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Unread 10-23-2020, 08:58 PM   #13
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Join Date: Oct 2020
Location: Queens NY
Posts: 17
Good to know about the AC/BC/CD/X, I figured it was mostly BS, wood and water don't mix no matter what the label says!

The L/720 is unfortunately not possible to calculate in my situation using the Deflecto, which does not work for engineered lumber or account for second layers of ply above the sub-floor. I'm trying to beef things up and re-enforce best I can where possible. like adding 3/4" as a second layer above the subfloor, more than the 15/32" required by F250.

Before I ripped it up, the floor was holding 12x12 ceramic tile, on an 15/32" CD underlayment nailed to the subfloor (<20 nails/sheet?), with no CBU or mortar of any sort, just some sort of yellow glue holding the tile to the ply for 30 years with no cracks.... I hope to improve in this!

I figure adding 3/4" quality ply will help stiffen the floor with lots of screws (no glue; learned my lesson there), plus joists being engineered or LVL, and 19.2 OC is not as far off of 16" as say, 24"

.... basically, my joists aren't 16" OC but 19.2; I can't change that, so I add an extra 1/4" of plywood thickness over the F250 minimum to compensate.

Is there a better IRL test I can do for deflection other than putting the laser level in the middle of the floor, marking the wall, then adding a 400lbs of live weight in the middle? (Also did the string version of this, could not see a noticeable change in the laser/string)

Any thoughts on self-leveling "liquid backer board" self-leveling type products? I'm not quite convinced they can replace CBU, but would make cabinet installation and tiling a lot easier if the floor was truly level!

Last edited by risotttto; 10-23-2020 at 09:21 PM. Reason: laser test
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