Deflect-O-Lator valid for treated lumber? [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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06-14-2019, 06:54 AM
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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06-14-2019, 07:47 AM

In my experience, PT lumber is treated (:yeah:) 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?

06-14-2019, 07:50 AM
different species of wood is used for PT, different grades, and also different processes.

what info do you have on the wood?

06-14-2019, 07:51 AM
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.

06-14-2019, 09:50 AM
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??

06-14-2019, 10:13 AM
For the framing of a deck well above ground, yes, I am, Mike.

06-14-2019, 11:53 AM
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.

06-14-2019, 12:34 PM
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.

06-14-2019, 03:01 PM
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.

06-14-2019, 09:39 PM
And since the OP is planning to tile this deck, it'll hafta be waterproofed anyway.

06-14-2019, 10:51 PM
Just for kicks... :devil:

What would you do differently if it were one foot or less off the ground, still getting tile over it?

06-15-2019, 07:10 AM
Depending upon the size of the deck, slope, drainage, ventilation, etc, I might consider framing with KDAT lumber, Kevin, or might not.

06-15-2019, 08:12 AM
You coulda been a politician. :gerg:

06-16-2019, 01:13 PM
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?

06-16-2019, 03:31 PM
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.

06-16-2019, 05:10 PM
What Kevin said. None of the wood is KDAT................ I'll wait till the wood dries out before I do the tiling.Good summer project then, Frank. Just not this summer. :shades:

You might get by with tiling over what you've got, but I wouldn't do it on a project I had to sign my name to. Just too risky, 'specially when none of that risk is necessary at all.

My opinion; worth price charged.

06-16-2019, 05:19 PM
I had some treated lumber stored in my garage. It took about a year for it to feel nice and dry. :D

06-16-2019, 05:56 PM
CX -- I'm not sure I follow you. Are you saying the risk is that I'll be tiling over treated ply, and it would be better to replace it with un-treated ply? What is the unnecessary risk I am taking?

If I cover the ply with Durock, what difference does it make if it's treated or not? (I am planning to wait till next spring to tile it.)

Tool Guy - Kg
06-16-2019, 06:37 PM
None of us like the treated plywood in the least. I would remove the treated ply.

The framing...the fact that it’s sopping wet treated lumber stinks. It could easily be dry construction lumber, but it’s not. Around these parts, the building inspectors would be perfectly okay with dry, untreated wood. I don’t think any of them would object to using treated...not because they favor the treated. But because the code is written in a non-prescriptive manner. And treated lumber doesn’t violate the code, as far as I’m aware. Passing code doesn’t mean that a project is being done with best just means that it doesn’t violate code and meets some minimal standards which may or may not play well with your intended tile assembly. If the inspector knew that you built to standards against the manufacturer’s instructions, they would have proper grounds to fail the inspection. It has been my extensive experience that some inspectors are aware and some are not...meaning that the onus is on YOU to build to best practices. It isn’t wise to turn a blind eye to a problem just because something “passes” an inspection.

If you choose to stay with what you’ve got, I’d wait for the framing to dry. I’d use a moisture meter once every few weeks to check its progress and wait until it got down below 19%. Then proceed with the proper plywood and tiling substrate.


06-16-2019, 07:15 PM
If you choose to stay with what you’ve got, I’d wait for the framing to dry... Then proceed with the proper plywood and tiling substrate.

Sorry, confused again. KG you say "if i stay with what I've got" (which is treated plywood already in place)....but then you say "proceed with the proper plywood" (by which I assume you mean put down untreated plywood.)

Doesn't the screen porch sit on TOP of the plywood? If so, the builder can't build the walls and roof until the plywood is down, so how can I let the framing dry out before putting down the plywood?

Is there an option to put non-treated plywood over the treated ply after it has thoroughly dried, and then tile? And/or won't the Durock over the treated ply allow me to tile? Does that cause problems? I'm just trying to find a way out of this mess I've walked into.

Tool Guy - Kg
06-16-2019, 08:34 PM
This is why I said it stinks.

I'm just trying to find a way out of this mess I've walked into.You're paying someone to do this, right? Maybe it's time to ask what your contract reads. Your builder is expected to be the knowledgeable party here. For instance, if I wrote someone a contract to build them something and it turned out that I made a mistake with installing a material that the manufacturer's said was a no-no, I'd be on the hook for correcting the mistake before proceeding.


06-16-2019, 08:43 PM
Frank, I don't think there's any way you can expect a successful tile job over treated plywood. I'd just bite the bullet and get rid of it.

