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Old 07-07-2011, 07:24 PM   #1
BobL43
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BobL43's 2011 Bath Reno

Hi guys,

I am in the middle of a bathroom renovation; everything is gutted already.

The bathroom I am working on is 7' 2" wide by 7' long. The 7 foot dimension runs the length of the joists. I have attached a rough sketch. The question I have is whether this floor is strong enough with 3/4 inch plywood over the joists to support ceramic or porcelain tiles (my wife hasen't chosen which yet). The deflection calculator here would work fine for me, but I have a situation where the rear 3 feet of this portion of the house is cantilevered over the foundation. Each joist is Doug Fir 2X8 (but they are only 7 1/8 tall) 16 inches on center for the total length of 14 feet 9 inches, but the span is supported 3 feet from the ouside end by the foundation wall. this is a one story house. I hope I explained this situation clearly enough. The other bathroom in this sketch "bath 1", I did over 20 years ago with ceramic 6X6 floor tiles and have never cracked a tile or any grout.

Thanks for your looking at this for me and any advice you can provide.

Bob
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Old 07-07-2011, 09:21 PM   #2
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Welcome, Bob.

I suspect your joist structure was designed to meet the code-required L/360 when built, even though it's a little shy of passing the Deflectometer, which is a pretty conservative creature by design.

The cantilever throws in some complication and I'm not really equipped to sort that out scientifically for you. They can be helping with your structural issues and trying to bite you inna butt at the same time.

Injineer Bob (bbcamp) will be able to 'splain you, but he won't likely be by until inna morning.

Your 3/4" subflooring satisfies the requirement for most tiling substrate manufacturers, but there are still those of us who'd try to get you to add another layer of half-inch ply before you get to your substrate material.

By the way, Porcelain tile is a type of ceramic tile. Your wife might need that information when shopping for her flooring.

My opinion; worth price charged.
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Old 07-08-2011, 05:06 AM   #3
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Thanks

Thanks for the welcome CX, and for the info. My concern with the cantilever is the the possible influence of a snow load on the roof causing some lift in the floor in the new bathroom. not much leverage with only 3 feet overhang, but maybe it is.

I apologize for leaving this out: There is another bathroom (in the basement) below these 2 bathrooms that has 2 walls supporting these joists; One is close to the foundation wall interior, and the other is 6 feet closer to the center house beam, making the maximum span length for the first floor bathroom joists 6 feet at any point. I updated my sketch to show this

I left that out, I guess, because my main concern was the cantilevered part on the rear of the house. This web sites deflectulator shows that the interior span is good even for a stone floor if I interpreted it correctly.

Thanks again
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Old 07-11-2011, 08:34 AM   #4
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Well, so far I have not got a reply to my cantilever issue. If Worse comes to worse, I'll build a beam under the overhang. I have a 6 inch thick poured concrete patio that I put in in 1973 that starts at the foundation wall and continues under the overhang and is now covered by a deck I built in 1988. The bathroom under reno now has its joist supported by walls in the basement which are of the basement bathroom construction, so the max span of the joists in question is just about 6 feet, which comes out very good in the deflectulator: L/ 1415.

Right now I am finishing the demo by removing the rest of the subfloor 1/2 plywood that was the deck of the house contruction before the walls were built on top of.
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Old 07-11-2011, 08:42 AM   #5
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Bob, it is the snow load that would bother me in a tile application. If you could support those loads with a new beam and post arrangement, I'd do it.
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Old 07-11-2011, 11:18 AM   #6
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Thanks Bob, I guess I will do that then. the hardest part of that will be removing the lattice deck skirt at the perimeter after convincing my wife that ai need to move her Hibiscuses and clematis, and whatever out of the way.
Man, trimming away the subfloor decking around the bathroom perimeter is a real pain, only because I am being SO, so careful not to hit any wires or plumbing with the Sawzall blade. Then I have to install nailers around the perimeter to attach the new 3/4" plywood to all around.
This room will only need 2 sheets of 3/4 plywood, meaning only one joint and it will be perpendicular to the joists. i am going to use tongue and groove plywood. Is that the correct direction for the joint, or should I lay the tongue and groove joint over one joist? The original subflooring was 1/2' plywood deck over the joists and 5/8 plywood over that. both layer had thier joints perpendicular to the joists and of course were not directly over one another, offset by about 18 inches.

Thanks again
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Old 07-11-2011, 11:32 AM   #7
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Dude, some marital advice. Do Not Touch The Plants. If they need moving, let her do it right. Same with pruning. I don't touch the bikes. Mr. Wendy doesn't touch the plants. Everyone is happy.
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Old 07-11-2011, 02:35 PM   #8
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Talking

Wendy,

LOL! You see, this bathroom reno is for her; it is her very own private bath with a new clawfoot slipper tub she wanted for SO many years. It was a fully functional 1/2 bath for the past 41 years, but now she wants me to redo it for her into a full bath. I am 68, so I guess she wants me to do it while I'm still able to. I am. We've been married for 46 years, so we do know each other very well. I would never mess with her plants under normal circumstances. The snow won't be here until the winter, so I can wait on messing with her flowers until the late Fall, but want to get the beam under the floor to support it BEFORE I tile. We'll work out something. This bathroom should give us both much pleasure
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Old 07-11-2011, 03:54 PM   #9
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What screws to use?

for screwing down 3/4 inch plywood to my floor joists, what is the proper length ("deck screw"?) to use? Is 1 5/8 inch long enough?

Thanks
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Old 07-11-2011, 08:37 PM   #10
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The "Liberry"

I found that the correct direction to lay the plywood down is perpendicular to the joists for best deflection strength. Now, I still need to know how long the srews should be to fasten 3/4 plywood to the joists; how much should the screws penetrate the joists? This, I had no luck in my searches.

Thanks
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Old 07-11-2011, 08:46 PM   #11
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if your joists are in good condition, 2 times the thickness of the plywood is normal (or a hair longer) Add more if the tops of the joists are in poor shape.
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Old 07-11-2011, 10:12 PM   #12
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Where I grew up one always wanted screws to penetrate one inch into the structural material when fastening sheet goods.

For sheet to sheet, you must penetrate fully through both sheets.

My opinion; worth price charged.
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Old 07-12-2011, 05:46 AM   #13
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Thank you Paul and CX
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Old 08-14-2011, 02:56 PM   #14
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Back to this job

OK, I am back to my project. When installing tongue and groove plywood, id it correct or not to apply glue in the tongue/groove joint?
There is only a little over 1 1/2 sheets in this small bathroom, and I am allowing 1/4 inch clearance all around the perimeter to the wall plates.


Thanks!
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Old 08-14-2011, 04:48 PM   #15
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no need for glue at the T&G
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