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Unread 06-14-2019, 06:54 AM   #1
Franko
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Deflect-O-Lator valid for treated lumber?

I'm building a screened in porch (8' off the ground) and it will have pressure treated floor joists. I plan to tile the porch floor. I have used the Deflect-O-Lator to calculate the deflection.
But a carpenter just told me that pressure treated lumber is less rigid than non-treated wood, which made me wonder if I can use the results of the Deflection calculation without making an adjustment for using treated wood?
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Unread 06-14-2019, 07:47 AM   #2
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Frank,

In my experience, PT lumber is treated () as standard lumber in the structural design tables but you can us the "unknown wood in good condition" selection in Deflecto if you want to build in a safety factor.

PT can be much wetter than standard lumber initially so you may want to let it stabilize for a period of time before setting tile.

What type of sub-floor are you using over the joists?
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Unread 06-14-2019, 07:50 AM   #3
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different species of wood is used for PT, different grades, and also different processes.

what info do you have on the wood?
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Unread 06-14-2019, 07:51 AM   #4
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Welcome back, Frank.

The pressure treated wood in my part of the world is usually SYP and while it is fully wet it, as it will be when you purchase it, it will be less rigid than when fully dried, but you could still evaluate it using our Deflectolator.

That said, I'd surely recommend you not build a deck that is to be tiled using treated wood unless it's KDAT-type treated wood. In actual practice, I'd not use treated wood at all for a deck structure that will be 8 feet above ground, but that's an entirely different discussion.

You'll also want to consider that deck structures are required by code to be built to carry more load than the interior floors of your house. And the fastening to the structure of the house is a critical point of the construction.


My opinion; worth price charged.
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Unread 06-14-2019, 09:50 AM   #5
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if not treated wood you are pretty much left with cedar timbers or redwood timbers.

you aren't suggesting he build an outdoor deck with pine/fir lumber are you??
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Unread 06-14-2019, 10:13 AM   #6
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For the framing of a deck well above ground, yes, I am, Mike.
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Unread 06-14-2019, 11:53 AM   #7
speed51133
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IRC specifies that decks be made from “approved rot-resistant material,”. So that would be against code that is in use or adopted in 49 states.
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Unread 06-14-2019, 12:34 PM   #8
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Kiln dried fir or yellow pine eight feet above ground is quite rot resistant enough for most applications to suit me, Mike. And the sap wood cedar we get herabouts these days rots quite handily, it seems.
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Unread 06-14-2019, 03:01 PM   #9
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To echo a point CX made, around here, a porch with a roof can be treated as part of the house structure versus a deck exposed to all weather. Just depends on the construction details and the local inspection authority’s view.
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Unread 06-14-2019, 09:39 PM   #10
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And since the OP is planning to tile this deck, it'll hafta be waterproofed anyway.
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Unread 06-14-2019, 10:51 PM   #11
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Just for kicks...

What would you do differently if it were one foot or less off the ground, still getting tile over it?
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Unread 06-15-2019, 07:10 AM   #12
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Depending upon the size of the deck, slope, drainage, ventilation, etc, I might consider framing with KDAT lumber, Kevin, or might not.
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Unread 06-15-2019, 08:12 AM   #13
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You coulda been a politician.
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Unread 06-16-2019, 01:13 PM   #14
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A lot of input on this one. The builder moved fast, so here's where things stand:

The deck/porch is already framed out. He used high quality 2x10 joists (I believe they're #1; they are approved for ground contact.) I caught the builder in time to tell him to space the joists 12" OC rather then 16. The span is 14', but he said he could put an additional beam under the joists at 7' if I want him to. The plywood is treated. None of the wood is KDAT.

So, if that's what I am working with, how would you recommend I tile the floor? I am guessing I can't use Ditra over treated plywood. If not, the only other option I know if is something like Durock over the ply, then add the tile.

Is there a better option? I'm guessing you'd recommend I have him add the extra beam at 7' to increase the stability, and I'll wait till the wood dries out before I do the tiling. If I do that, is there a reasonable chance the tile will not crack?
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Unread 06-16-2019, 03:31 PM   #15
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First thing I'd do is remove the treated plywood. I had a builder put treated ply on a covered deck and he was hopping mad when he had to remove it and replace it with regular ply, but I'd never tile over treated ply.
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