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-   -   designing 24 OC floor system specifically for tile (https://www.johnbridge.com/vbulletin/showthread.php?t=123716)

ginahoy 09-12-2017 02:20 AM

designing 24 OC floor system specifically for tile
Hi, I'm new to this forum. I'm a building systems engineer. I design mechanical and (previously) automation systems for high performance homes.

I'm finishing up the structural drawings for my own new home in SE Arizona -- a ranch over finished basement. Before I hand off to my truss designer & structural engineer, I want to flesh out a preliminary spec for the floor system. I plan to have tile floors throughout the main level.

My open web floor trusses will have very low deflection (better than 1/720) due to relatively short spans and my requirement for 18" height to accommodate the duct system. The basement ceilings will be suspended grid (lay-in tiles) for permanent and easy access to all plumbing, wiring the duct systems :yummy:

Here's the rub: I want the floor trusses to be 24" OC... not to save $ but to minimize interference for any future plumbing/electrical/HVAC mods. I'm willing to spend more than what I save over 16 OC to beef up the sub-floor, but I don't want to waste money out of ignorance. As you can imagine, I'm getting all sorts of advice from my contact universe (tile distributor, truss designer, GC, etc). I read SBC's guidance but it only addresses truss deflection. Of course, my concern is the unsupported span between 24-oc trusses, not truss deflection.

Searching through the forum, most of the threads on this topic deal with existing floor systems. But I did find a couple of comments that recommended 1-1/8" plywood subfloor with isolation membrane. I'd appreciate confirmation of that, or an alternative recommendation.

Also, there's a full bath on the main level and I'd perfer to avoid a transition in floor height (the entire main level will be tiled). Thoughts?

Kman 09-12-2017 09:26 AM

Welcome to the forum, David. :)

If we're talking ceramic tile and not natural stone, you can certainly get by with two layers of plywood, the first being a minimum of 5/8" tongue and groove properly installed, and the second being a layer of 1/2" installed over the first.

You can also use 3/4" for the first, and 3/8" for the second, but 3/8" is pretty difficult to work with, so we typically recommend 1/2".

Either of those setups will give you the strength you need for ceramic/porcelain tile over joists that are 24" on center.

ginahoy 09-12-2017 12:33 PM

Thanks for the feedback, Kevin.

Did you mean to imply that two layers is preferable to one? A local supplier carries 1-1/8 T&G plywood but I would have to source 5/8 & 1/2 T&G plywood in Tucson and pay a delivery surcharge. And with the additional labor involved in laying down a 2nd layer, I gotta ask if the performance of 1-1/8 would be any different than 5/8 + 1/2?

jadnashua 09-12-2017 12:56 PM

On 24"OC, first, you'll only get a residential rating for the tiled floor, and it REQUIRES a second layer of plywood. If you plan to have a big party, or maybe an island with a stone top, or a piano, or something similar, you should rethink the 24"OC. 19.2"OC might work better, if I remember, you can get a higher rating, and may be able to go with one layer of subflooring if you're using ceramic (not stone). Double-check the specs.

The issue is the between support deflection that causes the ends of the ply to act like small levers. A second layer of ply, installed properly, offset at the 1/4-span points with good fasteners, shields that jacking effect at the panel ends from transmitting into the rigid tiled surface.

Kman 09-12-2017 01:35 PM


Originally Posted by David
I gotta ask if the performance of 1-1/8 would be any different than 5/8 + 1/2?

That's something I can't answer. I started to suggest the 1 1/8" Advantech, but I figured it would be more difficult to locate it vs. 5/8" plywood. It depends on where you're located, I guess.

If you're planning on ceramic, I think you'd be fine with either choice. I can't say that one would perform worse or better than the other. If I had to guess, I'd say the difference would be negligible.

John Bridge 09-12-2017 01:41 PM

Hi David, :)

I've tiled quite a few floors consisting of inch-and-an-eighth Sturd-I-Floor ply or OSB over trusses set on 24 in. centers. I also agree with Kevin that two layers of plywood will do the trick, too. No glue between the two layers, and butt joints of the top layer overlap those of the first.

