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Unread 08-06-2022, 11:20 AM   #1
Brody
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plywood orientation to joists

Hi, Im renovating a kitchen/powder room in a 1912 heritage house.
the area were renovating is the old kitchen area (170 sq.ft) and a newer addition added in the 90s (160 sq.ft)
The old part of the house settled over the years (2 1/2" over 16') so we did some floor leveling by cutting the joists to create a level floor.
The newer part of the house only had to be cut down 1 1/2".
As per the engineers instructions we sistered the joists in the old part with LVL material. The new part didnt need sistering as the joist were cut to 8" with a 9' span.

the newer addition joists run perpendicular to the old part of the house, to keep the continuity of the subfloor layout we ran it all the same way, so the newer section doesnt have plywood running perpendicular to the joists.
the plywood is glued/screwed and nailed with .131 nails.

MY QUESTION: will this pose a deflection problem where the plywood runs parallel with the joists?

addtionally we will be using a schluter membrane for uncoupling.

hope this makes sense

Thx
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Unread 08-06-2022, 12:12 PM   #2
cx
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Welcome, Stew.

Somewhat surprised your engineer was happy with your ripping your joists, given that you now don't know what grade of lumber you're now working with. But if your unsupported span is only 9 feet, and if your joists are a full 8 inches deep, and if your joist spacing is 16 inches on center or less, you should be within the required design deflection of L/360 or better unless the joists are in rather poor condition.
Quote:
Originally Posted by Stew
...the plywood is glued/screwed and nailed with .131 nails.
Goodness! The screws and glue would have been plenty if the schedule was adequate.

But the plywood orientation is the problem. You don't indicate the thickness or grade, but the incorrect orientation makes it almost certainly insufficient and I'm surprised the engineer didn't point that out. Wish you hadn't done that.

Post 10 in the Shower Construction Thread in our Liberry will give you some insight into the problem.

My opinion; worth price charged.
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Unread 08-07-2022, 01:36 PM   #3
Brody
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Thx CX.

Joists are 2 and better fir, plywood is 3/4” t and g fir.
Everything feels pretty solid, that being said I’ll have the engineer take a look at it.
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Unread 08-07-2022, 02:16 PM   #4
cx
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Quote:
Originally Posted by Stew
The new part didnt need sistering as the joist were cut to 8" with a 9' span.
Again, Stew, the joists were 2 and better before you ripped them down from whatever size they were to begin with. They're still Fir, of course, but not necessarily 2 and better.

You don't indicate the joist spacing. Most manufacturers of ceramic tile substrate materials will accept nominal 5/8ths" exterior glue plywood over 16" joist centers as their minimum requirement (which I consider borderline crazy), but your incorrectly oriented nominal 3/4" ply, presuming a 5-ply construction, will have barely more than 3/8" of material oriented in correct direction. I have no way to do meaningful testing of something like that, but I'd wager your incorrectly oriented 3/4" material would test less rigid than properly oriented 5/8ths" material.

Entirely up to you, though, and it's all about risk tolerance. Keep in mind, though, that when the plywood is tested as part of an entire tiled package to determine those minimums, it's done using new material, in perfect condition, near perfectly installed over joists with zero deflection. And the test needs pass only once.

My opinion; worth price charged.
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