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Unread 01-11-2020, 12:52 PM   #1
syncrodriver
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Tiling over web truss 24" oc 25' Span

Hey guys,
Ideally this is best answered by an engineer but we are working on a project and we are building a master bathroom that was once a bedroom. We have removed the 3/4" existing subfloor and have revealed the truss's. They measure 24" oc, 16" height and span 25'. I have never dealt with these truss systems before and want to make sure I make the right move before moving forward.

My initial plan of action was to glue and screw 3/4" ply hardwood back down to the existing web joists. Then follow up with a second layer of 1/2" advantech osb offset spacing over top of the 3/4". For setting tile we planned to either use install cbu or ditra depending on surrounding transitions. Open to others past experiences with this type of situation. Yes somebody really did some framing sins on that back wall. That is being reframed! Thanks in advance guys!
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Unread 01-11-2020, 06:43 PM   #2
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Welcome, Charles.

No one can tell you the design deflection of that joist structure except the manufacturer. Can you find a manufacturer's name or perhaps a model number on one of the joists?

That's a very long unsupported span. Was the structure built within a code compliance jurisdiction requiring compliance inspections?

The original subflooring was not glued to the top chord of the joists?
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Originally Posted by Charles
My initial plan of action was to glue and screw 3/4" ply hardwood back down to the existing web joists.
Not sure just what you might mean by "3/4" ply hardwood," but you do not want to use any hardwood veneer plywood anywhere in your subfloor package.

I would want and appropriate plywood for your second layer unless you elect to use the CBU instead of the Ditra as your tiling substrate.

Your plan (presuming the joist structure is adequate) is fine, I question only your choice of materials.

My opinion; worth price charged.
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Unread 01-11-2020, 10:29 PM   #3
syncrodriver
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Thanks Cx!

We have a 3/4" 5 ply fir plywood for our base layer of 3/4" to be glued and screwed to the joists. What materials would you suggest if the joist structure is acceptable?

Definitely code compliance jurisdiction. Plywood was nailed down no glue. Removal was a breeze. Floors are squeaky all over the house!
Thanks for your advice!
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Last edited by cx; 01-11-2020 at 10:38 PM.
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Unread 01-11-2020, 10:49 PM   #4
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The reason for the question about the code compliance jurisdiction is that engineered floors can nearly always be counted upon to at least meet building code, which is usually a deflection of L/360 and is the same as the requirement for a ceramic tile installation according to the tile industry. Note the operative word above would be "nearly." I'd still recommend you try to find the joist manufacturer's identification and try to verify what you've got there.

A first layer of nominal 3/4" exterior glue T&G plywood with no face of grade lower than C is a good start. A nominal 1/2" layer of a similar product for the second layer would be my choice.

This good article
from our Liberry shows what I think it the very best method of installing those layers.

My opinion; worth price charged.
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Unread 01-12-2020, 06:55 AM   #5
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Interesting joists, have never seen them. You're certain there isn't a load bearing wall somewhere along the 25' span?

Looks like that back wall was reframed at some point to add a partition wall and a smaller window.
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Unread 01-12-2020, 10:13 AM   #6
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First time I have seen this style of joist as well. Engineer came out last week and confirmed that joists span from one party wall to the next. This is a townhouse.

I am attaching the spec sheet for the 3/4” ply my supplier recommended. Have no issue getting a different product if this is incorrect. I want make the right moves here and appreciate any of your expertise. Also would you suggest liquid nails between the 3/4” and 1/2” layer or brushing on titebond.

Cx thanks for that article in panel install. Great read and extremely helpful.
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Unread 01-12-2020, 11:38 AM   #7
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Hi, Charles. I don’t see the spec sheet.

In regards to gluing layers. If you decide to glue layers, construction adhesive would be super difficult to use and get good results. You want FULL coverage of the adhesive between the layers of ply (every square inch) and ribbons of heavy-bodied construction adhesive gunned out of a tube that wouldn’t want to squish absolutely flat would create a bunch of micro gaps between the layers that would allow downward deflection. That kind of deflection subtracts, rather than adds, to the integrity of this tile assembly. Ordinary white glue that is spread with a small notched trowel would work. But it would be an intensive process, as glue dries quick so you’d have to work fast and fasten with a tight pattern of screws...taking care to not screw-jack the plywood layers apart.

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Unread 01-12-2020, 12:16 PM   #8
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Sorry here is the spec sheet.

https://www.roseburg.com/Product/rid...-underlayment/
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Unread 01-12-2020, 04:22 PM   #9
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Be nice if those manufacturers would just tell you what they're selling. I would appear to be an exterior underlayment, which according to PS-1 would make it of face grade no lower than C Plugged and interior plies at least C grade (I think) and an exterior glue. That would make it suitable for your subflooring application, Charles.

My opinion; worth price charged.
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Unread 01-12-2020, 06:01 PM   #10
syncrodriver
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Thanks Cx! So I'm assuming if the engineer gives me the ok to use 1/2" I would assume a grade similar to the above. screwed ever 6" and not penetrating the joists, subfloor only.
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Unread 01-12-2020, 06:19 PM   #11
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I would say so, Charles. See my warranty information below.

What you want is an exterior glue plywood with no face of grade lower than C. T&G is not necessary, sometimes not desired, for the second layer of subflooring.

For installing the second layer, you want fasteners (screws only for my floor) long enough to fully penetrate the bottom subfloor layer. You don't need to be paranoid about hitting a joist, just make an effort not to fasten into the joists. If you stagger your top layer as indicated in that article and pay attention to your fastener spacing, you should have no trouble missing the joists for the most part.

My opinion; worth price charged.
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