Travertine tile installation: Questions on Joist sistering and subfloor [Archive] - Ceramic Tile Advice Forums - John Bridge Ceramic Tile


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01-17-2017, 05:19 PM

I have a few questions in regards to sistering joists and my current subfloor.

I am looking to install travertine stone tile in my entrance area, were I currently have ceramic tile. The area in question is above a crawl space the total area of the floor is 24'x 12' give or take, which is supported by the block foundation. However only 8'-8" x 12 of it is ceramic the rest leads into a den with hardwood flooring and will remain.

My joists are 2x10s (actual 1-1/2"x9"-9-1/8" must have shrunk), spaced on 16" centers, with an unsupported length of 12'. Which gives l/522-558, below the l/720 required for stone. The subfloor is 23/32 t&g plywood. I am looking to sister the joists from supported end to end with 2x8's under where I plan to install the stone tile. Will 2x8's give me the required l/720 deflection needed for stone tile? The reason for choosing 2x8s is because it is in a crawl space and given the electrical, plumbing and duct work in the area, along with the fact the current joists joists are not standard 2x10s.

Along with that is my 23/32 t&g plywood sufficient for stone tile? I plan on using Blanke Permat on the subfloor and tiling over. I am looking to minimize height difference between the tile and hardwood floor.

Thank you for your help!

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01-17-2017, 05:22 PM
Seems like it would be easier to cut the span down and build a support underneath the current joists instead of fighting the wiring and everything in the joist bays.

01-17-2017, 05:31 PM
As long as the sister covers at least the middle 2/3'rds of the span, you'll gain most of the advantage. The 2x10 nomenclature comes from the rough lumber ends up smaller because it is milled flat (and hopefully straight!), then, it tends to shrink a bit.

Try changing the width and depth of the joists by the thickness of the sisters and running that through the calculator. It will be a little less because you may not be going the full length, but a little more because part of the joist is taller than the part you're going to sister.

Note, for a natural stone, you also will need two layers of plywood in addition to the stiffer joists, the in-between strength must be higher, too.

01-17-2017, 09:22 PM
Thank you for the responses.

Ryan, the mid beam was a consideration and felt it would make a stronger floor, however some of the duct, plumbing and conduit is at the bottom or below the joist. I would also have to dig and pour footers allow to cure and so forth, and figure sister joist to be the better route.

Jim, yes I plan on sistering the full span, figure it wouldn't hurt and should not be too much of a fight, one joist might be a struggle to make it one full piece though. In my searching I saw mostly people beefy up existing 2x8 joists with 2x10 at 2/3rds or so , so I just wasn't sure how beneficial it would be in my situation stated above. I am familiar with the nomenclature, I was just stating the actual just in case it made a difference and figured it just shrunk in over the 30 years. I apologize I don't really follow what you are stating with the numbers in the joist calculator.

The two layers of plywood wood be an issue with the thresholds at three doorways, interfering with the door. How necessary is the two layers?

01-17-2017, 11:15 PM
Two layers IS a requirement when using a natural stone. It is VERY important. The problem with one layer is that the ends of two sheets at a joist will deflect up like little levers and crack the natural stone because it is not as homogeneous as a manufactured tile (in the scheme of things, nothing's guaranteed, but that's more the norm). By installing a second layer properly, you will mask that lever effect.

Look at the blue bar above...put your numbers for your joists into there and run the deflection values.

There's one method that can work, but is a bit harder to do right that can keep the overall height down to effectively one layer. Tear off all of the ply, install cleats on the sides of all of the joists such that you can insert pieces of ply on those cleats so that the top surface is exactly even with the top of the joists. Then, install a second layer (all grain going the same way on both layers) on top of the joists. This ends up supporting the ply across its entire span across the joists and virtually eliminates that jacking at the joist seam. It's more work to ensure the first layer is properly positioned, as both the cleats and the joists need to be straight and anchored exactly where it needs to be or you will have gaps, and lose the benefit. Most people either give up on the real stone and buy a look-alike porcelain one, or live with the room transition required to make it work.