The joists will dry out eventually, but you'll have to wait a while for that to happen. As they dry out, they'll start to twist and warp, which is likely to exert too much stress on your tile, Durock or not.

06-16-2019, 09:17 PM
KG - My builder is not to blame. The blueprints called for treated joists with 5/4 treated planking over them, which is what he was going to build. But he asked me if I would prefer that he do plywood and indoor/outdoor carpet instead (which he says he's done a bunch of times and people like it.) I said yes. Then my wife decided she would like me to tile it I figured I could just tile over the plywood as long as I had him space the joists at 12" OC instead of 16". I guess my options are to have him do the indoor/outdoor carpet over the plywood, or have him pull up the plywood and go with the 5/4 planking as called for in the original plans. Ugh.

Thanks Kevin for the answer to the specific answer about Durock. That helps me understand WHY I can't just fix it that way.

06-17-2019, 11:20 AM
Maybe vinyl planks rather than indoor/outdoor carpet? I have no idea what they require, but I imagine they would be more forgiving than tile.

06-17-2019, 11:44 AM
I have the same situation on a screened in porch; treated plywood over treated lumber. I installed primed and painted 3/4”x3” T&G Douglas fir floor over 15 years ago and it’s still going strong. As I recall, there is a 1/2” airgap between the doug Fir and the ply which keeps the T&G floor dried out to protect against rot.

This was a redo to replace 5/4 treated planking originally laid over the plywood and which looked terrible after a few years when the gap filled up with gunk (leaves, pet hair, etc).:noid:

06-17-2019, 12:50 PM
I asked the local flooring dealer about vinyl. They told me that there are a few products that the manufacturers say can be used outside, but they don't recommend it. They say it doesn't hold up. Of course, this is the same floor dealer who told me "of course you can tile the porch, people do it all the time."

Funny you said you didn't like 5/4 over the plywood....because that is exactly what we are thinking we'll do if we don't carpet.:stupid2:

06-17-2019, 01:39 PM
Your "floor dealer" wasn't wrong in telling you you could tile your porch, Frank. You just need to make some changes in your construction plans for it to work well.

06-17-2019, 01:58 PM
Funny you said you didn't like 5/4 over the plywood....

The planks tend to warps and shrink leading to big gaps that collect all kinds of nasty stuff. :twitch:

06-17-2019, 04:51 PM
So would a viable strategy be to put down outdoor carpet until the PT framing and plywood dries out, then rip it up, put down another layer of 1/2" non-PT plywood, then a tile waterproofing membrane (e.g. NobleDeck), and then tile?

Or is the PT plywood likely to end up way out of flat, making tiling a real pain?


06-17-2019, 05:07 PM
Seems like it would be less work just to remove the PT ply and replace it.

But to answer your question, I still wouldn't do it with the treated ply on there. You could wait all that time for it to dry out, and it might be flat, or it might look like a skateboard park.

06-17-2019, 07:39 PM
Agree with Kevin, pull up the PT ply and move out.

06-18-2019, 02:26 AM
OK, there have been so many statements that the treated plywood is terrible, I want to be sure I am clear on what you pros are saying I should do....

You're saying I should pull up the PT plywood and put nothing on the joists until they have thoroughly dried, and then put down non-treated ply. Is that right? Obviously that would leave the porch unusable for many weeks.

06-18-2019, 05:01 AM

No, they are suggesting you have the builder remove the treated plywood right now and replace it with plywood that is not treated. Then in a year or so after the treated framing (e.g. floor joists, ledger, etc...) has dried out (you can verify with a hygrometer), proceed to set your tile using durock or whatever tile underlayment you choose to install over the untreated plywood.

06-18-2019, 07:50 AM
Confirmation please, from one of the pros who has been recommending I pull up the treated plywood....

If I do what Radas suggests, the untreated plywood, after setting flat against the elements for a year, would be severely degraded, wouldn't it?

06-18-2019, 08:05 AM
I would never leave untreated plywood exposed to the elements for weeks/months, but that is me. The pressure treated wood could take 6 months to dry out. I know the stain I use recommends waiting a full YEAR before pressure treated wood is dry enough to take the stain and have a valid warranty. They are recommending you use a moisture meter to determine how wet the wood is and just wait until it is 19%. No idea how long that will take as it depends on your local humidity, temperature, sun exposure, airflow, rain, the lumbers initial moisture reading, etc. Freshly treated wood is sopping wet and "bleeds" liquid when driving a screw.

Are the joists and beams exposed to the air on the underside, or are they totally sealed in? Is there like a drainage system with gutters?

