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Old 02-17-2018, 11:05 AM   #1
Jeremychips
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Is my floor strong enough for mud shower pan installation?

The deflecto calculator tells me my floor is L/416. I assume that if this was one open space and all I was doing was floor tile I would be fine. However there is a wall perpendicular to the floor joist mid-span separating two bathrooms, both of which will have tile on the walls surrounding a shower and tub respectively.

My floor joists span 13.5 feet and are 2 x10 Douglas fir. Sub floor is 5/8 plywood with another half inch layer of plywood on top which will then get a quarter inch Hardie and tile. Shower is 32x68". Some of the joists have holes for plumbing drilled through them, one hole being an inch and a half from the edge rather than the required two inches. It has been that way for 50 years since the house was built with no noticeable adverse effects.

My concern is that both the bathtub and the shower which run parallel to the joists are supported by the same three floor joist directly underneath, and a 4th that is just a few inches outside of the shower area. The wall running parallel separating bathrooms from an adjacent bedroom is supported by it's own joist. No weight from that wall is on any of the joists supporting the bathroom floor as the subfloor throughout the rest of the house is Tongue & Groove where the bathroom is plywood. They are not connected in any way. This is significant because that takes about 80 square feet of wall tile and backer board from the two largest shower and tub walls out of the equation.

Will this floor be strong enough to do what I'm planning? The only alternative I can think of is to use a foam shower pan which will take about 400 lb from the mud pan away.
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Old 02-17-2018, 11:35 AM   #2
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Hi Jer,

Welcome to the forum.

What's below the bathrooms? Your drawing says 14' 5" between the top and bottom walls shown, and you state that the span is 13' 5". So there's a bearing wall under one of the walls shown, and another bearing wall that is 1' inbound of the other wall shown?

I haven't run any numbers for your situation, but in general there are two questions to consider: Will the joists be strong enough to handle the full load imposed on them (dead plus live)? And how much will the floor joists deflect under the imposed loads after tile installation, will they be stiff enough to meet tile industry standards?

The deflecto addresses the second question. But construction order also matters. If you install all the wall tile before any floor tile, then the floor tile won't be subject to the initial deflection caused by the weight of the walls. [It will still be subject to any additional long-term deflection, known as creep.] The same applies to the weight of a mud pan, the initial deflection caused by it will not affect your tiles on top of the a pan.

What waterproofing method do you plan to use? The single sloped mud layer required for a surface applied method (sheet or liquid) would weight less than the double layer required for a PVC liner.

Cheers, Wayne
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Old 02-17-2018, 11:42 AM   #3
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Welcome, Jer.

I'm going to presume the wall perpendicular to the floor joists near the center of the span is not a load bearing wall. Would that be the case?
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the subfloor throughout the rest of the house is Tongue & Groove where the bathroom is plywood.
I'm also going to presume your plywood subflooring is also Tongue & Groove style?

While you'll be adding a good deal of dead load to your joists, I don't think you have a problem according to your description. The one joist with a bore too near the bottom is always as consideration, but if the joist is otherwise sound and of good grade it may not be a problem. Having such a defect in the center third of the joist span makes it more problematic, but as you point out it's been that way for many years without signs of failure.

Adding 400 pounds of dead load to such a structure should not be a problem as I see it. If your floor can't support a mud shower floor, I'd suggest you not build a shower there at all. A foam tray isn't going to make or break the deal.

My opinion; worth price charged.
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Old 02-17-2018, 11:47 AM   #4
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The drawing is an approximation that I threw together real quick. The true span is 13.5"

I had planned on the PVC liner method with preslope but I'm open to suggestions. Whatever is the best for the situation is what I'll go with.
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Old 02-17-2018, 11:50 AM   #5
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The perpendicular wall is not load bearing.
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Old 02-17-2018, 11:58 AM   #6
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Use of a direct bonded waterproofing system (ANSI A118.10) for your mud shower receptor will save you more than half the weight of a traditional double thickness mud pan. It also makes a better shower if one of the sheet membrane types is used. My current favorite is the USG Durock Shower System and the material is available on Amazon.

My opinion; worth price charged.
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Old 02-17-2018, 12:18 PM   #7
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P.S. If you want to repair the joist with the hole too close to the bottom, the fix is relatively easy since you still have 1-1/2" of solid wood at the bototm. Just add a 1-1/4" wide metal strap along the face of the joist, with the top edge of the strap at the bottom of the hole. Screw the strap to the joist, avoiding the area directly under the hole.

Calculating the proper size strap and number of fasteners would take me longer than it would take to install the strap itself (since I don't do such calculations every day). But anything you properly add would increase the capacity of that joist, and something like a Simpson ST18 with SD10112 screws would certainly help.

Cheers, Wayne
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Old 02-17-2018, 12:52 PM   #8
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I think I'll add some Simpson reenforcements and move forward with tiling the Walls before the floor. Thanks for all your help.
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