01-18-2017, 03:17 AM
My neighbor had their master bath renovated last year and the installer did not add the second layer of plywood. Within the first month, the really nice 18"X18" travertine tile developed a single crack across all of the tiles in a line parallel to the floor joist. It is close to where they stand at the vanity so they now have a really nice runner in front of the vanity to cover up the crack.

01-18-2017, 02:31 PM
While I know a lot of guys wouldn't install stone with a single layer of t&g there are some underlayments that warranty installation of natural stone over a single layer in certain instances. Blanke permat would be one to look into. I have never used it so I can't say one way or another.

01-18-2017, 03:25 PM
If you read through the back of the Ditra installation manual, Schluter says that they tested Ditra with one layer of ply and stone worked MOST of the time. It depends on how conservative you are and your risk tolerance. Because they want to ensure success, they follow the industry guidelines, and require that second layer because they have tested it and KNOW that it will work if you do it right. That's the big difference. How risk adverse are you? Can you live with a cracked floor if it occurs? That jacking effect between sheet ends over a joist is real, testable, repeatable. Whether it will affect your installation, nobody is going to say one way or the other.

01-18-2017, 03:46 PM
Jim, I am pretty sure I follow you on the cleating, essentially the second layer of plywood would be between the existing floor joists correct? The problem I have with that is I fear I will not be able to tear up the existing subfloor because of existing walls, therefore I would have to accomplish this from below.

The transitions in the interior space I am sure I could live with but with an extra layer of plywood and added thickness of the stone the finished floor would be above two exterior doorsills. If laying a second layer of plywood on top of my current what is the minimum thickness for the second layer of plywood?

I understand your points clearly, and I never like to cut corners, as you said there is a risk involved. Obviously the manufacturer and you for that matter would like to see a satisfactory result and I appreciate that, I am just trying to juggle the best solution in my case.

Michael, thanks for the feed back.

Ryan, the blanke permat is the underlayment I am looking into installing for that reason.

01-18-2017, 04:52 PM
Daniel, I believe Blanke also advertises their Permat as reducing subfloor deflection, which I find ludicrous. Do be cautious when accepting the product advertising, keeping in mind also that there is no industry standard at all for uncoupling membranes.

My opinion; worth price charged.

01-18-2017, 05:07 PM
While tearing off an existing subfloor can be a major pain, sometimes it is the best solution. It was a number of years ago, but I tore all of the subflooring off of about 500sq feet that included four rooms. While I had it off, I planed and shimmed all of the joists so that I'd have about as perfectly flat and level floor as possible. To ensure I didn't have flapping walls or hanging sheets, I had to add some cross-blocking in various places. It was a lot of work. But, I ended up with about as flat and level and solid of a result as you can normally get. I was putting in radiant floor heating along with engineered, floating wood that required the same results as a tiled floor - less than 1/8" in 10'.

It can be done, but might require installation of a temporary wall prior to finishing things off. To me, it was worth the effort, but I'm not up to doing that type of work any more (that was nearly 15-years ago). It will be much harder to ensure you have the intimate contact required of the recessed layer, at least in one way. Gaps will defeat the end result. I suppose you could try to screw the piece in place, then install the cleats verses tearing things off, then installing the cleats. Maybe even a few screws from below first (that you'd take out later) to sandwich the two layers together, then attach the cleats. WOrking over your head is painful if you're not used to it! I'd rather make a jig to position the stuff and install it from the top.

Dave Gobis
01-18-2017, 05:56 PM
The reason "uncoupling membranes" with no standard claim they lower deflection is due to the constraints and reporting requirements of the test used. They don't do any such thing in reality. They just doesn't register as much deflection due to the air space.

01-23-2017, 09:01 PM
Having a tough time figuring how to fit the sister joists on the sill plates with being two blocked at 12' between the masonry foundation, the sill plate pockets are only 3" and 1-1/2" deep. Is there a trick I am missing by chance?

If it is not possible to fit on the sill plates I have been thinking of other options. I prefer to sister the entire joist to strengthen the entire floor. My thought is to add 2x10 blocking on/in the sill plate pocket flush with the block and support the 2x10 sister joists with double joist hangers, attaching to the blocking and existing joist and new joist. Best case I could do that to one side while the other I rest the joist on the foundation sill. Worst case add blocking to both foundation sill plate pockets. Any idea if this method would be in violation of any code?

