Joist span worries [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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11-20-2017, 06:54 AM
Hi thanks for reading my post. I am a DIYer and a tiling novice. I am gutting a first floor half bath (approx 5'x7') in my 1920 home, remodeled in 1954 which is when this half bath was installed. Due to a leaking toilet seal and carpenter ants I am replacing most of the 1x10 diagonal subfloor, cutting out from joist to joist across 3 joist bays and replacing with 3/4" plywood subfloor. I was planning on using 2" porcelain mosaic tile. The floor joists are 2x8 (im guessing redwood) approx. 16" oc and span about 13.5'. When I plug this into the deflectometer it says not recommended for tile. There is some blocking in the one bay, but the other two bays have 8" ducting in them so there's little I can do to block. Am I totally screwed on using tile?

I was thinking of putting a layer of 1/2" ply over the 3/4" and topping with 1/4 cement board, would this be sufficient or is the joist span just too great?

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HooKooDoo Ku
11-20-2017, 08:22 AM
The recommended subfloor for tile is 3/4" TNG plywood attached directly to the floor joists, and then a 2nd layer of 1/2" plywood with offset joints attached directly to the 3/4" layer (not the floor joist). In part, the idea is that the 2nd layer of plywood covers all the joint of the 1st layer... THEN use thinset to apply a 1/4" layer of CBU to the 1/2" plywood. In part, the idea here is that any deformities and pockets in the subfloor get leveled out so that you have a solid surface to start setting tile.

But that is the standard for a floor that is already passing the deflecto calculations.

Realize that as you walk across a floor, the floor joist flex to some degree or another. The smaller the joist or the longer the distance between floor joist supports, the more it will flex. Adding additional layers on top of the floor will not do anything to lessen the amount of flex the floor joist will experience.

To lessen the amount of flex, you must do something to strengthen the floor joists. If there is a basement or crawlspace under the floor joists, you could build a wall mid-span to reduce the span. If utilities are not in the way, you can strengthen a floor joist by sistering another joist to the existing ones (i.e. "simply" attach another joist to the side of the existing one). If you can't get access to the sided of the floor joists due to utilities, but the bottom of them is completely exposed (i.e. no utilities crossing under the joist), you can attach a 2x4 (or larger) laid flat to the underside of the floor joists to create a partial I-Beam.

John Bridge
11-20-2017, 09:07 AM
Hi Boone, :)

Joseph's idea is a good one. You might also "sister" the joists (screw new joist material alongside the old). Use the largest 2 by you can squeeze in.