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Unread 04-29-2002, 12:38 PM   #5
Bud Cline
Tile Contractor -- Central Nebraska
Join Date: May 2001
Location: Central Nebraska
Posts: 7,567

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APA OSB and plywood subfloor sheathing

Hey Guys (Gals?), I’m an Engineered Wood Product Support Specialist working at the APA Help Desk. I am not an experienced tile setter. So why am I on your thread? Well Dog Paws asked us (APA) to read the conversation and comment on gapping issues as related to OSB subfloor sheathing. So here goes, hopefully I’ll add to understanding rather than confusion.

As stated in APA literature: 1/8 inch spacing is recommended at all panel edge and end joints (subfloor, wall and roof) unless otherwise indicated by the manufacturer. Spacing is an APA recommendation intended to minimize the risk of panel buckling when panels are exposed to moisture during construction or acclimatize to a higher moisture content after installation.

When inspecting an existing structure it may be very difficult to determine if panels were properly spaced at the time of installation. If the panels were dry at the time of installation and subsequently acclimatized to a higher moisture content the space (gap) will be diminished. The gap may even be fully closed if moisture conditions were high enough to cause significant panel expansion.

(Properly manufactured tongue and groove joints will provide a gap between the faces of adjoining panels when the tongue is snugly fitted in the groove.)

If the subfloor has been exposed to moisture during construction, there is no gap between panels and the panels are not buckled it is likely that the panels were properly gapped when installed and subsequently expanded to close the joints. Unless you expect the panels to further expand providing additional gaping is not necessary. If you choose to create a gap between panels care needs to be given to any tongue and groove edges. The tongue and groove assembly is a structural component of the floor system. If it is cut through blocking or structural underlayment (minimum 1/4-inch plywood or OSB) or 3/4-inch wood finish floor must be installed. It is also important to avoid cutting into underlying framing.

Ridging at panel joints may occur for a variety of reasons, edge swelling due to moisture exposure; twisted, uneven or misaligned framing, debris on the framing, over driving tongue and groove edges during assembly, compression of panel edges due to expansion, buckling, etc. In most cases ridged joints can be made flush by sanding.

I hope this information is helpful. Please let us know if we can supply you with further information. You can submit inquiries about engineered wood products to us at:
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