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Unread 03-28-2006, 04:31 PM   #1
T-E-K
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Question Subfloor Basics

I'm Tim -new to the tiling game (but eager to learn) I'm starting on my kitchen and have some subfloor questions. First the basics:12 feet wide w/9"-9.25" x 1.5" joists, 16-17" O.C., spanning 16 feet. Defecto says I will need to add more support and I am planning on running a beam halfway to cut the span in half to 8' which brings me to question #1 . The beam will be about 12' long mounted in an unfinished basement, would I be better off using a steel I-beam (8" tall x 4" wide) or going to some wood joists which I assume I could use a couple 2 or 3 or 4(?), 9x1.5's sandwiched together and then held up with the typical basement steel posts, what is your expert opinion? Assuming (correctly I hope) this will gve me the support I need I then go to the subfloor and question #2 which currently consists of 3/4" C-D interior plywood glued and nailed to joists, followed by 5/16" plywood stapled to the 3/4" and then covered w/sheet vinyl. This setup still leaves me about 1/4" below my next room with carpet so I can still add thickness to the floor which I would like to do anyway to even everything up. Do I need to remove that stapled 5/16" and vinyl first and then go to some new 2nd layer of plywood attaching to the 3/4" I still have, then add hardibacker (1/4" or 1/2" ?) finally followed by the 1/4", 12" square tiles? Thats a mouthfull

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Unread 03-28-2006, 11:12 PM   #2
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Hi Tim, welcome! The beam could be a 2x10 doubled or a steel beam like you said. What ever head room you can go with. Yes you need to remove the vynil and 5/16" underlayment. You can add another layer of 1/2" ply and use Ditra instead of the cbu to get the height you need..
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Unread 03-28-2006, 11:27 PM   #3
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Welcome, Tim.

Kinda what MMike said. But if you use a support beam of doubled 2x10s you'll need a center support under it. And all the support posts will need to be adequately supported at the bottoms. What have you got for a floor in that basement?

I've never seen CD plywood that was interior grade. You made that up, didn't you? Just to test us.

It's not good stuff for subflooring in either case, but with the half-inch ply (exterior glue, AC or BC) over it you should be OK.

My opinion; worth price charged.
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Unread 03-29-2006, 09:03 AM   #4
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I'd suggest using Microllam, it's lighter than steel and stronger than sawn lumber (it's also very stable with humidity and temp). I'd guess using two pieces of 1 3/4" x 9 1/2" nailed (not glued) together will do the trick!

_______pete
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Unread 03-29-2006, 09:32 AM   #5
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Thanks guys!! 1) I'll probably go w/steel beam (4X8) as I would like to support from ends only to avoid obstuctions in the center which sounds like this should work OK The basement floor is poured concrete - hoping I can set the steel support post on top - is this OK or a NO-NO ?? I could actually tie one end of beam into the foundation wall by cutting a notch and shimming/mortoring in - this would avoid a post on one end altogether and maybe look a little more professional 2) Believe it (!) the 3/4" ply is stamped "INTERIOR C-D" ! So I'll go with the 1/2" ply but how to fasten - nail or screw, do I need thinset here and do I need to fasten to hit joists below the 3/4, and how far to space ?? Then I'll go with the 1/4" Hardi as I think the 1/2" is going to set me up to high. WOW, if you can help me with this I'll be close to pulling the trigger on this project
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Unread 03-29-2006, 10:08 AM   #6
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You'll need to get someone else to calculate the deflection of the steel beam, Tim. I can't say if the 4x8 (of unknown thickness) is sufficient or no. Microlams or a gluelam might work, too, as Pete suggests, but you'd hafta get the specs from the manufacturer on that.

One end into the wall is good. Other end on the floor may or may not be. I'd want to at least sit the bottom on a bigger pad, like a 12x12x4" concrete block or some such bedded in some thinset to make a good footprint.

I'll take a lesson on the interior-grade CD. Not my first time.

Read this article on installing the plywood.

Install the Hardi per manufacturer's instructions.

My opinion; worth price charged.
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Unread 03-29-2006, 10:14 AM   #7
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Tim!

