2nd Layer of Plywood into Joists - Subfloor Question [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


View Full Version : 2nd Layer of Plywood into Joists - Subfloor Question

12-30-2016, 07:53 AM
Longtime lurker on this great forum. I'm remodeling my kitchen and entryway (Roughly 300 sq.ft.) which is over a crawlspace. The floor joists are 2x10's and 16" O.C. The joists are supported every 8-10 ft. by Irons beams which are mounted on the concrete crawlspace floor. The joist structure is solid and passes the deflection calculator.

Much to my surprise when I pulled up the 3/4" Oak hardwood floor; I realized that there is a 1/2" plywood subfloor which is then topped by a 2nd layer of 3/4" plywood. BOTH layers of plywood are nailed into the joists. I plan to put 1/4" cement board over the two layers of plywood.

My question:

Since I will be laying stone (Travertine), do I have to pull the 2nd layer of plywood and then reset with screws to avoid the joists OR can I just sink screws into the 2nd layer of plywood and just avoid the joists? Any other suggestions?

Sponsored Links

12-30-2016, 09:43 AM
Welcome, Jim. :)

Unfortunately, what you have there is a single layer 3/4" plywood subfloor installed over a 1/2" spacer.

The Marble Institute of America requires a double layer plywood subfloor, properly installed, regardless the joist spacing, for a natural stone tile installation. You, of course, can install your Travertine over anything you want.

In your situation I would recommend you install a second layer of nominal 1/2" plywood subflooring, installed as shown in this good article (http://www.johnbridge.com/images/mike2/For%20Liberry%20Stuff/Underlayment-Nielsen-Woeste-0604.pdf..pdf) from our Liberry. On top of that would be your CBU or tiling membrane.

As to the joist structure, the joists would be suitable for a natural stone installation only if they are known to be of good grade and species. It's rare for a joist structure to be designed to meet the required L/720 deflection requirement unless the original plans called for a natural stone floor covering.

I'm a little confused about just what you have for joist supports. You say they are mounted on "iron beams which are mounted on the concrete crawlspace floor." But if you have a crawl space, the "iron beams" couldn't well be setting on the floor, seems to me. Can you clarify that a bit?

My opinion; worth price charged.

12-30-2016, 10:06 AM

Sorry, I realize that was confusing with the way I typed it.

The joists are supported by iron beams. The iron beams have iron posts that are roughly 4' tall and are mounted into the concrete floor.

Would I be better off removing both layers of plywood and then then re-using the 3/4" layer and adding 1/2" plywood on top? I am a little worried that I will have a lot of weight with 1/2" layer, then 3/4" layer, then another 1/2" layer.

12-30-2016, 10:15 AM
Possibly, but I'd be more concerned with your steel beams and wood joists being designed to meet the necessary L/720 deflection requirement. You'll be adding a lot more weight with the Travertine, tiling substrate, bonding mortar and grout than with the extra layer of half-inch plywood.

I wouldn't bother removing the bottom layer of half-inch plywood unless overall height was a serious concern.

My opinion; worth price charged.

12-30-2016, 02:04 PM
Thanks, I will add another layer of 1/2" plywood then. I will just use Ditra instead of the cement board so I can save some height.

The current structure far surpasses the minimum deflection requirements using the site calculator so we should be good there!

12-31-2016, 05:32 AM
So the 1/2" plywood is on the joists and the 3/4" is on top of that. Is that correct? I'm not entirely familiar with the Marble Institute's specification (I really need to order that book!) but I certainly do concur that the weight of a travertine tile floor will be substantial compared to the old wood floor. In this case more is better. Go with another layer of 1/2" (glued and screwed into the realm of overkill (every 6" o.c.) is my preferred method.). Schluter-Ditra membrane is my choice for a stone such as travertine, over a mechanically fastened cement board, over the top layer of plywood. It allows for lateral movement where the backer-board doesn't. In my experience the lateral movement is a partner to deflection in sabotaging you plans for a beautiful travertine floor.

Also I think I remember something in the TCNA Handbook about only the bottom layer of subfloor should be fastened to the joist. Maybe I'm "mis"-remembering though. I'm not sure how much trouble I'd go through though to correct something like that at this point though but I'm no engineer. :corn:

01-03-2017, 08:35 AM
The current structure far surpasses the minimum deflection requirements using the site calculator so we should be good there!

Worth noting that if you have any of the steel supports beneath the travertine, make sure the tops of the opposing joists are tied together well. I have seen many times where the abutting runs of deflection compliant joists results in cracking of stone finishes because the ends of the joists will rotate unless anchored across the support well.

06-08-2017, 07:09 AM
Much to my shock, I noticed a four foot long crack in my new Travertine tile installation and it is extremely bothering; of course! I literally am dejected after the time/expense/thought process that went into this.

Details on install:
- I installed the tile, 12x18 (300 sq. ft.)
- Install is over a crawl space with a concrete slab
- Joist system met the deflection requirements
- 1st layer of plywood is 3/4" screwed to joists
- 2nd layer of plywood is 1/2" screwed in field
- Used Ditra, installed with Ultraflex (modified) and then tiled with Kerabond (Unmodified)

The install was four months ago and I now have a crack that runs through three tiles for about four feet and is perpendicular to the joists. I can take a picture if needed. Obviously there is movement in the subfloor but my question is why and what can be done to prevent more cracking. Keep in mind the install was in the Chicago winter and now it is the Chicago summer. I'm not sure if I was supposed to do anything with the crawlspace to minimize shrinking/expansion of the subfloor?

