Hello. Last weekend I started my tile project, of prepping my front porch for tile. (I framed porch with 2x10" joists, 16" OC with a run of 6'). I have 3/4" plywood ontop of that.

Ontop of the plywood I installed 1/2" Durock, and here is the mistake I made...I liquid nailed it down instead of using thinset and screwed the cement board 8" OC with 1 5/8" square heads. I thought it would be OK to use liquid nails instead of thinset, until I started reading the forums...which I should have done prior to starting this.

I really do not want to tear off the Durock, as I doubt it will be reusable. I was thinking since I will have to buy more Durock either way, can I just apply thinset ontop of the Durock I laid incorrectly, and add second layer of 1/2" Durock ontop of the thinset (screwing this second layer as well)? Will that eliminate any moving that may occur because of the gapping caused by the liquid nails?

I realize 1" of Durock doesn't add anything structurally, but I am hoping the thinset between first and second layers will eliminate the movement issue which could crack the tile.

This is an open porch, therefore exposed to weather, so I want to make sure what I do will keep the tile secured and weather tight. If the only option is to tear off the Durock and start over, I will...I am just hoping I do not have too...

A few points (And you're not going to like any of them):

1. The big problem with using subfloor glue instead of the thinset is that the glue will remain flexible. This means that if a concentrated weight is put on one spot, the Durock will compress. As you know, this could make any tile above this pop off.

2. Adding a second layer of Durock with a thinset layer underneath, will help a little, but will not eliminate this problem. Meaning that your tile could still move and pop, but now you have a higher subfloor and you have spent move time and money. Truly a case of pouring good money in after bad.

3. All a moot point anyway because your tile job will only last between 5 and 8 years or so. Tile is not waterproof, nor is grout. Same for durock - it is not damaged by water but it will not repel water. Net result is that anyplace on your porch that can get wet will have water seeping through the tile, thinset, Durock and subfloor glue. All that moisture will remain in the plywood subfloor beneath all that. Basically, the water can get in but it will never get out as there is no ventilation or any way for it to dry. Then, the plywood will rot. You may eventually see some rot on the joists, but usually the subfloor will rot through first.

Really. I've seen this too many times and have rebuilt countless tile porches that were made this way. I would either just go ahead and finish out the rest of your tile - knowing that you are going to completely rebuild everything in a few years or rip everything out and start over with either a floating subfloor on top of a rubber membrane or a mortar base over a waterproof membrane.

Thanks Jaybee. I had that feeling in the back of my mind and was hoping there was a solution. The porch is covered, but in a heavy rain it does get wet on the sides.

I bought porcelein tiles and an outdoor grout, and I was going to do very small grout lines, to help reduce the amount of water that could seep through. When I tear off the durock, if I put down felt paper before the thinset and durock will that eliminate the chance for rot? I rebuilt this entire porch ground up because of rot and do not want to have to do it again.

Maybe the best solution it to remove the entire subfloor, and just put down that composite deckboard. I was going with tile thinking it would be unique and low upkeep, but it does not appear to be working out that way

Your best solution - meaning most durable for your money spent - will be to go with a composite type decking product instead of tile.

The felt paper route will not work. First, the felt will just redivert the water but not necessarily away from the plywood subfloor. Second, if you put down felt (or even a rubber membrane) then you have to construct your subfloor to be a completely connected unit. This involves either a double layer of subfloor bonded together with overlapping seams or a subfloor that is elevated on composite joists. If you simply added a layer of felt and then a layer of Durock, the Durock would not stick to the felt and would move around.

It's also not a matter of type of grout or of grout joint size. All tile and all grout will not be affected by water. The problem starts with the fact that no tile and no grout is waterproof. Water will soak through both tile and grout (and Durock and thinset) which is why whatever is under all that needs to be impervious to water. Even grout sealer is not waterproof, it just protects the grout from getting stained and changing color.

The above problems are directly related to how much water any section of the porch gets. The areas around the perimeter on the weather side of the porch will have problems while places up near the house will last forever.