# Replace plank subfloor with plywood



## Hammerswinger (Jan 21, 2017)

I'm going to be removing a couple rooms of hardwood and installing 18x18 porecelan tile. The current subfloor is 1x8 planking ran at a 45° angle. Subfloor is squeeky and I'm aware it's not suitable surface for cement board underlayment. I do not want to install plywood over existing due to height issues. It will be meeting a 12 ft long room of hardwood and I'd like to have that as level as possible.
Questions....I'm sure the planking is ran under walls. Has anyone had to deal with this? Suggestions?
I know if I cut planking flush to wall, I will have to install blocking under wall for support and for edge of plywood.
Do I cut to next joist over and leave remaining section to wall? Given the area, blocking is going to be a pain and the 12ft strip where it meets existing hardwood, I guess I need to block both sections there?

My plan...3/4" t&g plywood, 1/4" cement board, porcelain tile. I'm meeting 3/4 planking and 3/4 hardwood.

Would 3/8" over the planking, detra, and tile be sufficient to keep height without so much pain in the butt?
I don't wanna pull subfloor, blocking, and replace if I don't need to.

Suggestions from anyone who has dealt with this before?


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## joed (Mar 13, 2005)

Why is it not suitable to run cement on top of planking?
Screw all the planks down tight and install the cement board.


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## joecaption (Nov 30, 2011)

No way would I run tile over just planks!!
There going to be full of cupping, gaps, movement.
Sure way to have cracks in the grout and tiles.
#1, need the joist size, and free spans.
Just adding screws to the boards over under sized over spanned joist is just not going to work.
Tile is a one shot deal, difference in height is my last consideration, easy to deal with that compared to a cracking floor.


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## joed (Mar 13, 2005)

Not tiles direct to the boards. You put cement board on top.


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## Hammerswinger (Jan 21, 2017)

Of course I would put cement board over planks. The planks are 1x8 currently nailed to 2x10 joist spanning 12 ft. Height is the reason I would consider removing planking. I know I can get a solid base by putting plywood over planking but I believe I have heard that 1/4" cement board is no good over only plank subfloor. Should I screw planking down, 1/4" cement board, and then tile? Will this be sufficient? Even with large tile? Should I go that route or install new plywood down to joists?

I can easily make a solid base for the tile, but really want to keep that height. This is an open concept area and where kitchen meets living room....I don't want a 3/4" height difference.


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## Msradell (Sep 1, 2011)

The planks are going to be a stronger floor than I would ever will be! As suggested earlier screws them down and if you have some rough areas I would consider running a floor sander over those areas prior to putting down the cement board.


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## joed (Mar 13, 2005)

> I would consider running a floor sander over those areas prior to putting down the cement board.


I like that idea.


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## Hammerswinger (Jan 21, 2017)

I guess I'm a little concerned about under building it. I have a habit of over building things...not a bad thing I guess. It seems hardibacker and durock require their product to be set on minimum 5/8" plywood. Other forums I'm getting mixed info. I'd rather have an over durable base and a height difference than a bouncy subfloor that cracks these pretty tile. I do really really want them to be flush as its a long seam in a open room. I'd like to grout the seam (if that's acceptable. I've done it in smaller transitions with good results) I'd screw down 1/2" exterior ply and cbu and be done with it if I wasn't so concerned about height that's why I was/am almost considering ripping down to joists and starting new.

Am I over thinking it?
Can someone say with certainty that with adequate joist strength and deflection that 3/4" pine planking in excellent shape, screwed down, followed by a mere 1/4" backer set in thinset is strong enough?


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## carpdad (Oct 11, 2010)

I can't give you the exact numbers. You can search with words like ceramic tile on plywood or deflection allowed for tile floor, etc, for published ratings. Personally 12' span for 2x10 may be too much. There is a chance for heavy appliance or bookcase, etc, to go on these floors and that is when the floor can flex. True porceline, they say, is stronger tile and can stand more load but the grout lines can break.
You don't have to focus on installing cement boards. It's not as if they are water barriers. They have better surface for the thinset or other cement to stick but membrane layer or even redguard may be an option. Some modified thinset, I think, is supposed to stick better. 1x8 subfloor is too variable for tiling, I think. Even cement boards will have to be leveled in such cases. They used to put down 1" layer of cement to level the old floors and then changed to cement boards, but thin boards have no strength for flex.
If joists you have is good, then you can cut the 1x8 with a saw about an inch away from walls, put in blocking half under old and half under new. Cut does not have to be flush to the wall. Keep in mind the thicknesses may be different and may have to sand it down. All perimeter should be blocked. Diagonal or double/triple blocks, whatever it takes-don't engineer it. If you block the ply joints in the field, that may give you better flex ratings as well.
As I said, I can't give you the exact numbers, but it starts with finding out if your 2x10 joists are good. Adding t&g ply does not do much. You can use the regular ply. T&G ply came into diy use with the i joists and it is engineered fine line. T&G ply does not suddenly give you a stiffer floor for all purpose. Some ply is just made better and that may make the difference but you will have to find out.


