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Unread 02-26-2008, 04:59 PM   #1
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Join Date: Feb 2008
Posts: 2
Subfloor deflection advice (1st timer)

Hi all:

Was impressed with previous posts that alluded to the structural deflection--tile interaction, so I thought I would get some help for my own project...

I want to tile a 15 sq ft. bathroom using 12"x12" ceramic tile. After removing the existing underlayment and flooring I find I have 2x8" tongue and groove subfloor, resting on top of 4X6" beams spaced 6' oc perpendicular to the T&G and spaced 4' oc parallel to the T&G.

This situation does not fit neatly into the "Deflecolator" and my inexperience with this subject necessitates that I seek advice. From what I learned already I should use a minimum of 5/8" plywood (structural) and some 1/4" backer board (moisture). The floor seems solid (jump test), but the crawl space would allow easy placement of 2x6" joists, 16" oc, over a 6' span (this translates to a deflection of L / 839). Is it needed?

Also is there an alternative to backer board that will minimize the height increase and still provide moisture protection?

Any information is appreciated.

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Unread 02-26-2008, 05:22 PM   #2
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Join Date: Nov 2003
Location: LaConner, Washington
Posts: 13,694
You must be out here on the West Coast. That type of post and beam construction is quite common out here. Have it in my house as well. Deflecto does not measure deflection for that type of construction but we know from many past threads on this subject that a layer of 3/8" minimum plywood over the T&G planks (AKA car decking) works just fine for tile. Make sure the plywood is CC grade or better. No CD grades should be used under tile. I'm assuming you will go with either a ceramic or porcelain tile. That correct?

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