Screws in Durock...tool late?

[coopns]

Finished the Durock with thinset at 2. Cleaned up, thought about screws. Started screwing some in at 3:45. Started to panic, got more screws and put some every six inches all over - done at 4:15 or so. (Small bathroom 32 sq. feet). Could see it was still being pulled down in spots. Before that I put some tile boxes and weights on some spots I felt were a little loose.

Do you think that will be all right?

[jadnashua]

The pot life of thinset varies depending on the type, how it is mixed, the humidity, and how damp the surface you apply it to. If the thinset still mushed when you screwed it down, maybe. If it were tile you were trying to adhere, absolutely not, but the goal of thinset under cbu is to fill in minor imperfections so the board is fully supported. IF that happened, you should be okay. Otherwise, you've got problems. Hard to tell without being there. No guarantees. Safer to remove and replace, but it could be fine. You'll get more and better results where you started this over at www.johnbridge.com - those guys deal with tile every day.

[TWEAK]

As long as you have enough thinset under there to support the durock, it will be ok.

The problem you might have on a floor, especially if using typical 1/4 inch backer board, is if the resulting durock suface may have become wavy - that is, not flat. This isn't a fatal problem, but it IS a major annoyance and will make the tile setting much harder. The problem is that you'll have a hard time with lippage (that is the term for the effect you get when one tile is at a different height from an adjoining tile, creating an annoying "lip"). I've dealt with this in the past by locating the high and low spots in the durock surface, and using a size or two larger notch size trowel in the low areas.

If you put a long level on your floor (durock surface doesn't have to be truly and perfectly level, but it should be flat... a 4 of 6 foot level is a good way to check for "flat") and discover that you have an out-of-flat problem that's pretty severe.... say 1/4" or more.... it will be way, way easier in the long run to build up the low spots with a skim coat or two of thinset, or medium bed mortar. Or, if it's really bad, even self-leveling concrete. The tile setting will go MUCH easier if the floor is flat, especially if you're using 12" or larger tiles.

[jadnashua]

The reason for cbu installation over plywood is so that you have a surface that is compatible with the tile, expands and contracts the same way as the tile, so it all works together. What actually happens over time (and it doesn't happen quickly), is that since the cbu (or tile if adhered directly to the wood) expand and contract at different rates, and the bond can be broken. So, if you follow the manufactuer's instructions and use an unmodified thinset, it has no elasticity to it, the bond will break and you have the cbu and tile disconnected from the subflooring. When using the specified fastener, what really happens long-term is that the tile and cbu move, and actually crush the cbu around the fastener so the cbu and tile are a floating floor, held down by the fasteners. this motion is very small, but exists. If you choose to utilize one of the approved methods of tiling directly to (two) layers of plywood, you are required to use a highly modified thinset (which has a little give to it). Use a lower-compliant thinset, or have a less than perfect installation, and you risk failure. There are organizations whose sole purpose in this is to evaluate tiling installation methods and come up with methods that work long-term, under adverse conditions. The manufacturers run their own tests. Trying to second guess all of that research with a few personal experiences is fraught with room for failure in the general use situation. Plus, except maybe in a home situation where you DIY, this failure can take years, often gets disconnected from the installation method utilized...in other words...things could fail, and you would never know why or if.

RTFM, and follow it. You might get lucky, you might not, if you don't.

The thinset in a cbu installation is there to fill voids, NOT to hold the panel in place. The fasteners hold the cbu in place, and a second purpose of the fasteners is to smush it into the thinset to minimize the notches (i.e., spread them out to fill in the gaps - providing the desired 100% coverage).

[TWEAK]

Trying to second guess all of that research with a few personal experiences is fraught with room for failure in the general use situation.

Jim, I am curious about what you're reading in the above posts that leads you to believe that anyone is "second guessing". OP has thinset under his CBU, has screws every 6 inches, and while he waited too long before screwing, it was still was wet enough to mush when he drove the screws. Should be fine. Skim coating the low spots in CBU to get it flat is nothing new or particularly controversial.

[jadnashua]

The pot life of most thinsets is in the order of an hour (some are longer, some are less). Once spread out, it starts to tack up much quicker, especially when applied to wood, and the cbu (depending on the type) can be quite thirsty, too. So, after two hours under the board, you are at risk. Note, he said it pulled down in some areas, implying there were areas that didn't. This could lead to uneven coverage, which is the whole purpose of using the thinset in the first place.

[TWEAK]

Gotcha.

Custom blend has an open time of 45-55 minutes (no stated pot life), but this would make the pot life pretty long.. Versabond has a pot life of 4 hours. I've spoken to the folks at Custom and they see no problem with using modifieds under CBU - in fact, they recommend using whatever you have convenient. The "Under CBU" application is about the least demanding one for any thinset.

As I see it, the OP was noticing some mushing - so the material hadn't hardened. As you know, thinset (any flavor) stays pretty soft for at least a few hours, although it looses its tack after 20 minutes or so. It takes between a day and several days thinset to cure to rock hard. Yes, it won't stick as well if it's dried out too much... but not really a problem under CBU. As you very correctly noted, it's only purpose is to provide vertical support.

I'll stand by my comments that he will be ok as long as he has enough material under there to support the CBU. Next time he should get to the screwing a bit quicker, mostly for his own peace of mind.

Many pros use a pneumatic nailer with galvanized roofing nails to speed up the process - this seems to be recognized as perfectly adequate and easier on the knees, too. Unfortunately for me a roofing nailer is about the only pneumatic I don't have!

[jadnashua]

The spec sheets call for hot dipped galvanized nails, which are hard to find in strips or coils for a nailer. The more commonly available electroplated stuff does not meet the manufacturer's instructions, so be careful. In fact, at a big box store, it's hard to find hot dipped nails - most of them are the electroplated variety. I really hate the hardware that is generally available at a big box store...cheap, but you get what you pay for...

[andy_m]

+1 on the hot dipped roofing nails.

There's "some" very good stuff and some very good pricing at Home Depot & Lowes... but there are a lot of specialty items where you just have to go to a regular industrial or trade supply place.

I keep my eye on **** for items like nails and such. Often contractors will have or half a box box left over that you can get for relatively cheap. I've bought stainless, 21 degree stainless plastic collated framing nails this way... way too expensive to pay regular price for, but definitely nice for outdoor projects.