06-18-2019, 08:58 AM
Frank, I'm not completely clear on the final destination of this deck. You indicated in your opening post that it will be a screened porch, which gives me to think it will be fully roofed, probably with a suitable overhang. Would that be correct?

If so, you might get by without the deck being sloped away from the building, which is also not mentioned, best I can see, but you'll still want to fully waterproof the surface before you install ceramic tile.

If all that is correct, I see no reason you could not remove the treated plywood, replace it with an exterior glue plywood of suitable grade, build the remainder of the structure, install your CBU or other tiling substrate, and waterproof the installation now, waiting 'till next Spring to install the tile as you have indicated was your plan anyway.

That still won't completely ensure that your treated framing is suitably dried, but it would have done a good bit of its shrinking and changing shape by then if it remains exposed from below. You would not have use of the deck in the interim unless you covered it with something to protect the floor surface, but you should be able to determine how flat the surface has remained and base your floor covering options at that time.

Or you could completely disregard everything you've read here about the dangers of tiling over what you currently have and take your chances with a ceramic tile installation. We can't guarantee failure any more than we can guarantee success here on this forum, best we can do is tell you where the smart money is likely to be betting, eh? :)

My opinion; worth price charged.

06-18-2019, 10:51 AM
Thanks CX. The last thing I will do is disregard the advice you and the others are giving me. I really value the help you give all of us.

Yes, about half of the deck will be screened in, fully roofed with a 2' overhang. The other half of the deck will be 'open to the sky.' It will not be sloped away from the house because (per the builder) the roof line of the porch needs to tie in to the house roof line and the spaces between the deck boards will let rain water drop through. The floor of the deck (and porch) will be 2-3" below the floor level of the house and they have installed a heavy rubber type barrier between the house and the deck's ledger board. The entire deck/porch is 8' above the ground, so plenty of air can get to it.

Two questions. You said: replace [the treated ply] with an exterior glue plywood of suitable grade, build the remainder of the structure, install your CBU or other tiling substrate, and waterproof the installation now, waiting 'till next Spring to install the tile

1) Would it help the joists to dry out if I also waited till next spring to put the CBU down? (or maybe it is needed to shield the plywood?)

2) What do I need to do to 'waterproof' the installation? Thanks again.

06-18-2019, 12:02 PM
so is the underside of the deck exposed?

06-18-2019, 03:32 PM
...the roof line of the porch needs to tie in to the house roof line and the spaces between the deck boards will let rain water drop through.But wasn't that before the decision was made to tile the deck, Frank? The other half of the deck will be 'open to the sky.And that portion of the deck will/will not be covered in tile?

06-18-2019, 04:32 PM
CX -
Yes, the decision about not sloping the deck and matching the roof line was made before any decision about tile. Actually, the builder said they just don't slope them as a matter of course.

The only portion of the deck that would receive tile is under the screen porch roof. The open part of the deck will have 5/4 decking planks.

Attached is a picture of the joists and you can see the black rubber shield and the 3" drop from house floor to deck floor. (The dreaded plywood is in the foreground.


06-18-2019, 04:50 PM
See right there?

That's a picture of Satan. Your plywood is evil. :D


06-19-2019, 05:50 AM
I'm kind of late to this rodeo. Having fresh view at this situation I think you need to reevaluate what structure and materials you have what your original plans were. there are lots of new decking materials of the Trex type variety but some of them are $10 per square foot just for the materials. But then you wouldn't need plywood on top of your treated joist. Screened-in patio only two foot overhang it would get half wet in these parts. Half covered half not. This just does not seem like a good surface for tile no matter what. I'm thinking it freezes in all but southern Florida. I say forget about tile and think of something else but you still may have to take up the plywood. Perhaps carpet is the way to go.

06-19-2019, 09:28 AM
FWIW, around here, you'd have a 50/50 chance of getting red tagged at inspection for short joist hangers, but that's on your contractor.

06-19-2019, 10:06 AM
Simpson Strong-Tie, at least, for 2x8 through 2x12 joists publishes allowable load values when using a hanger one size smaller than the joist.

Cheers, Wayne

06-19-2019, 07:49 PM
Thanks Wayne for that comment. At this point, the last thing I need is more concern about the way this is being built. I have a lot of confidence in this builder....he has a Class A license, has been building for 40 years, lives locally, has TONS of happy customers in the area, so I really want to believe he knows what he is doing. And I do believe that.

06-20-2019, 07:35 AM
I do not recall ever getting an answer with respect to the the joists being exposed below the deck. Are they?

06-20-2019, 08:22 AM