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Unread 11-24-2022, 12:17 PM   #1
robbieallan90
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Joist Span for Porcelain Tiling Advice

Hello everyone. I've spent a few days searching for and reading posts on this forum that address situations similar to mine, and I have learned a lot about joist type, sizing, spans, and deflection. This forum is an excellent resource, and I appreciate that members address questions from newbies like myself with details that are particular to their unique situations.

In my case, I am considering installing 24"x24" porcelain tile through most of the main floor. My house was built in 1951, Alberta Canada, currently has wood and carpeting, with some tile in the bathroom. Joists are dimensional lumber, fir, that measure 9" depth to a little greater in some areas, and 1.5" wide.

The joists are spaced at 12" on center and the span is 16'. On top of this for a subfloor are tongue and groove 3/4" 8" wide planks arranged diagonally across the top of the joists. These can either remain (be screwed down, then 3/4" ply over top) or be removed and replaced with ply. Ceiling height is not a concern on the main floor.

The deflectometer gives me L333 for 9" tall, 1.5" wide, 12" on center, and 16' long with deflection of .576 inches.

Deflection isn't noticeable when walking on the floor, but things do shake or vibrate if someone jumps on the floor. I am not sure how best to address this floor to make it suitable for tiling, and I would appreciate some advice about how best to move forward.
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Unread 11-24-2022, 01:12 PM   #2
cx
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Welcome, Robbie.

If you don't put that geographic location into your User Profile, so it appears with each post, the information will be lost before we leave this page.

The ANSI standards and TCNA ("governing" bodies south of the 49th parallel) require a deflection no greater than L/360 for floor structures to receive ceramic tile, and that deflection coincides with building codes for wood framed structures down here. I'm not sure what the requirements might be in your neighborhood, but I suspect they'd be similar.

Can you tile over the joist structure you have? Sure. You can tile over anything you think is appropriate. We can tell you only what our standards require and where the smart money is likely betting. I can also likely find you a deflection calculator that indicates your joists, as described (except for the unknown grade) would meet the L/360 requirement. Our Deflectometer is a rather conservative tool, which is good because our visitors are almost always dealing with used, sometimes very old, lumber in less than perfect condition.

If you want to improve your joist deflection, we'd need more information, such as access, type of space below, etc.

Your subfloor structure would require the addition of a minimum of nominal 1/2" exterior glue plywood, with no face of grade lower than C, over the sawn boards. With 12" joist spacing, that would make a very rigid subfloor.

My opinion; worth price charged.
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Unread 11-24-2022, 02:44 PM   #3
robbieallan90
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Thats very helpful, thanks for your response. I have already removed my basement ceiling (about 8ft in height), and revealed underside the joists. The lumber actually appears to be of very good quality. There are very few knots, and no cracks, splits, warping, or any visible problems.

There are however an HVAC run and air return to each room such that any joist sistering would be inconsistent across the length of the room. There are also electrical wires run through holes in the joist. Not many of those, and nothing I could not take care of.

I am not opposed to laying down additional sheets of ply atop the tongue and groove subfloor. My plan was to lay down 3/4" ply, then probably another 3/4" with channels for hydronic heating. If I do not lay down tubing, I can do 3/4" ply, then 1/2" ply glued and screwed.

Without addressing the joists, might this result in sufficient rigidity for tiling?
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Unread 11-24-2022, 04:15 PM   #4
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Robbie, there are two distinct areas of deflection to consider when tiling a wood framed floor; one is the joist deflection, and the other is the subfloor deflection between joists. They must be addressed individually, as changes in one do not necessarily affect the other.

Your joist deflection, if you want to improve it, might be more easily addressed by installing a framed wall or support beam to shorten the unsupported span of the joists a couple feet. If that is an option, the famous (well, sorta...) TYW basement closet against one wall might be a good solution for you, while at the same time providing a bit of extra storage space in the basement.

Your subfloor structure is not gonna require any help at all once you've installed the minimum layer of plywood over the board subfloor. Nothing wrong with using thicker plywood, but in your case it's really not gonna buy you anything important. A single half-inch layer is more than adequate, even with the orientation of your board subfloor.

My opinion; worth price charged.
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Unread 11-24-2022, 04:43 PM   #5
robbieallan90
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That sets me off on the right foot then. Thanks a lot for taking the time to reply to my questions, I appreciate it.
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