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Old 08-27-2004, 07:49 AM   #1
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why backerboard...is it needed?

I'm looking for some perspective as I've only done 4 floors to date (in my home. I'm a DIY'er). In the past, I've laid them on plywood with thinset designed for use on plywood (I believe it was flexbond).

I'm currently building a master bath in which I'm going to lay marble tile (walls and floor)(joists are now sistered so deflection is fine). So now I'm looking at what to use on floor and shower walls.

For the floor - what are the pros and cons of using thinset for plywood versus backer board? I figure there must be a strong reason for backerboard or people would avoid the extra work and stack up in floor height associated with backerboard (1.25" of plywood + backerboard = high floor)? is the extra work and increased floor height that comes with backer board a necessary evil to disconnect the tiles from the natural movement/expansion of the wood subfloor?

For the shower walls - what's the best build up from the studs to tile? 30# roofing felt, 1/2 backer board then modified thinset and tile? Do you tuck the felt/vapor barrier into the shower pan (yes, I'm going with a composite/veritek shower pan for the floor)?

Thanks, in advance, for any and all assistance.
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Old 08-27-2004, 09:31 AM   #2
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Good morning Mike. Welcome.

Re. tiling directly over plywood vs. backer board, looks to me like you have it all figured out. Yes, the backer board will help isolate/uncouple the tile from sub-floor movement that will take place. Some experienced professionals still tile directly to plywood using special installation techniques but we do not recommend this method to our general membership and readers here on the Forum.

If 1/4" backer board on the floor creates a height problem, there are isolation membranes, Schluter Ditra being one, which will only add an additional 1/8" or so of height.

And yes again, 1/2' backer board on a shower wall over a vapor barrior, 15# felt is good, using a good modified thinset is a good construction method. I'm not familiar with the pre-made shower pan you mention so really can't comment on how the vapor barrior shoud interface with it. But the pan installation instructions will cover this detail.
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Old 08-27-2004, 06:41 PM   #3
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Hi Mike, Welcome.

We have one professed plywood guy in our membership. There may be other closet dudes we don't know about.

The main guy is Bill Vincent, and that's his username. Look him up in the member list and shoot him an email. He'll be more than happy to talk about tiling directly to plywood. He's been doing it for years.
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Old 08-31-2004, 10:29 AM   #4
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Hi All:

I was going to start a new thread but my issue is also about the need for backer board -- but over radiant heat. Here's the set-up. 5/8" plywood is glued/nailed to 2 x 12's @ 16" o.c.; 3/4" sleepers are screwed to the plywood @ 12" o.c. and 1/2" pex tubing is laid between the sleepers; light-weight concrete was poured between the sleepers, and 3/8" plywood was screwed into the sleepers. The system is running and seems to work fine. The original thinking was to lay down a vinyle sheet good over the plywood after leveling out any uneveness with a self-leveling concrete. My wife nixed the sheet good idea, I nixed wood (too many accidents waiting to happen), and so tile is the compromise.

In looking at tile at Lowe's I was told that I had to put down backer board because there would be too much flex in the floor. The tile guy at HD said that it would be okay because in total I have 1" of plywood given the concrete between the two pieces of plywood. (The floor IS very solid when you walk on it.) A tile installer at a local tile store said that I would be taking a chance that the plywood would delaminate unless it is A/C; in fact it is B/C. He recommended backer board, which I suppose I could use if I am very careful to only screw it where the sleepers are. I truth, I am very reluctant to take the risk, becuase if I put a screw into the tubing that's not the only thing that's screwed. While at the tile store I saw a mock-up using a red/orange plactic mesh which I now believe to be Schulter's ditra. What I object to with ditra is the series of channels that act as insulators to impede the flow of heat. Hence, what I really want to do is place the tile on the plywood, but only if I gain enough confidence it will work long term. Maybe I am being too purest about using ditra. Why is it recommended, as Mike2 seems to suggest, to isolate/decouple the tile from sub-floor movement?

I will, as you advised Mike, seek advice from Bill Vincent, but would certainly welcome guidance from any of the other experts that frequent these pages.

Thanks! Sorry if this is an overly long post.
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Old 08-31-2004, 11:00 AM   #5
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I wouldn't worry about the thermal isolation from the Ditra. The waffles are filled with thinset, so there is mostly concrete in the heat path from the tubing to the air.

I'd worry about tiling directly to plywood, since you only have a single layer (I know about the one under the gypcrete) of 3/8" over the sleepers. If the gypcrete does not support the plywood perfectly, you could have movement. This would be an area where bedding the plywood in thinset over the gypcrete/sleepers would be beneficial.
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Old 08-31-2004, 02:34 PM   #6
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Thanks for the reply, Bob. The material between the sleepers is light-weight concrete, not gypcrete, and it is already in place and covered by the plywood -- so if there are any voids their is not much to be done about it now. When you say "bedding the plywood in thinset over the gypcrete/sleepers would be beneficial" did you think that the plywood was not in place as yet, or would the bedding be on top of the plywood?
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