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Old 09-12-2008, 02:45 PM   #1
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Quick sanity check because I obviously don't do this every day.

Existing subfloor is 3/4" Advantech OSB glued and nailed to 2x8 joists 16" OC.

To get the height right and prep for tile I was going to:

1) Screw down a layer of 3/4" BC plywood. No glue; no T&G; same direction as subfloor, offset joints; I plan to use 2" screw to fully penetrate existing subfloor over the joists.

2) Layer of 15# tar paper.

3) Screw down a layer of 1/2" Hardibacker bedded in thinset. Alkali resistent screws to penetrate both the ply and OSB.

Is my memory serving me right or am I suffering from another "senior moment"?

Thanks.
Sam

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Last edited by samjack; 09-12-2008 at 02:48 PM.
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Old 09-12-2008, 04:51 PM   #2
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Like me....you're getting older Sam!

With two layers of 3/4" you're going to have a very solid floor, from joists to joists, anyway. Please go down and measure the unsupported span of the joists. Measure from one support to the other, face-to-face. This is important.

1. Good plan. 1 3/4" screws, even better.
2. Absolutely NO tarpaper. Where did that come from?
3. OK, but change to rust resistant screws, alkaline resistant mesh tape and thinset. No need for 1/2" backer, 1/4" is for floors, but if for height, OK. No need to penetrate the OSB at all or much. Use the special Hardi screws.

Jaz

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Old 09-12-2008, 04:59 PM   #3
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Quote:
Originally Posted by JazMan View Post
Like me....you're getting older Sam!

With two layers of 3/4" you're going to have a very solid floor, from joists to joists, anyway. Please go down and measure the unsupported span of the joists. Measure from one support to the other, face-to-face. This is important.

1. Good plan. 1 3/4" screws, even better.
2. Absolutely NO tarpaper. Where did that come from?
3. OK, but change to rust resistant screws, alkaline resistant mesh tape and thinset. No need for 1/2" backer, 1/4" is for floors, but if for height, OK. No need to penetrate the OSB at all or much. Use the special Hardi screws.

Jaz
Wouldn't 1 1/2 or 1 1/4 scres work as well? I always thought that when laminating underlayments, you did not want to penetrate into the floor joists.
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Old 09-12-2008, 05:35 PM   #4
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Glad you brought that up. Inch and a quarter would definitely NOT work very well. Only penetrate the sub by 1/2", YIKES that would be a disaster. Plus there is NO lamination going on here.

Inch and a half would be much better. Problem is that the first 1/8-1/4" of the screw is pointed, which does nothing once set. I recommend going a little longer for max bite. Especially when the sub as in this case is OSB, which does NOT hold a screw as good as plywood.

Also most CBU companies like you to use a screw that is 1/4" "too long" simply to verify fasteners placement in case of a complaint someday. Quarter or 1/2" penetration of the joists does nothing, no affect.

Jaz
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Old 09-12-2008, 05:47 PM   #5
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The joist span is under 8 feet. I remembered to include the joist size and spacing so I wouldn't be reminded to check that but neglected to include the span. Doh!

Quote:
Originally Posted by JazMan View Post
Problem is that the first 1/8-1/4" of the screw is pointed
Which is the only reason I would go over the joist. I have a nasty scar on top of my head from an errant nail.

The 1/2" CBU is to get the height only.

Tarpaper? Why did we do that? I have no idea. Maybe we did that over 5/4 diagonal planking. Or maybe that was how cavemen uncoupled tile from the underlayment. At anyrate, I'll skip the tarpaper.

Alkali tape. That's it! I told you I was getting old. I'll use the Hardi screws.

Thanks for keeping me straight.

-Sam
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Old 09-12-2008, 05:49 PM   #6
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Quote:
Originally Posted by JazMan View Post
Glad you brought that up. Inch and a quarter would definitely NOT work very well. Only penetrate the sub by 1/2", YIKES that would be a disaster. Plus there is NO lamination going on here.

Inch and a half would be much better. Problem is that the first 1/8-1/4" of the screw is pointed, which does nothing once set. I recommend going a little longer for max bite. Especially when the sub as in this case is OSB, which does NOT hold a screw as good as plywood.

Also most CBU companies like you to use a screw that is 1/4" "too long" simply to verify fasteners placement in case of a complaint someday. Quarter or 1/2" penetration of the joists does nothing, no affect.

Jaz
By lamination I just meant one layer going over the subfloor. Good expanation of the screw requirements. One thing though, I know the Backer-On screws come in 1 1/4 and 1 5/8. How can they expect penetration through the CBU and 1 1/4 to 1 1/2 of flooring to verify screw placement?
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Old 09-12-2008, 06:01 PM   #7
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The screws won't penetrate in this case because of 2 layers of 3/4", you're right. Most of the time the CBU goes over a single 3/4".

So, in this case Sam should use 1 5/8" or use roofing nails instead. My choice is roofing nails, works great, faster, cheaper. Plus you can easily drive them flush.

Jaz

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