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Old 12-13-2005, 03:56 PM   #1
PeterW
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Water Barrier

I calculated the deflection of my floor using the Handy Deflectolator and it said that the current floor is not acceptable for Ceramic tile (2x10 joists, 16"oc, with 14' spans), so I am going to beef up the current 3/4" shiplap subfloor with an additional 3/4" plywood.

I have read some recommendations to put a water barrier between the plywood and the shiplap to avoid swelling of the shiplap. Other recommendations say to glue the plywood to the shiplap. It would seem silly to do both since you would effectively be glueing the plywood to a peice of plastic and not the subfloor.

The shiplap is exposed underneath (crawlspace with good ventilation), and the area is an entrance way/bathroom (tiolet and sink only), so I am inclined to go with the glue, but wanted to solicit opinions.
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Old 12-13-2005, 04:00 PM   #2
Davestone
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The problem isn't the subfloor, it's the unsupported long span, you'll need to cut that down if possible by supporting the joists somehow,or sistering.
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Old 12-13-2005, 06:23 PM   #3
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Welcome, Peter.

If you've got good wood in those floor joists, your span would be within specs for ceramic tile. Did you not try to determine what kind of wood you had, or go with the Unknown Wood criterion because you know you don't have SYP or Doug Fir or equivelant?

Three-quarter ply over your board floor would be fine, if the joists meet the span requirements. You can get by quite well with only half-inch ply in that application if height is a consideration. More is always better.

Adding plywood does nothing to improve the joist span requirements, though.

There is no need to glue or install any kind of membrane over the boards, neither will do much good. Just screw the existing boards down to be sure they are tight to the joists, then screw the plywood to the boards and not into the joists. Will make a good floor for tiling.

After you install a suitable substrate on top of the plywood, of course.

My opinion; worth price charged.
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Old 12-14-2005, 07:44 AM   #4
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Did you not try to determine what kind of wood you had, or go with the Unknown Wood criterion because you know you don't have SYP or Doug Fir or equivelant?

That's correct, I wasn't sure what type of wood it is, and didn't know how to confirm what type it is. Is it likely to be Douglas fir, how could I tell?

Also, why don't you want to screw through the plywood in to the joists, I thought that would be a good thing.
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Old 12-14-2005, 08:02 AM   #5
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There may be mill markings on the joists that will give you information, Peter. Otherwise it's color, grain, and such to an experienced eye. Maybe you have such an eye available to you?

Tell us what part of the country you're in, and post good pichers of the joists, and we may be able to give you a hint.

Fastening the underlayment is not intuitive. For tile floors you want the subfloor as rigid as possible while isolating the top surface as much as possible from the joist structure. You want to lay the second layer with the end joints past a joist by one-quarter of the joist spacing (about 4" in the case of 16" centers), and the long edge joints at mid-span of the first layer. Then fasten only to the bottom layer and not into the joists.

Yeah, I can prove it.

My opinion; worth price charged.
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Old 12-14-2005, 08:25 AM   #6
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Hi Peter......

Un-coupling subsequent layers of floor underlayment from possible joist movement is a good thing fer ceramic/stone coverings.. IE not screwing underlayments solidly to joists.

Now... iff'n the idee is to stiffen up a floor that is borderline...... screwing an additional 1/2" or more underlayment solidly to joists and blocking is just one of many ways of tying a floor together... not the best way alone but.... one way.

Cementious Backer Underlayment (CBU) products used as a final underlayment before tiling assist this important un-coupling effect cause... they are not connected (screwed into) the joists at all, are kept away from walls, and..... unmodified thinsets beneath em don't adhere solidly to various ply woods.

There is a "Ditra" product which may be used in place of CBU which is well thought of here-a-bouts but..... excessive joist span may remain an issue in all cases.

Hope this helps :--)
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Old 12-14-2005, 08:36 AM   #7
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Oh ya......

What CX said.... He's quick
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Old 12-16-2005, 02:12 PM   #8
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Yeah, I can prove it.

CX, I would be curious to hear/see the proof/rational, just for my own edification. Is there somewhere I can read.

I have put a telepost under the span, cutting the max distance to about 9 feet.

Would you put building paper between the shiplap and the plywood?
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Old 12-16-2005, 04:52 PM   #9
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How'd you support all the joists with one post, Peter?

Here's a paper on the theory of placing the second subfloor layer at the quarter-span of the supporting joists.

The part about getting some isolation from the joist system by not fastening into the joists is another theory, and that one is allegedly supported by test results. I believe the better results to be more a factor of the proper positioning of the sheets (as described in the paper) than of not fastening into the joists, but that I cannot prove. And I further believe that still better results would be achieved by the proper positioning of the sheets and properly gluing the top sheets to the bottom sheets. Schluter, for one (one of he co-authors of that paper), specifies a tighter fastener schedule in an attempt to more closely approximate the rigidity of gluing, while claiming that gluing is "not necessary." That's another argument for another place. Again.

And in your situation with the board subfloor, gluing is not a good idea (even for those of us who are large fans of gluing) because there really isn't a good way to do it properly in real life.

No, I would not install anything between the subfloor layers, no matter what each layer was made of. Friction is your friend there. If you have moisture issues under the subfloor, you should address them in the space below the floor.

My opinion; worth price charged.
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Old 12-16-2005, 10:22 PM   #10
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My area for tiling is approx 60'' x 14', so there are only four joists that need support. (joists at 16, 32, 48, 60, 65)

I am going to either use a 4x4 or two 2x6 on top of the telepost. I have a 5/8 steel plate to distribute the weight at the bottom as I am not going to put a full footing in. I was thinking that the floor was close to meeting the spec for tile, so it doesn't need a lot of extra support and, the weight of tile I am adding is fairly small so a single telepost would suffice. Do you think that would be ok?

I am going to run the plywood in the 60" direction, which is perpendicular to the joists, so spacing 4" off the joists is a moot point for me, since the plywood will span all the joists.

I read to space the plywood 1/4" between the sheets, do I need to fill that space? It would seem bad to fill a space that is there for expansion, but I have found a few things in tiling that run counter to my intuition so I thought I would ask.

Comments on any of the above?

ps
Thanks for your help
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