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Unread 03-03-2021, 03:24 PM   #12
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Join Date: Jul 2001
Location: Boerne, Texas
Posts: 92,804
While the sawn board subfloor doesn't meet the letter of the NSI requirement, we've always held that it complied with the intent. Back when they were still the MIA I attempted a number of times to get an official reading on that without success. I consider it adequate. See my warranty information below.

The reason for the requirement is to eliminate any places in the subfloor where a joint above a joist top extends all the way through the entire subfloor. Your second layer of plywood, properly installed, will provide that.

As for the joist deflection, there's a good story behind that that says a group from the MIA was sitting about considering the requirement and one or more suggested that it needed to be at least twice as rigid as the requirement for ceramic tile. I'm told alcohol was involved.

Will your L/480 deflection be fatal to your stone installation? I dunno. Laticrete allows that when one or more of their products is used for bonding, but I don't know the basis for that, either. Come back in 20 years and report on your installation. Unfortunately, I'll not be able to attend.

But don't cheat on the subfloor requirement.

My opinion; worth price charged.

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