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Unread 11-27-2020, 10:28 PM   #11
Veteran DIYer- Schluterville Graduate

STAR Senior Contributor

Join Date: Jul 2004
Location: Nashua, NH
Posts: 14,233
Originally Posted by Paul
I thought the adhesive would bridge the voids between plywood and uneven floorboards and work like a shim. Does that not work?
The reason why there's a minimum thickness of ply is so that it can support itself as long as it is screwed to the existing subflooring material. But, that first layer of planks must be anchored well to the joists first, if required. As was said, the reason to use ply over the planks is because plywood is MUCH more stable than dimensional wood, so it doesn't change as much with temperature or moisture content changes. What tends to happen if you try to glue solid wood to plywood is that the differential movement can put a lot of stress on the joint, and literally break it over time. Prior to that, it tries to transfer its movement through to the other layer, which you are trying to avoid.
Jim DeBruycker
Not a pro, multiple Schluter Workshops (Schluterville and 2013 and 2014 at Schluter Headquarters), Mapei Training 2014, Laticrete Workshop 2014, Custom Building Products Workshop 2015, and Longtime Forum Participant.
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