06-08-2017, 07:56 AM
Welcome back, Jim. :)

I merged your new thread with the old one so we have some of the history of your project with it.

Did you use exterior glue or exposure 1 plywood with a face grade of C or better? What kind of screws did you use?

06-08-2017, 02:53 PM
Is the crack aligned with anything underneath the floor?

How big is the area that was tiled?

Does it get a lot of direct sunlight?

It sounds like you removed the existing subflooring to get a 3/4" layer on the joists. Was the 3/4" ply T&G? Did you leave a gap at the ends?

06-08-2017, 03:52 PM
To answer the questions:

- Yes, the 1/2" plywood was appropriately rated.
- Screws were Deck Mate exterior star drive screws
- I can check the crawl space but the crack may be in line with a support beam that I added in order to meet the L/720 requirement
- The tiled area is 300 sq.ft.
- No direct sunlight for the most part there
- The 3/4 ply was existing; I just screwed it down since it was nailed when the home was built. I believe it was T&G but can't be 100% certain

06-08-2017, 06:56 PM
I can check the crawl space but the crack may be in line with a support beam that I added in order to meet the L/720 requirement
Did you use kiln dried lumber for your support beam and posts? What size is the beam?

Cheers, Wayne

06-08-2017, 07:40 PM
Did you install bracing and what kind of plywood did you use b c exterior?

06-09-2017, 07:53 AM
My wife called me and said there appears to be another small crack in another section of the kitchen which also happens to be right where the 2nd support beam was installed. I am going to measure where the beams are tonight in relation to where the cracks are.

The beams were 4x4 posts (Douglas fir) with a sistered LVL beam on top (9.25"). I then used shims above the beam between the joists since all of the joists were not touching the beam. Possibly the posts expanded with the warm weather in Chicago?

06-09-2017, 07:54 AM
Yes, I used metal bracing between the beam and the joists and used exterior bc grade plywood for the underlayment. I also used metal caps underneath the supports posts so that wood was not touching concrete.

06-09-2017, 09:08 AM
Sounds like you did everything right its highly unlikely the posts would of expanded. I see you even stripped it all down and started clean. did you use glue? thats especially important for the first layer. Other than that its possible you may of needed to sister a few joists to achieve enough flatness and support in a certain area.

As well cross bracing is especially important installed like every 4 feet. Did you use metalor wood bracing all along the joists?

Hopefully its just a lack of mud under those few tiles.

06-10-2017, 02:31 PM
Yep, it is the support posts as the cracks are literally right under the posts that were added. They must have heaved or expanded. I probably would have been better off with the existing support structure. I am at a loss as the remaining tiles in line with the beam are likely to start cracking as well. Really disappointing for following all the requirements for stone...


06-10-2017, 04:28 PM
Jim, did you actually remove the existing first layer of 1/2" plywood subflooring and start anew with 3/4" T&G plywood and a second layer of 1/2" plywood? Or did you just add 1/2" plywood over the original package you had?

I've never actually encountered one of those crawl spaces with a concrete floor in it, but the ones I've read about over the years here on the forums didn't sound as though they were very structurally sound at all. Did you provide any additional concrete under your new support posts or did you just sit the posts directly on the existing concrete?

You did all this structural improvement before installing the tile, yes?

The cracking you have is directly above the existing support beam under the joists, or only under the newly added beam?

06-10-2017, 07:43 PM
I dont get how 4' tall post could expand that much compared to everything else. was it very cold and in an uninsulated space? it would seem if you had that post and lvl beam system in 3 different points it would of been all tied together and not a problem.

very interesting to find out that that can actually happen. is there anyway you can install a temporary beam and lower the supports for that other one and then readjust so that they're just touching the beam (relieve and tension)

theres a good discussion about bridging in this thread. really just the bracing on the ends by the iron supports like john was saying is all you need. I assume cross bracing down the span (evey 4-6') of the joist may reduce noise in a wood floor.


Hope you get it figured out

06-11-2017, 08:22 AM
CX - No, I did not remove the existing 3/4" subfloor. I just screwed it down every 4-6" into the joists since it was only nailed. I then added the 1/2" plywood. I did not pour any new concrete under the posts; I just set it on the existing concrete. Yes, I did the structure improvements before tiling. The cracking is ONLY above the newly added beams. There are now three total beams: One existing steel beam which has no cracked titles and then the two new wood beams, which have cracked tiles. How is this for oddity: The one beam is only four feet long whereas the tile spans about six feet in this area. The cracked title are only about four feet long!

Colby - Yes, it is an un-insulated crawlspace. This is how they are in the Chicagoland area. Just a poured concrete foundation wall with a poured concrete floor. The 2x10 joists sit on the concrete foundation wall. It is not heated/cooled. I installed in winter and it is now summer. What is the "bracing on the ends of the iron supports?"

At this point, I am probably just going to pull the beams. I never should have installed them. The only risk now is that I forget how snug I made them and if the joists drop, I may have more cracking. However, my philosophy now is that the tiles are going to crack anyway so I would rather pull the beams and then replace any cracked titles and then I will be good to go. A ton of extra work, but I don't have much of a choice.

06-11-2017, 04:34 PM
When you installed the new beams, could you tell if any joist was not exactly in line with the others? Plus, are you sure that the beams themselves were straight? How did you end up anchoring it in place? If one was a bit low, and you tightened things so all were in contact, that might account for a crack only in part of the span.

If you lay a long straightedge across the area that cracked tiles..is it flat, or is there a peak there?