Replies

1. 
   NJkwan | Jan 27, 1999 06:13pm | #1

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   Some say that 3/4 inch ply subfloor is OK if you use the thinset with additives. But I would check the floor joists first. Is it bouncy? If there is no bounce, then either apply 1/2 inch cementboard screwed to the subfloor or, the latest recommendation from a tile supplier, nailed metal reinforcing sheets over moist barrier, skim coat of thinset and then install with thinset. Personally, I would never install right over plywood. Good luck.

2. 
   Guest_ | Jan 28, 1999 02:52am | #2

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   Hoo boy, sorry NJKwan, but do you know what you are talking about? Example: "...nailed metal reinforcing sheets over moist barrier, skim coat of thinset and then install with thinset." What the heck is that???

   So, you wouldn't install right over plywood, okay, but thinset over metal? Oh yeah, like, sure. Besides, whatis a metal reinforcing sheet?

   Anyway the chap asked about min. T. of subfloor. A subfloor should be 5/8-3/4" thick. Of course, the thickness is consequent to joist spacing. You could have 3" thick subfloor, but with joists 40' apart you'll not have a very tight floor. The more proper question would concern joist spacing. The maximum floor joist spacing for interior floor to be tiled is recommended at 16" o.c.

   Sorry for being such a hard a--, but I don't buy into the "let's make it up as we go" school of construction. I mean, check for bounce? What does this entail, the kids jumping up and down while you watch from the side?

   I know enough to know I don't know everything,; that I have alot to learn. But I know when questionable advice is given too.

   Now, I better duck 'cause the flak I'll get for this will be immense. It's just that I get burned up about questionable methodology freely given.

   Sure, it's not rocket science, but when I go to sell my house, I'll know that I'm not putting another s---house on the market

1. 
   Guest_ | Jan 28, 1999 03:08am | #3

   *Rich,Your points are well taken. I, too, am familiar with the quick answer that really doesn't inform. No flak from this quarter.

1. 
   Guest_ | Jan 28, 1999 03:29am | #4

   *I use this as a rule of thumb for tile over subfloor installation. Cut your joist span in half. And go no more than 16" o.c.5/8" min. plywood under a full float. 3/4" min. plywood under backer board. 1 1/8" min. plywood for membrane installation. (I do not do tile directly on plywood.)

1. 
   Guest_ | Jan 28, 1999 03:34am | #5

   *Rich, Ditto, What is heck is “… nailed metal reinforcing sheets over moist barrier…?” On a more serious note. This is bad as floor underlayment gets. We installed a 5/8” subfloor over 16” on center floor joists. Over the subfloor we installed a 5/8” plywood underlayment. We laid an 8”x8” ceramic floor in thinset mortar as the finish flooring. After several months the grout cracked and the 50% of the ceramic tiles became loose. The homeowner, rightfully so, was furious.Here’s what happened. The underlayment was laid exactly over the subflooring, all the butt joints and running joints were in the same place. The glue that was used was pathetic; there was no wood transfer when the underlayment was taken up. The underlayment was nailed on 12” centers at best. Even thought we had 1¼” of subflooring + underlayment, the floor failed. To make matters worse, this was in a kitchen. We removed all the appliances, cabinets, base, ceramic flooring and underlayment, just to get back to a starting point…. What a job!What I learned: Stand over the underlayment installers! I now use ¾” tongue & groove plywood sub-flooring, glued and gun-nailed with 8d nails. We have screwed the subflooring in the past, but I think its overkill. We install ½” Durrock underlayment, heavily glued and nailed with coil galvanized roofing nails. The underlayment must be staggered over the subflooring. It is also important to vacuum the subflooring so that the glue bonds. 5/8” plywood subflooring is marginal, use ¾” tongue & groove, its worth the money & effort.JJ Hamer Hamer Construction, Inc.

1. 
   Guest_ | Jan 28, 1999 10:54pm | #6

   *JJ,1) Do you find Durrock the best, or do other cement backer boards perform well?2) Does you method differ when 3/4" tg OSB is used as a subfloor?3) What kind of glue do you use?4) What Nailing Pattern do you use?5) For wet areas, like bathroom floors and tub surrounds/backsplashes, do you put down a moisture barrier under the backer board? I've heard the horror stories of green board under tile and moisture problems. It seems that backer board would allow moisture penetration also. What are your thoughts?Thanks, Steve Wright

2. 
   Guest_ | Jan 29, 1999 02:28am | #7

   *Steve:I like Durock the best, haven’t had much experience with other types of concrete underlayment, but they are probably ok if installed correctly.I personally don’t like OSB, I prefer ¾” Fir Tongue & Groove plywood, but I’m sure others have different opinions on OSB. We use PL-400 subfloor structural & sub-floor glue. It’s easy to find in Chicago, there are probably other comparable brands. We make sure the floor is clean, no drywall clumps, and no dust. (shop vac all dust.) We also run a minimum ¼” bead of glue at 8” centers, between the subfloor and durock. As per above make sure that the subfloor joints never match up with the underlayment joints. Don’t skimp on the glue and don’t let it sit for an hour before attaching the two pieces together. Also watch the basics, house temperature and humidity can effect wood and glue.Nailing is on 8” centers in the field and 4” centers at all butt joints. Durock recommends 1¼” to 1- 5/8” screws or 1½” galvanized roofing nails. Screws are better, but I don’t think they are necessary. I do not put a moisture barrier down before the durock, since the glue is bonding the durock to the plywood subfloor. Hopefully the floor will not be subjected to the same amount of water that a shower is. Showers / bathtubs are an another topic entirely.My main concern is eliminating tile cracks, grout cracks, and floor squeaks.All this discussion is also based on proper floor joist engineering, make sure you’re solid to start with before going on.FYI: JJ Hamer

3. 
   Guest_ | Jan 29, 1999 05:19am | #8

   *The tile setters that are doing my latest project are bedding the durorock in thinset. Looks strong.

4. 
   Guest_ | Jan 29, 1999 06:32am | #9

   *Steve:I use the same thinset Im going to use to install the tile. Well, maybe I'll use gray instead of white. 1/4" notch on the trowel, full coverage under the sheet. I use galvanized screws, 1-5/8", 6-8 inches in the field and 3-4 along each edge. I will only install durock over 3/4" minimum subfloor. Anything less gets a full float. I like Durock. The only other brand Ive used is Wonderboard and I think it just smells worse. I also like the rough textured surface on the Durock.Dave

3. 
   gary_gilbert | Jan 30, 1999 12:55am | #10

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   I just extende my kitchen a little bit and I'm planning on continuing the tile in this area. The old stuff has lasted for years. It has 3/8" CDX ply over 3/4" fir (or southern pine) floor boards. All this is over solid wood subfloor. What's there works so I'm thinking to just continue on with 3/8" ply over the other fir flooring I've discovered. Am I headed for trouble????

1. 
   NJkwan | Jan 30, 1999 01:43am | #11

   *Soooooooorry! I thought this was discussion and advice and anyone can contribute. Did I give advice that will end in a total botch job? No sir. Even the professionals on this site know that how detailed the advice is, there can be many unforeseen problems and the best advice is being there. My motto is study, check and ask, double check. If I don't know the name of everything there is, this is because I know my limitations but won't let it stop me from learning. You must be aware that such disdain for less than professional knowledge and nomenclature keeps the customers from becoming involved in the work we do for them and sometimes even used to intimidate them by bad apples that give the trade industry such bad reputation.

4. 
   Guest_ | Jan 30, 1999 01:58am | #12

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   What is the minimum thickness for the subfloor when installing ceramic floor tile?

   Thanks