Replies

1. 
   Guest_ | Jul 14, 1999 02:12am | #1

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   With the 1/2 inch plywood glued and screwed down, there is no question that you will have to leave it there. Besides, it probably gave you a much stiffer floor, which is a plus.

   I think the concern is that some time down the road there might be a hairline crack in the grout between tiles, and moisture could go down there. Concrete backerboard is not impervious, so the moisture could continue down into the plywood, and if it's not exterior grade the plywood might swell or delaminate or rot or something dreadful like that.

   I've heard of putting down a layer of plastic vapor barrier on the plywood first with some mastic goo, then the backerboard with some more mastic, then proceed with normal thinset to bed the tiles.

   Instead of concrete backerboard, you could also consider the 1/4 inch hardiboard material which is alleged to be impervious to moisture.

1. 
   Guest_ | Jul 14, 1999 05:12am | #2

   *RC,I think your existing subfloor will be OK. Use 1/2 inch thick cement board if the total combined thicknesses of the backerboard, thinset mix and the actual tile will not pose a height problem. If it does pose a problem, use 5/16 thick backerboard instead.I would not use the plastic vapor barrier like Jay mentioned. Instead, I would install the backerboard directly to your existing subfloor using a construction adhesive or thinset mix, along with using galvanized drywall screws. Space the screws about 8 inches apart around the edges and perhaps 10 inches apart in the middle. You need to trowel thinset into the seams and also embed the mesh type drywall tape into it. This thinset, when dry, would help control any moisture that might otherwise penetrate the backerboard.Allow this backerboard to dry overnight before installing the ceramic tile. Install the tile accordingly, and then afterwards, use a latex additive with your grout. The latex will help keep your grout from cracking.My buddy's house contains ceramic tile flooring in almost every room and was installed using this method and he has no problems to date. His house is now 10 years old. Infact, his foyer area was tiled using 1/2 inch AC plywood instead of the cement backerboard because at that time the ply was cheaper. Your main goal is to have a floor that will be rigid and have little or no flex. Wood can accomplish this goal as well as the cement board. However, the AC ply was exterior grade and was coated with an additive to keep the wood from soaking up too much moisture from the thinset. Jay mentioned something about a waterproof backerboard. One such product which is mentioned in FINEHOMEBUILDING Issue #120 on page 107 is "Dens-Shield." Georgia Pacific handles this. Their phone number is 1-800-284-5347. I don't much about this product myself so I can't really comment about its performance.Good luck,Davo

1. 
   Guest_ | Jul 14, 1999 05:33am | #3

   *RCman,You bet buddy, go for it.Ed. Williams

2. 
   Tripp_Huey | Jul 16, 1999 05:12am | #4

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   The only backerboard thet is waterproof is DensShield. Hardie Backer is not.

1. 
   manja | Jul 18, 1999 08:44pm | #5

   *we're doing a new bathroom with a custom shower. the floor, walls and ceiling of the shower willbe tiled.we have the cement board ontop of 2 layers of exterior grade 3/4 ply with screws every 10 inches on the floor. (we will use it on the walls and ceiling as well.)my question is about the floor. we want it to slope towards the drain which is in the center ofthis 4' x 6' shower. do we use thinset on top of the cement board floorand slope it? Or should we leave this for the tile guy to do?would it matter if we put the cement board on thewalls and then did the floor, or better to take care of the floor first?i know that fine homebuilding had an article on this years ago. (i have the one where there is a retro being done. but i need the article that described this in new construction.)know the issue?thanks

1. 
   Philip_Lawrence | Jul 20, 1999 03:47am | #6

   *Manja:Try the "Search" on the top of this page. Look for the FHB Index and perhaps go look under bath or tiling.If not, let me know and I can do a bothersome search of all the indexs and issues w/ my hard copies of FHB (hav'em all except the first few).OK

1. 
   Philip_Lawrence | Jul 20, 1999 04:04am | #7

   *Manja:Are you aware that you should first put down a base in this shower, usually an EPDM product? It is most important for waterproofing the whole set-up. Since the slope to the drain is so little, you would have the tile contractor do the thinset installation atop the backerboard which is giving the floor the rigidity nec. for tile.The backerboard, like Durarock, can be used on walls and is likely to be done at the same time as the floor. It is the base that follows, then the thinset and tiling.

1. 
   Frank_LaPiana | Jul 25, 1999 09:11am | #8

   *Latiscrete (spelling?) sells a product which is a latex-rubber compound. You paint it on your backerboard, or any other material that will be tiled. They guarantee that thinset and mastic will adhere to it. It's also supposed to be waterproof enough to allow "ponding".I used the product on my tub surround walls and bathroom floor (both concrete backerboard) and haven't had any problem with adherence.

3. 
   RCman | Jul 25, 1999 09:11am | #9

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   I keep reading how cement board for tile has to be installed over minimum 5/8 EXTERIOR grade plywood subfloor. Well, my bath floor has that, then a 1/2 inch layer of regular 'ol plywood (not particle board) over it, in the bathroom, as an underlayment. It is glued and screwed together, laminated quite well; surface is clean of old flooring. Can I just put the cement board over that, even though it is not exterior grade? I don't think it will be a problem, but I thought asking wouldn't hurt. . .