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01-12-2014, 07:34 PM
First, thank you to all that post here. Your advice and patience are astounding and appreciated.
I have a 30 year old starter home and the wife wants to tile a 5'x8' bath floor, which is currently vinyl. My joists rate a deflection number of L/297. It has unknown species joists in good condition, 7" height x 1.5" width, 16 O.C. The joist span is approximately 10'6". My subfloor measures 5/8" thick over unknown underlayment. The house flooring, in general, has some flex or bounce obviously. Again, like most, I have some height matching issues to transition carpet in the hallway flooring.
My question is this...if I remove the 5/8" plywood floor and replace with 3/4" T&G plywood + Membrane to manage the height issue, will that improve my deflection number enough to lay porcelain tiles safely? Do I have to add blocking or sister to the existing joists to improve the deflection.
Thank you for your support!

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01-12-2014, 09:01 PM
Welcome, Mark. :)

There are two areas of floor deflection you must deal with when planning a ceramic tile installation. The joist deflection and the between-joist deflection.

The type and thickness of your subflooring determines the between-joist deflection and has nothing much to do with the joist deflection. The joist deflection has nothing at all to do with the subfloor deflection.

To bring your joist structure up to the required deflection standard you can either add more joists, usually by sistering, or add a support beam or wall to reduce the joist span. Only you can tell which is most feasible in your situation.

I'm confused about your subfloor. You say: My subfloor measures 5/8" thick over unknown underlayment.More hints. What's under the 5/8ths" subflooring?

My opinion; worth price charged.

01-12-2014, 09:22 PM
Thanks CX for the reply. I'm new to this so let me try to clarify.

I have a 1/2" plywood underlayment on top of my joists and on top of that I have a 5/8" plywood subfloor. I've read that It is recommended to have a minimum of 1 1/4" subfloor before tile is installed. I really don't want to go that thick once I add the membrane, thin set and tile, it will be way higher than the flooring in the next room.
I guess I need two answers. First, I need to fix the joist deflection so is blocking an acceptable method? Second, will 3/4" subfloor be enough to support porcelain tile?

01-12-2014, 09:39 PM
OK, little confusion in terminology here. The first layer of plywood is your subfloor. Or, in your case, called a subfloor. The second layer could be an underlayment, but more likely another layer of subflooring in your case.

In any event, the 1/2" layer is essentially worthless as a subfloor. It's too thin to be of any real value and it's not a tongue in groove style that is required for that first layer unless each joint is properly blocked, which yours are not.

If height is a serious concern for you, your best bet is to remove everything down to the joist tops and start over with a layer of nominal 3/4" T&G exterior glue plywood with no face of grade lower than C. That alone would meet the minimum requirements of any tile substrate manufacturer. I would rather see you add another layer of nominal 1/2" plywood, but it's not a requirement.

There is no such general requirement in the ceramic tile industry that you must have any particular thickness of subflooring. There are industry recommendations and there are substrate manufacturers' recommendations and they vary.

If by blocking you mean between-joist blocking or bridging, no, that adds nothing at all to the rigidity of your joists. It helps the joists maintain and display their full design deflection capability, but it does not improve it.

My opinion; worth price charged.

01-13-2014, 05:44 AM
Again, thanks CX for the answers. Hi ho, hi oh it's off to work I go. I'll let you know how this turns out but I plan to take this area down to the joist and replace with the minimum 3/4" T&G. If I can stand more height I will do so.


01-19-2014, 06:51 PM
When I reinforced my 2x7 floor joists instead of adding a full 2x4 and creating an inverted "I" beam, I ripped the 2x4's down the middle and instead glued and screwed the ripped pieces (essentially 2x2's) to the bottom creating an effectively 2x8.75 joists. Do I still get the benefit of reinforcement?

Houston Remodeler
01-19-2014, 07:01 PM
With the quality of 2x4's available I'd guess; not one little bit.

01-19-2014, 07:16 PM
In theory you might, Mark, but in the real world it's unlikely you bought yourself much. It would require an extraordinarily good job of gluing those pieces together and you haven't got a prayer of doing that onsite with a subfloor in place and a very, very small chance with the joists fully exposed.