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## Hammerswinger (Jan 21, 2017)

carpdad said:


> I can't give you the exact numbers. You can search with words like ceramic tile on plywood or deflection allowed for tile floor, etc, for published ratings. Personally 12' span for 2x10 may be too much. There is a chance for heavy appliance or bookcase, etc, to go on these floors and that is when the floor can flex. True porceline, they say, is stronger tile and can stand more load but the grout lines can break.
> You don't have to focus on installing cement boards. It's not as if they are water barriers. They have better surface for the thinset or other cement to stick but membrane layer or even redguard may be an option. Some modified thinset, I think, is supposed to stick better. 1x8 subfloor is too variable for tiling, I think. Even cement boards will have to be leveled in such cases. They used to put down 1" layer of cement to level the old floors and then changed to cement boards, but thin boards have no strength for flex.
> If joists you have is good, then you can cut the 1x8 with a saw about an inch away from walls, put in blocking half under old and half under new. Cut does not have to be flush to the wall. Keep in mind the thicknesses may be different and may have to sand it down. All perimeter should be blocked. Diagonal or double/triple blocks, whatever it takes-don't engineer it. If you block the ply joints in the field, that may give you better flex ratings as well.
> As I said, I can't give you the exact numbers, but it starts with finding out if your 2x10 joists are good. Adding t&g ply does not do much. You can use the regular ply. T&G ply came into diy use with the i joists and it is engineered fine line. T&G ply does not suddenly give you a stiffer floor for all purpose. Some ply is just made better and that may make the difference but you will have to find out.


Sounds like you would suggest removing down to joists? Given the area that will be a pain but I'm willing to. I have ductwork all over there in this room and when I do the same in kitchen , the kitchen leads to a small hallway to stairs and a half bath. I say it's gonna be a pain because I only have from 9pm on to work on it at night. Some blocking may not be able to be 2x10 due to 6" duct ran in joist cavities.
I guess I find with this tile to stay flush to a 3/4" hardwood floor I can only use 1/4" product on top of subfloor. I havent had much experience with the membranes other than waterproofing a couple of shower walls with it. I guess my mind deterred from it because it's pretty expensive isn't it? Is it a better product? I'm using fusion pro brand epoxy grout from HD if that makes it more or less likely to crack? I know the stuff is fun to work with. Lol I understand grout cracking is due to surface below and not the grout or tile.

I guess I just need confirmation from a couple people that I need to rip up to joists to make floors flush...that adding 1/4" of any type of material on top of plank subfloor isn't enough? No mircle product? With pulling cabinets and half bath and doing this all in my free time, I was looking to make some part of it quicker. Ahh


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## carpdad (Oct 11, 2010)

Your questions got me thinking I need some reminders so I searched with a question: can I tile over 12' span kitchen floor? Lead me to one site (Ask the Builder) and another (AWC spancalc). The builder says you need min 1 1/4" of flooring for tiles and awc says #2 2x10 dougfir can have as much as about 15' of span with deflection of L/480, which the builder prefers. I didn't search more for board subfloor but it looks like if you replace the boards with 3/4 ply and put 1/2 hardibacker, you are good for the tiles. Make sure from basement that the span is supported by load bearing walls. Wall on slab is good for this. 
Perimeter blocking can be 2x4, even on the flat side for 16" joist spans. If on flat side, use dry 2x4. Predrilling will make nails/screws much easier and screws make the adjustment/mistakes easier. Nailing can move the blocking:smile: so use screws for the subfloor.


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## Hammerswinger (Jan 21, 2017)

Thanks carpdad,
Your post got me thinking as well. Yeah I believe you are right about the 1 1/4 under tile. My hardwood I'm meeting is on 3/4 planking so height of 1 1/2 off joists. The tile that I already purchased and is in another part of the house as well is almost 3/8" thick.

Your comment led me to the conclusion...to do it properly, there is no way I can meet that 1 1/2" flush.... 3/4" ply + 1/2" cbu + 5/16" tile + thinset.... Won't be a big difference but still some.

So based on the information I'm receiving here, I'm going to leave 3/4" planking, cover with 1/2" plywood, overscrew the hell out of it, and follow up with 1/8" thick ditra and tile. By my math....demoing to joists would save me a mere 1/8" in height. I can bite the bullet and say it won't be flush either way and I'll just set in a pretty oak transition.


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## carpdad (Oct 11, 2010)

If you make your own threshold, using wood, the height difference will disappear over everyday use. Problem with store bought threshold is that they are made to go over the uneven ends of the flooring. If you can even out the wood flooring, and lay the tiles straight with finished edge (diamond file), the threshold can go in between. If needed, shim the threshold and install with real brass screws, predrill and countersink. Practice on countersink so you don't bury the head. Remove the door stops for jamb to jamb threshold.
Remembering, that threshold (oak) was rabbeted to go over the engineered floating floor for expansion. I used shims so the screws did not clamp down on the flooring. Butts to the tiles. But the overhang thickness was less than 1/4" somewhat beveled. You can use the bevels to adjust the height difference as well. Plane or bevel on the table saw. Thinner overhang will split off. That floor was 2x10, about 13' span, actual bath floor about 6', and I used double joists under the jet tub. 2x2 porceline floor tiles, 10 yrs and no grout cracks.


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