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Old 06-14-2019, 07:51 PM   #12
jadnashua
Veteran DIYer- Schluterville Graduate

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Join Date: Jul 2004
Location: Nashua, NH
Posts: 12,944
The reason for two layers is to prevent the joints over the joists from bending upwards, potentially producing a stress crack. Adding a second layer, properly offset, prevents that first layer seam from projecting upwards enough to preserve the natural stone with potentially numerous internal imperfections. A manufactured tile is more consistent and can usually handle a single layer..

Think of the ends of the panels as little levers...a weight in the middle between the joists makes the edge of the joist act like a fulcrum, pushing the end up slightly even with the construction adhesive considering it's only attached by typically about 3/4" overlap.

IOW, use two layers to mask that joint's movement.
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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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