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Unread 06-05-2014, 01:33 PM   #12
Veteran DIYer- Schluterville Graduate

STAR Senior Contributor

Join Date: Jul 2004
Location: Nashua, NH
Posts: 14,989
No, you do not want a hardwood ply, either (although a good high ply birch plywood can be used)!

You want something that says exposure I or exterior rated glue, underlayment, with the worst side at least grade C (or A or B)...IOW, no D faces.

If you want to know more about this, go to
and near the bottom, download the file: Data File: Selection, Installation and Preparation of Plywood Underlayment. It's free, but you have to register on the website to get it.
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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