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Unread 02-25-2005, 11:51 AM   #1
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one more guy needing subfloor advice

I am working on replacing a vinyl floor in an upstairs bathroom with 12" x 12" ceramic tile. I have tiled before, but it has always been directly to concrete, so most of my concerns are related to preparing the subfloor. I thought I had the process down, but reading through the posts here has raised many questions and made me less confident. I just want to run it by the experts. Please point out any errors or oversights in my plan. Thanks in advance for any advice.

Step 1 - remove existing vinyl
Remove quarter round and peel up vinyl starting at corners. This will leave the subfloor and a luan substrate used for vinyl installations. Should I leave the luan or get rid of it? I am inclined to get rid of it because I can't believe it is doing any good structurally it is probably covered in adhesive, and I don't really want the extra height.

Step 2 - prepare subfloor
Height is a concern, so I want to minimize it as much as possible while still providing an adequate base for tiling. I have fir 2x10s, 16" on center, spanning a maximum of 14', covered by sheeting with the following characteristics stamped on the underside (24" OC Rating - 3/4" Floor Span - T + G Max Width 47 1/2). The deflecto calculation gives me an L / 380. which sounds very marginal to me. The deflecto doesn't take into account the subfloor which seems to be better than average with the 3/4 T + G material. Do I need to add additional plywood on top of the subfloor? Can I use either 1/4" or 1/2 durock? It seems like the 1/4" hardibacker is popular on this forum. Is there a distinct advantage of hardibacker over durock? If I need extra plywood, should it be glued to the existing subfloor or just fastened? When installing additional plywood and the backer material I should make sure that the fasteners don't penetrate the joists, right?

Step 3 - add the backer material
Use 1/4" x 1/4" trowel to lay down a bed of thinset and then screw or nail the backer material per the mfg specs. Keep backer board 1/4" away from edges to allow for expansion.

Step 4 - tape the joints
Use nylon mesh tape and thinset to tape the joints in the backer board similar to taping drywall joints.

Step 5 - lay the tile
Use the same 1/4" x 1/4" trowel lay a bed of mortar, placing tiles acording to a predetermined pattern, tapping each one into place to insure good adhesion with the thinset. I have read that it is a good idea to clean the thinset out of the grout lines as you go to make a better grout job. Is this really necessary? Keep tile 1/4" away from edge to allow for expansion. Edge will be covered by quarter round.

Step 6 - grout

Step 7 - install quarter round
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Unread 02-25-2005, 01:35 PM   #2
Dave Taylor
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I can get you started while you wait for the pros to respond....

(Quote: Should I leave the luan or get rid of it?)

Remove it.... I've still got lauan marks on my body from one of my own previous posts.

(Quote: Step 4 - tape the joints
[Should I Use] nylon mesh tape and thinset to tape the joints in the backer board )

Brent, If you use Hardi CBU... I believe it should go over plywood and be sure to follow their installation instructions at:


part of which are

1 Prior to setting the tile, fill all joints with the same mortar used to set the tiles.

2 Embed the 2" wide, high strength, alkali resistant, glass fiber cementitious backer unit tape in the mortar and level

Lot of luck

Dave T
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Unread 02-25-2005, 01:45 PM   #3
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Brent, your joists are just fine for ceramic tile. The deflectolator has some conservatism built into it, so if it says OK, then, it's OK.

Your plywood is fine, no need for any more. 1/4" backerboard will work. Any brand will work, pick one and follow their directions.When taping, remember to use backerboard tape, not drywall tape. They look similar, but are made of different materials.

When setting the tiles, pull a tile every now and then to check the thinset coverage on the back of the tile. If not nearly 100%, you need to use a bigger notch or backbutter the tiles. Do clean the thinset out of your grout lines as you go, but you do not have to get it down to the backerboard, simply less than about 1/3 full is good enough.
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