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Scruffy 10-27-2010 09:22 PM

Subfloor for slate tile on a screened in porch floor
I've read several posts on this topic but I'm not sure that I have found the answer. I have a 12x20 screened in porch (attached to my house) with 16" knee walls topped by triple track storm windows. I remove the glass in the summer exposing the screens and some rain water can get on the floor. I have a pool and dripping bodies have been known to come in and drip water on the floor. The floor joists are 2x10X12 16" on center and about 3 feet off the ground. The decking is 2x6 boards topped by 1/2 inch plywood and currently topped by indoor/outdoor carpeting. I want to replace the carpet with slate tiles. I'm not sure about deflection but I can jump up and down on the floor and it doesn't appear to move. What do I need for subfloor? After removing the carpeting should I clean the plywood and lay the tile on that? Remove and replace the plywood? Add Duroc on top of the plywood? (this option would be tight at the door threshold into the house) Or should I lay Ditra on top of the plywood and follow their instructions? Or is there some other option? And, if it's important, I live in Atlanta so I don't get long severe winters.

gotogregg 10-28-2010 02:45 PM

Hello scruffy,
I am Gregg with The Home Depot in Chicago. Sounds like you are on the right track. I will be doing something very similar to my porch next summer! For starters the ½” plywood isn’t strong enough to directly tile over. Just because you can’t feel it flexing and moving when you walk on it doesn’t mean that it isn’t. The best way to do this project is to remove the existing ½” plywood and put down a ¾” cdx plywood. Since the porch is only 3’ off the ground you not only have to worry about the moisture from the pool soaked people walking on the tile, but from the ground underneath it. I would then lay an ice and water shield over your new ¾” subfloor. The ice and water shield will protect the floor from any moisture below. After the ice and water sheild is on I would put down a ¼” cement board. This will add strength to the floor and make a perfect surface for the tile. Let me know if this helps and what your game plan is going to be. My way isn’t the only good way so I am also very interested to hear other ideas. Thanks -Gregg

JazMan 10-28-2010 03:57 PM

Hi guys,

I have to disagree with Gregg and his method. Several errors. Will you please refer us to the source of your installation method? Certainly not in the Tile Council of North America Handbook.

The first thing to look at is the joists system to make sure they are stiff enough for natural stone. They're fine for regular ceramic or porcelain, but just a bit under spec for slate. Need to know the species and grade of those 2x10's. Stone requires a much stiffer floor than porcelain or regular tiles.

2x6 decking tells me it may have once been a deck. Yes, no? How's the condition, are you convinced it is built well? Are the 2x6's still flat or are they cupped at all? If the 2x6's are in good shape, theoretically 1/2" exterior grade underlayment C/C or better should be ok. It's always better to go thicker. Either remove the 1/2" and install 3/4" (Not CDX though), or even another 1/2" over the top.

Then install 1/4" "real" concrete backer followed by Ditra. Since the walls prevent water run off, you need to treat this like a shower floor, so additional waterproofing is necessary. Treat the seams of Ditra and flash up the walls several inches. (Ask)

Do not place any membrane over the plywood. The concrete backer gets thin set'd direct to ply. Ice and water shield is for roofs. Cement backers do NOT add structural strength to the floor, it only give a tile-friendly surface for the thin set mortar.

The ground should probably be covered with a barrier of some type, but I don't know what is recommended in your location. You must have cross ventilation too and of course good drainage away from the building.


Scruffy 11-02-2010 09:21 PM

Hello JazMan,

The joists are No. 2 s-dry, SYP. There is some coding before the SYP but it's too blurred to read.

The porch was originally built as a deck with railings around the sides and a roof over it. I'm convinced it was built well because I watched the builder build it. I closed it in as a screened porch in 1990, two years after it was built so I know that part was built really well :laughing:. From underneath, the deck boards lay flat on the joists so I assume there is little or no cupping.

I have about an inch to play with before the floor is too high for the threshold of the door to the house. The tile is 7/16" thick and I don't know how much to add for the mortar. If I add a 1/2" plywood, the backer, Ditra and tile I come up with 1 5/16" without the mortar. Would I use Kerdi-Band as flashing or metal flashing?

Several years ago I laid down 1 mil plastic sheeting on the ground under the porch for a vapor and dust barrier. The front and side openings underneath are protected by lattice so the cross ventilation is good and the drainage is excellent.


Bud Cline 11-03-2010 03:57 PM


JazMan 11-03-2010 08:26 PM

To do the job right you'll need to raise of cut the door. The finished floor is gonna be about 1 3/8 to 1 1/2" higher off the 2x6 subfloor.

The Southern Yellow Pine #2, 2x10 joists spanning 12'0" do meet standards for natural stone floor with your double layer of subfloor method if installed correctly and if they're still in good condition.

I can speak to the condition of the ground under this porch. Have you considered any insulation under or between the joists?


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