Floor Joist and Subfloor Questions


Old 01-22-07, 08:29 PM
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Floor Joist and Subfloor Questions

I'm remodeling a 9' x 12' bathroom and want to use 1/2" thick x 16" square travertine tiles on the floor. I currently have 10" I-joists on 24" centers. The unsupported span is 12' in most areas and 8'6" in others. I'm adding 2x10" dimensional joists in between the existing I-joists. This will then be 12" on center joists. What kind of subflooring will I need to support the tile properly? I would like to keep the floor as low as possible so I don't have transition problems between the adjoining carpet. Would one layer of thicker subflooring (say 1 1/8") plus cement board suffice or should I plan on 2 layers of 3/4" t&g plywood plus cement board? Thank you for your input.

Last edited by wiebster; 01-22-07 at 08:32 PM. Reason: clarification
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Old 01-23-07, 07:35 AM
Join Date: Jan 2006
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Im not sure how to go about calculating deflection for a floor that has a mix of Ijoists and dimensional lumber. Before you do that, check with the Ijoist manufacturer to see if there will be any issues. If you ignore the fact that you have this mix and calculate deflection as though its all dimensional 2x10's 12" on center with 12' unsupported span you just make the L720 required for natural stone. The Ijoist manufacturer's usually have good information web sites and phone numbers for questions about their product. Check their website and if you cant get answers there call them. Concerns would be mixing and matching the I joists with the dimensional lumber and if you will meet L720.

Use two layers of plywood totalling 1 1/4" or better. Use exterior grade bc or better plywood. From there you could use 1/4" cement board bedded in thinset and screwed, or you could use an uncoulpling membrane like ditra. Sometimes when you just meet minimum deflection, the uncoulpling membrane is a little added protection and the way to go.

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