01-23-2017, 09:26 PM
Daniel, it would help if you'd post a photo or two to be sure we understand exactly what you see a the problem.

If I understand the situation, there is no reason at all for you to go to extreme lengths to get the ends of your sister joists supported on the sill plates. I favor sistering full length, but you can stop the sisters at the walls/beams/whatever you have for support of your joists. You should still have sufficient bearing surface for your joists and you'll still benefit from full length sistering if you properly attach your sisters to your joists with construction adhesive and mechanical fasteners.

My opinion; worth price charged.

01-24-2017, 12:00 PM
Thanks CX, I will get a few photos this evening, are you looking to see the general area or specific points?

I understand what you are saying about still benefiting without resting on the sill. I just figure if I could take the extra steps now and install them the full length, I won't have to kick myself later for not doing so.

01-24-2017, 02:03 PM
Testing has shown that sistering the middle 2/3'rds of a joist run provides nearly the same benefit as resting it on the supports IF it is properly bonded to the original joist.

01-24-2017, 03:45 PM
See the photos of the area I am dealing with, it's approximately a 12'x9' area in my entrance which also has a powder room. I could temporarily remove the duct work but the plumbing must stay as it feeds the powder room and a second floor main bathroom.

I do understand that about properly bonding the sisters to the joist, as I stated before just looking for an option to strengthen the entire floor with it support of the foundations sill.


01-24-2017, 03:46 PM
Second photo

01-24-2017, 03:53 PM
This is looking towards my garage, with my back to the entrance in the crawl space from my basement.

01-29-2017, 01:22 PM
I am still toying with the idea of adding blocking between my joists on the sill plates and using double joist hangers to span the full length. Just would like to know if that is anything against code.

In the mean time I was looking more into cutting the 12' span in half, there are some obstacles with drain lines that cannot be moved, clearance between bottom of joist and drain is 6-1/2" and duct work in a joist space which sticks 1" below joist. I have come up with constructing a beam out of double 2x6s (or triple) spanning 24' (10' and 14') with a 1x on top to get me under the ductwork. I would support it in 6 places with 4x4s on precast footers.

Thoughts and opinions on this?

02-20-2017, 05:50 PM
I have decided to add mid beam support to reduce the deflection. I am installing a double 2x6 supported by 4x4s columns on 5'-4" centers. I will reduce the 12' support span of the 2x10 joists in half to 6'

I have a question in regards to taking up weight, do I need to actually take any weight of the joists? Some joist are slightly lower/higher. Or simply get the beam snug to the lowest floor joists cut 4x4 columns to fit and then shim any floors joist not touching the double 2x6 beam?

02-20-2017, 09:41 PM
Daniel, this might be a good time to try to flatten your floor a little. I'd want to unweight the low joists a bit when installing the beam and shim only as necessary under the others that are still high after that.

I don't think doubled 2x6s are gonna get you where you need to be. I've not done any math on your beam, not knowing even what wood it might be, but I think you'll need at least 2x8s under there for your purpose.

My opinion; worth price charged.

02-21-2017, 09:05 PM
The 2x6s are SPF #2 the reason for choosing the 2x6 over something larger was because of limitations of drain lines, duct work, and I figure all I need to do is shorten the span to limit the deflection. There was a chart I came across that if I recall with the double 2x6 would be good for 6'-6" ish support spacing, which I cannot find at this time.

You mention unweighting the lower joist, by how much? The reason I ask is one of the lower joist is a double 2x10 joist with a steel plate, the unsupported span of that joist is 9' which also carries a load bearing wall thru the second floor. That double joist is directly below where the transition between the proposed travertine and existing hardwood. In reality I only would need to support under the entrance, but I figure while I was In there to carry it through the entire floor.

02-21-2017, 09:27 PM
Problem you have with your beam is that you're now looking to it to support half the total load on that floor and do so with little enough deflection for natural stone. Yeah, if you used your 2x6s on 16-inch centers for your entire floor with supports ever 6 1/2 feet, they would be fine. But you're not doing that.

A beam in the center of your 12-foot span must support the floor load for half the distance in each direction to the next support and deflect less than L/720 while doing so.

Entirely up to you what you're comfortable with.

My opinion; worth price charged.