Tell us just what you did and let's see if we can decide what you might have. Photos would help. :)

01-19-2014, 07:47 PM
Thanks again for the reply CX. Not sure how to attach photos but here is a little more description...
I used kiln dried SPF 2 frame grade 2x4's and made sure they were straight as arrows. Once they were ripped, I applied Titebond wood glue to the underside of the joist as well as the top side of the 2x4 using an old credit card to make sure the glue was spread evenly and with a good thick coverage on both surfaces. I then used 3 1/2" deck screws spaced 7" apart starting 2" from the end along the entire 10' span. The glue oozed out along the entire length pretty well.
Sistering will be almost impossible due to the water and electrical lines running between the joists. I didn't realize that I was supposed to create an "I" beam and now I'm afraid the glue has set and I can't go back.

Houston Remodeler
01-19-2014, 08:07 PM
To attach photos, click on the "paper clip" icon at the header of the reply box


Scroll down a bit and click on "manage attachments"

01-19-2014, 08:58 PM
That might have been effective if your mating surfaces were very flat and square and smooth and clean, Mark, and if you were able to achieve good bonding pressure without splitting your added piece (your spacing is a bit wide for my taste), you might have done enough good to achieve your goal if you let it all cure for a couple days before loading the floor at all.

Tough to say from over here, but if you gained the equivalent of just another half or three-quarters of an inch of joist depth, you would have done substantial good.

My opinion; worth price charged.

01-19-2014, 10:56 PM
Mark, was the piece you applied continuous for the entire 10' or is there a joint in the middle?

01-20-2014, 06:37 AM
I made sure that I cleaned the existing joist face well and both surfaces were straight (no twists or cracks). The new piece ran the full length of the joist and didn't split at all. If I continue I'll decrease my screw spacing to 6". Again, the bathrooms are very small with effective walking space of about 5'x5' so not huge areas.
Thanks for the replies.

01-20-2014, 10:34 AM
Mark, the size and shape of the area to be tiled has no bearing on the requirement for the joist structure. It's the unsupported span that governs.

01-20-2014, 03:46 PM

I can run the deflection numbers for your custom sized beam of 1.5" x 8.75" if you can let me know the max unsupported span. I read 10' in a couple place but was not sure if that is the full unsupported span.

The calculations will need to assume perfect lamination but will give you an idea of best possible performance and then it will be down to CX's tried and true "it's your floor and your money" rule.

01-20-2014, 05:25 PM
Thanks so much PC. The span is 10'5" and the joist depth will be exactly 8.625". I have two plywood subfloor layers. Next to the joist is a 1/2" layer with a 5/8" layer on top if that. There is a felt layer between the two.
I do not have enough depth to use a 3/4" plywood sheet as the transition would be too high. I might pull up the 5/8" layer and replace with another 5/8 T&G layer, glued and screwed if ya'all recommend.


01-20-2014, 09:09 PM

I ran the calculations for 10.5' unsupported span for 1.5" x 8.625" rectangular beam.

Calculated deflection (inches) = 0.195"
Deflection Factor = L/647

Unknown wood / laminate offset (minus 25%) = L/485

So it looks like you are good for Ceramic assuming your lamination is solid but it's your floor and your call.

As a FYI, given your house is 30 year old, it is very likely your joists are SYP or Doug Fir which yields a deflection rating of L / 389 on Deflecto. The wood type can vary by area and I didn't see your location in your user profile.

If you are building any more joists up, use the entire 2x4 attach to the bottom of the joist. Have you used the TiteBond glue much? I use the LOCTITE Pl 8X ( for build up of beams like this.

Your sub-floor configuration is usual to say the least. Can you pull up both layers and replace with a single 3/4" T&G layer? One good layer is much better than two suspect ones.

01-21-2014, 06:08 AM
Thank you PC. I have two bathroom floors I need to support and I've only done the one with the ripped 2x4's. I'll definitely use the full 2x4 on the second floor. I think on the one I've already done I'll go back and sister a second joist to the joists I can get to. Can't go full length but should be able to cover 2/3's the length.
I can also get to both subfloors so I will definitely replace the layers with one good layer of T&G. Do you know if they make anything thicker than 3/4"?

01-21-2014, 08:39 AM
You can get Advantech OSB in 1 1/4" thickness but it's heavy and hard to get. I'd probably just do two layers using the 3/4 t&g glued and screwed to the joists and a second 1/2" layer screwed to the first layer at the 1/4 and 3/4 points of the joist spacing (read this doc in the liberry ( for details).

01-21-2014, 08:45 AM
Can't go full length but should be able to cover 2/3's the length

You can also glue and screw a 2x4 to the side of the joists along the bottom edge (making a stubby L shape) to increase the strength. That would also held to ensure the strength of your original glued section.

