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Unread 03-05-2006, 09:16 AM   #1
John and Susan Foster
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Seeking Basic Advice on Tile Underlay

Hello; this is our first post here; and we are looking for help planning the tile floors in a new house we are constructing this spring. The house will be a bungallow with a 11 7/8 LP joist floor ( max. span 15' 6"), with 5/8" spruce T and G plywood subfloor. We are planning a large area to be ceramic tile flooring. In our present house with similar construction we tiled four bathrooms and a laundry room using 1/2 select plywood over the 5/8" subfloor. We applied the tiles directly to the plywood with thinset. After 3 1/2 years we have had no problems. However, many sources have indicated that this is not the correct way to do it. Should we be using a different product instead of the 1/2 select plywood? What are the best products? We are aware of Wonder board and Hardie board products. Floor thickness is important to us as we are trying to keep all floor types level with each other ie hardwood flooring and ceramics. This is because the house is being built for in-laws where one is in a wheel chair.Would someone be able to enlighten us what exactly is wrong with using 1/2 select plywood well screwed down? We will be doing this project ourself so ease of installation is a factor.
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Unread 03-05-2006, 09:47 AM   #2
cx
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Welcome, Susan and/or John.

You can do what you're proposing so long as your subfloor and underlayment are installed exactly right. That method provides no forgiveness at all in the installation. We have very few pros hereabouts who will do it at all and only one who likes it. I'll ping him for you.

I'm not familiar with your plywood designations. For subflooring, you must have exterior glue plywood of at least CC grade, plugged and sanded, or better. The underlayment layer must be the same. We generally recommend BC grade because it is readily available, but for new construction you can order what you want. Not sure what your "Select" grade plywood might be, but be sure it's an exterior glue lay-up at least. It should say EXT or Exposure 1 or similar on it somewhere. I recognize that you're using it indoors, but you're gonna put thiset on it and the exposure 1 rating is the minimum allowed.

All that said, I would still recommend you drop the floors in the tile areas to allow a mud floor (first choice), or plan to use a CBU or isolation membrane over the plywood underlayment for long-term assurance on your installation.

On new construction it's very simple to plan the subflooring layers for different floor coverings to allow them all to finish at equal heights. The mud floor is the easiest to adjust and is the flattest and best tiling surface.

My opinion; worth price charged.
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Unread 03-05-2006, 03:27 PM   #3
Bill Vincent
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Hey Folks--

There are several things to watch for when setting over plywood. First, what you're proposing-- 1/2" over 5/8" is the bare minimum. I'd actually prefer to see another layer of 5/8" on top of the original layer. Secondly, the seams from one layer need to be staggered from the seams in the subfloor. Also, you want to make sure that the bonding layer is either Spruce or Douglas Fir. Pine has too much oil in it to get a really good bond, especially yellow pine. When laying the plywood in, the grain must lay across the joists, and the sheets should be gapped a strong 1/16", and then caulked with anything cheap, so long as it stays pliable. The object here is to keep the thinset out of the joints between the seams, giving the sheets of plywood room to expand and contract under the tile. The plywood should also be gapped from the perimeter by about a strong 1/8". You also don't want to use anything longer than 1 1/4" fasteners (screws or nails, although I prefer screws), because you don't want them going into the joists-- only into the subfloor layer. You want to be screwing it down every 6" throughout the sheet. You also want to be using an alkali resistant self adhesive mesh tape-- the same mesh tape used for cement board-- to go over each and every seam, once the caulking is dry. They'll get thinsetted down when you set the tile. Once this is done, you want to use a good quality, high latex content thinset. I stick to using unmodified thinsets and mixing them with liquid latex additives, such as Mapei's kerabond mixed with their Keralastic liquid, Laticrete's 317 thinset mixed with their 333 liquid, or Hydroment's Tilemate mixed with their Flexalastic liquid. These thinsets will give you the best bond you can get, even though just about any bag of modified thinset you see will say that you can use it over plywood (don't believe it-- it won't last NEARLY as long).
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