..get some "real" help on that beam. Granted steel is stronger than wood but, it also deflects under it's own weight. Hmmm, I'm still purdy sure Microllam is the way to go...

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Unread 03-29-2006, 11:37 AM   #8
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Tim, here's a list of common steel shapes that will work for your beam, and will not require a center support:

Wide Flange: W8x10 or W5x16. These will weigh 120 lbs and 192 lbs, respectively.

Standard I-beam: S6x12.5, 150 lbs.

Channel: C6x13, 169 lbs.

Misc. Channel: MC6x12, 144 lbs.

Tube steel: TS8x3x3/16", 159 lbs.

Also, your steel yard or building supply house may stock bar joists. These may be lighter than the solid sections listed above. Ask for one with an "I" value of 17 or higher.

One last thing, I would not recommend the channel sections, as the tend to roll or twist under load. It shouldn't be a problem, but it is easy to pick another section that is symmetric through the vertical axis.

Another last thing: when you get your beam, look for the camber (similar to "crown" or bow), and set the beam with the camber facing up. That way, the beam's own weight and your floor loads will act to straighten the beam.
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Unread 03-29-2006, 12:21 PM   #9
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Well, see, that's just what I was fixin' to say, Bob, on accounta I thought you was still dilly-dallyin' down to Belize.
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Unread 03-29-2006, 01:09 PM   #10
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OK ! Thanks so much for all the detail. I will check the beam out a little further here with my steel yard. "CX" thanks for the Plywood article (definitely do not screw 1/2" subfloor to joists, just to the underlying 3/4" subfloor). However, the article doesn't say - is there a rule of thumb about how many screws to hold the 1/2" ply to the 3/4", wouldn't this have to be minimal since the hardibacker will be screwed like crazy (!) and I could use screws long enough to penetrate through the 1/2' ply and into the 3/4" - thereby tying all together?? Lastly, do I need to thinset the 1/2' ply to the 3/4"or is the screwing OK - I know I need to thinset the hardibacker to the 1/2" ply. Again, many thanks!
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Unread 03-29-2006, 01:13 PM   #11
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Screw the plywood using screws just long enough to penetrate the bottom layer, and placed 4 t0 6 inches apart along the edges, and 8 to 10 inches apart in the field. Do not use thinset or anything else between the plywood layers.

Yes, there will be nearly a brazillian screws holding everything together!
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Unread 03-30-2006, 07:51 AM   #12
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One last question, going with the 1/2" ply on top of the 3/4", I am getting pretty high on the thickness by the time I add 1/4" backer + 1/4" tile. Is there any problem with going to something less than 1/2" on top of the 3/4"? - either 1/4"-3/8" (I do have that inferior INTERIOR C-D 3/4 )? Thanks again for the tremendous help!
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Unread 03-30-2006, 10:29 AM   #13
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You could use 3/8" plywood and Ditra. This combination will save you about 1/4".
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Unread 03-30-2006, 11:56 AM   #14
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Lightbulb

Humm,..I've been reading the exchanges and thought I'd just add that once I had to improve a floors "bounce" (for a hardwood floor application) prior to laying a Brazillian IPE hardwood floor. A lot was wrong with the subfloor and joist design and installation. I ended up using (a lot of) PL premium Urathane glue between the the original floor and the 5/8" T&G plywood. I made sure I hit the joists with 3" sub floor screws and 1 1/4" sub floor screws everywhere else. Oh yes I would also mention that the old floor was screwed down at 4" ctrs on the joists with 2 1/2" subfloor screws. I found the resulting improvement was significant and allowed ne to install the hardwood with confidence.
I'm new to the forum so I'll appologise up front if I'm being repetitive but the number of renovations I do that involve floors and the equal number of floors I have to fix that use drywall screws to screw the floor down It's enough to make me wonder who started the rumour that this was a good idea? Subfloor screws have a unique non threaded portion at their top that allows them to spin and pull the plywood down tight to the joist (in this case). Drywall screws will maintain the gap that may exist between the two materials and will eventually allow movement.
Anyways all feel free to comment
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