01-22-2014, 11:18 AM
Thanks again for the help PC. I have one more question and I'll stop bothering you.
Is it better to use one thick layer of plywood or two separate ones that equal the same thickness? If I use two sheets, does it matter whether the thicker sheet goes on the bottom or top? Logic says to put it on the bottom, but?
Lastly, I do have a transition issue that I need to manage so I Ned to keep the subfloor and underlayment to a max of 1 1/8" if at all possible.

01-22-2014, 11:28 AM
Thanks again for the help PC. I have one more question and I'll stop bothering you.

Not aproblem, keep em' coming....

Is it better to use one thick layer of plywood or two separate ones that equal the same thickness? If I use two sheets, does it matter whether the thicker sheet goes on the bottom or top? Logic says to put it on the bottom, but?

Need the thicker layer on the bottom, minimum of 5/8" thickness; seconds layer must be a minimum of 1/2". Personally, I wouldn't do less than 3/4 on the bottom layer.

Lastly, I do have a transition issue that I need to manage so I Ned to keep the subfloor and underlayment to a max of 1 1/8" if at all possible

Does this include the subfloor+DITRA (or CBU)+tile?

01-22-2014, 06:44 PM
PC, I just saw the post with your bath reno and...WOW it looks great! I plan to use similar size tiles which is why I'm so concerned about the deflection.
Actually it sounds as though we had similar problems with our joists. After doing similar work (reinforcing and sistering) my joists rate L/520 and feel solid. However my concern now turns to the subfloor.
My floor consists of 1/2" plywood next to the joist with 5/8" on top. I need to transition to carpet and only have another 11/16" to work with. So, if I go with 3/4" plus two layers of thin set, 1/8" Ditra and tile I'm well over the 11/16". That's why I was hoping to take up the existing 5/8" sub, leave the 1/2" but screw it down well then replace with 5/8" T&G on top (glued and screwed) giving me the same height but hopefully a stronger floor.
If you think this is too risky I'll have to live with the greater height of the 3/4" and use a threshold. Also, will I have any structural issues removing the 1/2" if the thicker sheet goes on the bottom? Both walls are on double joists.

01-23-2014, 10:59 AM
Mark, Thanks for the compliment on the bathroom, fun project and I was amazed at how much goes into a good bathroom reno.

My floor consists of 1/2" plywood next to the joist with 5/8" on top. I need to transition to carpet and only have another 11/16" to work with. So, if I go with 3/4" plus two layers of thin set, 1/8" Ditra and tile I'm well over the 11/16".

Re your floor thickness, I'm not sure where the 11/16" goal starts from. As a reference point, my "thinset - ditra - thinset - 5/16 thick tile" layer (top 2 layers of picture below) are very close to 11/16" thick.

I personally do not like the 1/2 - 5/8 combo you have and would strip it out and replace with a single layer of 3/4" T&G subfloor. That will give you a strong and flat base layer ready for install of the "thinset - ditra - thinset - tile" layers for a total approximate height of 1.5 inches from the top of the joist to the top of the tile.

The attached image illustrates the methods to level and build up a floor, your would not need the second layer of plywood as shown unless you want the extra height.


Re the height match up, I wouldn't get too concerned about a perfect match. My hall bathroom tile floor is 1/2" higher that the adjoining floor, I just used a threshold cut to 1 3/4" to align with the bottom of the door when closed. There are a lot of wood thresholds you can buy (or make) that work great too.


03-26-2014, 04:49 PM
I've just finished laying a restroom floor with large (12x24) porcelain tile. The subfloor is constructed of a sheet of 3/4" plywood glued and screwed to a sheet of 1/2" plywood. A separation membrane of ditra was layed on top. All tiles were back buttered and I used a 1/2" trowel to spread the Versabond thinset.

I am placing a 48" vanity with granite countertop on the tiles. The vanity sits on four feet with four 3/4" rubber feet used for leveling. I would estimate the weight at about 350 to 400 lbs. My question is, do I sound safe from the tiles cracking or should I remove the rubber feet? The vanity has been sitting on the floor for about a week with no apparent problems.

Thanks for the opinions.


Houston Remodeler
03-26-2014, 04:53 PM

I ain't no MIT graduate, but I can't see the rubber feet making a difference when it comes to point load. Af long as there isn't a void under the foot / feet, you should be good to go.

03-26-2014, 05:19 PM
And if your estimate of the weight of the vanity is correct, that's less than 100 pounds on each foot. It's heavy, but if the tile is supported, where can it go?

I'm assuming if you remove the feet, the vanity would be supported by the ends. I think I might make my decision based on what looks best, if you can even tell the difference.