Hi all, this is my first post. I'm looking for advice on a small tile project. I've tiled the landing on a flight of stairs, roughly 3 1/2 feet x 3 1/2 feet. I spent a day under the stairs supporting the floor from underneath, and another day up top screwing the floorboards to the framing, then applied a 1/4 inch sheet of cement board using construction adhesive and drywall screws every 3 or 4 inches. Here's where I'm running into problems. First I used a pre-mixed mortar from Home Depot, and 48 hours later it's still not fully set. It's a high-traffic area so it's really inconvenient. Would a heater speed up the drying? Second, the tile is 3 inch square with an uneven surface. It's similar to stone, with small inclusions and holes. Is there a trick to grouting to prevent the grout filling in the holes on the tile? I was considering taping the tiles ... Any help would be greatly appreciated! Thanks in advance, Geoff

There's a reason pros never use pre-mixed mortars. hopefully it was not mastic. Also drywall screws should not be used in flooring applications. the construction adhesive should do the trick even if the screws fail though. a heater will speed up drying, especially if you heat the floor from below.

If you don't fill the holes with grout, they will fill up with dirt. So you might as well grout it all. If you really don't want the holes filled, use a mustard squeeze bottle to fill the grout lines, then smooth and wash. Use sanded grout.

Thanks, dirt I can live with. I don't want the grout to smooth out the tiles. No it isn't mastic, and yes it's a sanded grout.

If it's "pre-mixed", it's the same as mastic. It's an organic adhesive. A lot of it depends on the tile and substrate as to how fast it will dry. But after 48 hours it should be dry if it ever will.

You've got a lot issues on your installation. No thinset under the CBU, mastic for your tile, and as Pete mentioned, drywall screws.

If you want to wait longer, that's your choice, but personally I would be taking it up to get the mastic off and resetting them with thinset. You might get away without thinset under the CBU, but I would add deck screws to keep it secure.

The reason we don't use construction adhesive to set CBU is because it's way more expensive than thinset. If you used enough, it should work as well or better.

Pistolpete is right about the CBU. Drywall screws were a mistake, but the construction adhesive should hold.

Like, Kman, my biggest concern is the mastic-based "thin-set". What you want for floor tile is a Portland Cement based Thin-Set Mortar - not a pre-mixed "thin-set" or "Floor and Wall Tile Adhesive". Anything that is pre-mixed and comes in a pail is not a cement based product. A cement based product will start to set-up and harden even if it is in a sealed container, so it is impossible to be sold in a ready-to-go state.

There are a number of reasons for using cement based thin-set. For a start:
1 - Drying Time - Mastic can take months to fully cure. Consider yourself lucky you didn't use a larger tile. With 3"x 3" tile, at least you have air getting through your joints.
2 - Strength - glue vs. cement (obvious)
3 - Exposure to Moisture - even when hard, mastic based products can re-emulsify if exposed to water or moisture.

If your tile is in and you want to give it time and take your chances, it's up to you. As a DIYer, what the hell? Why not? But as a professional, if one of my guys did the job, I know that we'd be there ripping it out and re-doing it.

Live and learn. You'll know better next time.

Hi Geoff, and all,

That pre-mixed stuff is mastic. Not good on floors.

No thin set under the backer board is a mistake. Using construction adhesive will not prevent the board from giving a "hair". You're not trying to bond the board, you're using the thin set to fully support the board. The fasteners hold is down. Drywall screws is obvious.

The holes in the tiles should be grouted.

This sounds like tumbled travertine, not porcelain tiles. If so, the pre-mixed is twice as worse even if it dries a few extra days. Also natural stone requires a much stiffer subfloor system which you may not have.

Plus: You said you spent;

Does that mean the 1/4" CBU is on plank subfloor? If so, wrong. You need plywood too.



No, we use thin set cuz it's the right thing to use.

Jaz