bond it in place...the screws do that. So, just that factor alone, regardless of the type of tile, would be problematic. Because stone can have (maybe lots of) internal weak spots versus a manufactured ceramic tile that is more homogenous, that is why they call for the floor to be stiffer.

A single layer of plywood where the end joints are over a joist or web, act like small levers as you flex the subflooring in between. That small lever tends to put a lot of stress on any rigid tile above it, and radiates forces that can crack it. Putting a second layer of ply over the first, when done properly, essentially prevents that from telegraphing into the tile.

What is critical in any installation, is to read, understand, and follow the installation instructions if you want a long-term, successful result. That includes achieving or exceeding the minimum industry guidelines. For stone, that's L/720 and two layers of subflooring, properly installed. CBU does not count as a layer of subflooring.
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Old 04-30-2019, 03:42 PM   #15
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Ok, I updated my profile to show my location. I will not lay a tile without consulting here first, promise. If we live through this install our upstairs bathroom is next.

I love this slate, it reminds me of every place I've been. I see Yosemite... Glacier and Yellowstone in the colors, anyway the tile is here. And we thought we had met deflection before we bought it...but now doubt.

We are going to put another layer of subfloor on this weekend. I think we will probably just reuse the hardie backer. I read an article that is here about how to install the second layer and why it's important so we will follow that method.

I attached the documents weyer, sent over.

Seriously thank you, you have prevented some major mistakes.

It's a mess, I admit.... but it's going to be worth it....And now
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File Type: pdf NER 200 march 88.pdf (2.81 MB, 12 views)
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