tile subfloor question

bubblahMarch 1, 2011

I have 2x10 joists @ 16" OC. My original subflooring is 1x12 planks laid perpendicular to the joists.

I'm tiling two different spaces--

Bathroom, approx. 4'x8' of tile, all of the subfloor except for about 5-6 sqft at the doorway has been replaced by 3/4" 'dryply' because of a moisture leak. I'm using 6"x6" porcelain tiles.

Entryway, approx. 4'x10' including a small closet, the subfloor is the original 1x12 plank, the orientation of the planks/joists is such that the planks run the "length" (10') of the space and the joists run the "width" (4') of the space. I'm using 12"x12" porcelain tiles.

I'm trying to match up (height) as best I can to strip hardwood laid directly onto the subfloor planks in adjacent hallways

My questions are regarding the best setup to put on top of the existing sub-floors--

1) 3/8" ply + 1/2" hardi?


2) 1/2" ply + 1/4" hardi?

I figure both of those will end up at approx. 7/8" thickness when including some space for thinset (and the fact that 1/2" hardi isn't actually 1/2")

Also, what type of ply is best? Since I have 1/2" hardi already, I decided to buy some 3/8" ply, I went to the orange big box, explained it is going under cement board for tile, and was pointed to some Georgia Pacific product marked simply '3/8" Rated Sheathing', at the time I was in a bit of a rush because I was approaching the cut off for renting their van for the day, when I got home I noticed that the sheets seem to "crackle" and flex an awful lot, they are also somewhat warped, but do lay flat with weight on them. I couldn't find the stuff on their website to verify the grade, however when I went to the blue big box site I found something that appears to be basically the same and it's rated "D-D". Is there some way to figure it out by looking at the sheets? I didn't see any obvious marks that would lead me to the grade. Also, I assume D-D grade is not OK? I would want A-C or something similar?

My plan is to screw the ply I put down to the existing subfloor (not the joists), then lay down thinset and hardi on top (with backer board screws where indicated).

My GC for the kitchen "glued and screwed" 1/2" hardi directly to the subfloor planks... at the time I had an inkling it might not be the best, but I assumed he knew what to do... since then what I've heard and read leads me to believe what he did was wrong.

I'm behind on this project which is what lead to me buying the materials when I had time and access to the van, but I'd rather be late then do it wrong... I don't mind eating the costs for any material I can't return, I just want to do things as correctly as possible :)

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I would think any grade plywood would be fine but I would like to see it be 3/4". The plank subfloor that you have can move independently of each other and you need plywood screwed to that to make a solid unit. Then I would feel comfortable putting either 1/4 or 1/2" hardi. By the way, I use 100% glue down of my Hardi board instead of the thinset. I have found thinset sometimes peeling off the plywood. Glue stays flexible and holds much better to the plywood.

    Bookmark   March 1, 2011 at 3:02PM
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in the GW bathroom forum you will get more answers.
The official story is that you need a lot of high quality plywood.
It may work well with less than the TCNA standards but you may as well read what the standards are.

Cement board could be 1/4" or thicker.
It could be replaced with a waffle membrane.

2x10 joists but mention how long the unsupported span is. 16" OC is standard.

    Bookmark   March 1, 2011 at 6:43PM
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5/8 CDX is a good standard grade for subflooring. However, under either hardwood or tile, I would recommend that a floor should be two layers of 5/8 T&G plywood screwed and glued. For hardwood, this provides a needed high-grip nailing substrate that will lead to much fewer problems with squeaks down the road. With your 1^12 subflooring, one layer of 5/8" t&g screwed and glued is fine. For tile, if you do not want movement and grout cracks, you need a very rigid substrate. If the tile is for a wet location, I would cover your subfloor with of 5/8 or 3/4 inch T&G plywood with 3/8" or 1/2" of concrete backer board above it-- again screwing and gluing all sheet goods is really important. As long as you maintain the minimums, you can adjust materials thickness to achieve a match of finished flooring. For example: 3/4" t&g + 3/4" oak matches 5/8" t&g + 3/8" concrete board + 1/2" tile very well.

As mentioned above span of the 2^10s is important. For anything over 12 feet, 2^12's or 12" TJI's are called for to prevent bounce. What is important is not the size of your room but the span of the floor joists which you can determine by going into the basement or crawl space. Of course your statement about 1^12 subflooring makes me think your house is old. If that is the case the 2^10's may actually be much stronger than today's are.** One thing that confuses me, however, is that plank subflooring was normally laid diagonally to provide better shear resistance and to allow normal layout of the finish floor. With the way your subflooring is laid, you really need to use plywood above, since otherwise you may be tempted to lay the flooring parallel to the joists to avoid ridges in the hardwood at the joints between planks-- that would be a really bad idea.


** The roughsawn 2 by 10s in use 100 years ago are actually twice as strong as today's 2 by 10's if made of the same wood species since the bearing strength of a joist is proportional to the square of the thickness and the cube of the depth of the joist. (2/1.5)**2 x (10/9.5)**3 = 2.1

    Bookmark   March 2, 2011 at 11:32PM
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