There are a couple of good articles in our Liberry. Scroll down to Posts 14 and 15.

ginahoy 09-12-2017 02:12 PM

Thanks, Jim. Your explanation re: double layer makes sense.


Double-check the specs.
What specs are you referring to?

BTW, a local tile distributor insists on installing 1/4" Hardibacker throughout instead of an isolation mat (that solves the bathroom transition issue). So would you still advise against a single layer of 1-1/8 in that case?

@John, thanks for chiming in and for that Liberry link. I'll review that before posting any more q's.

(edited after reading JB's reply)

jadnashua 09-12-2017 03:29 PM

I do not remember where it is spelled out, but there are specs for various situations from light residential to industrial, and what installation and structure methods are required to meet each of those variations. If I remember, 24"OC supports, at best, gets a light residential rating. One of the pros will know and can point you to them. As I mentioned, if you have any significant point loads, or heavy objects, or lots of people, or say a big bookcase, you can easily exceed the rating for light residential. It would likely work for a typical bedroom, but maybe not one that had a big water bed, for example. The 10/40 dead/live loading doesn't work if you're going to have a tiled surface everywhere...IMHO, you need a higher load bearing structure. Wood creep happens for a very long time, but most of it happens in the first year. Tile doesn't like to be bent, so both short-term and long-term needs to be accounted for, and you should build for the actual loading you are introducing. It is really easy to exceed the 10#/sqft dead load when dealing with tile, especially if you have an open floorplan, and non-load bearing walls. Just look at a typical bookself after a few years...it didn't break, but it took a big set...your floors, if overloaded with a high dead load can do that, too. Short-term, live loads, they'll bounce back.

houndzilla 09-13-2017 03:16 PM

Hi David,

I'm a commercial architect so I have a different slant on some of your questions. I'll leave the thickness discussion of plywood spans to the forum experts though. I think you would be better served with a tighter spacing and unless you are doing something really crazy with your building utilities or have plans for wholesale reworking of your floor plan I can't envision the concern justifying 24" OC.

Maybe you've already accounted for all these things, but I don't know what you know so I'll just list out some of my thoughts/experiences
-the biggest issue I see with open web joists like these are coordination with the HVAC contractor. This is in instances where a large clear opening is provided in the joist to accommodate duct work. If this is your plan then I suggest you get on your GC to make sure to heavily coordinate this as to avoid a potential headache.

-are you planning on having plywood specs clearly on the drawing? If sticking with a 24"spacing ring it in big freaking arrows as it will absolutely critical for a successful tile installation

-what are these joists over basement/crawlspace/something else

-These days I'm having a hard time getting COMMERCIAL GC's and Subs to read the drawings and specs I provide. I would be really concerned in your case with a residential framing contractor working with 24"OC spacing that requires a bit of care and more importantly thought put into how the deck sheathing is installed and how the joints are layered.

-would it be easier to put in a little preliminary thought and $$$ to figure out where your future needs might be and stub services to those locations?
It sounds like you are acting as the designer for your home, I'm willing to bet you have some ideas about how you would want to expand.

cx 09-13-2017 03:48 PM

Welcome, David. :)

Originally Posted by David
Did you mean to imply that two layers is preferable to one?

Absolutely better. The whole object in the double layer is to eliminate the possibility of having an end joint over the joist extend all the way through the surface of the subflooring. With a double layer you can always cover the joints on the bottom layer with a section of the top layer.

The most common point of ceramic floor tile installation failure due to structural weakness will occur over the floor joist tops.

With your 24" spacing I would want nothing less than a first layer of nominal 3/4" material and a second layer of nominal 1/2" plywood.

In addition to the articles previously linked, this article explains what I think it the best way to install that second plywood layer and I'd recommend you follow their recommendations in your installation.

My opinion; worth price charged.

Dave Gobis 09-14-2017 08:18 AM

What CX said, and I have looked at and participated in study and discussion on this issue over 20 years. There is an APA study floating out there someplace that says 1 1/8" on 24" works. It did work, within the very short duration of time the testing took to perform. Long term performance is a different matter. The tile properties are also a variable.

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