Old House w/Varied Joist Spacing > 24" O.C. [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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12-16-2017, 07:50 PM

I'm working on a bathroom remodel in an old (1830's) stone house. I have the bathroom demo'd to the studs and subfloor and have a concern about the spacing of my joists. I have some history on the place from the seller (who grew up in the house) who told me that the joists and main center beam were replaced by his dad years ago, I believe in the 1950's or 60's. The joists are very solid but are not evenly spaced - under the bathroom the spacing varies from 25" up to 35" o.c. I think the irregular spacing may be due to fact that the outside ends of the joists are set directly into the stone foundation wall and they didn't want to dislodge any major stones...just a theory I really don't know. Anyway, here are some details on the joists and subfloor:

Joists are 7.25" D x 3.5" W. I couldn't find any identifying stamps on the wood but they are in great shape. Spacing varies under bathroom from 25" to 35" o.c. as mentioned above. Span between supports measures 9'-3". I ran Deflecto but obviously was only able to check 24" spacing - I got L/647.

Subfloor is 3/4" T&G planks that are 4.5" wide. They are also in good shape although I will need to repair some large holes that were left in the tub and toilet areas after demo.

I want to install 12" x 24" porcelain tile. After reading a number of the threads on here regarding subfloor prep and joist spacing, etc., I planned on patching the subfloor and installing a layer of 1/2" plywood (Exposure 1 rating), followed by Ditra and tile. However I could only find this recommendation for joists on 24" centers and can't get a comfort level that will allow me to proceed.

Am I worried over nothing, or do I need to address the joist spacing before proceeding?


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12-16-2017, 11:08 PM
Jon, if not for the greater-than-24" spacing in places, I would tell you to move ahead with your plan, but I can't tell you what your joist deflection would be over the entire floor, nor if the subfloor and proposed plywood you have in mind would be enough for the deflection between the joists on the wider spacings.

CX may be able to answer that for you when he gets back online, or you might just have to have an engineer to calculate that for you.

For what it's worth, my guesses would be "no" on the subfloor and "maybe" or "probably" on the joist deflection. :D

12-16-2017, 11:24 PM
How many joist bays have more than 22.5" clear between the joists? Are those joist bays free of obstructions, so you could add a joist in them if required? You could do that by adding short 2x8 joists perpendicular to and between the common joists, at either end of the area to be tiled, and then running a new 2x8 joist between the short joists.

Cheers, Wayne

12-17-2017, 01:26 PM
Thanks Kevin, your guesses are worth more than mine.

Wayne - there are 3 joist bays that have clear spacing greater than 22.5". I would have to work around some wiring but that sounds doable. 3 2x8's would be cheaper than a structural engineer...

12-17-2017, 03:02 PM
I think adding the 3 joists is the simplest solution. Use kiln dried lumber to match the moisture content of your old lumber. Since your common joists are 4x8s, I would use joist hangers to attach everything. You only need to use the short joist hanger nails when nailing into a single 2x material; for nailing into the 4x8s you can use regular nails of the proper diameter.

On the wiring, if it goes through bored holes in the 4x8s, best to deenergize and disconnect one end and pull it out of the joists bays you are working in. Then after the new joists are in, drill holes to rerun the wires. If the wiring is stapled to the bottom of the 4x8s, you might be able to work around them if you are careful not to pull on them.

Cheers, Wayne

12-17-2017, 08:15 PM
The wires are run through holes in the joists so I'll disconnect and rerun as suggested. Thanks for the input, glad I checked here before proceeding.