Why?? dry pack mortar bed for showers
Question here why do tile layers use the dry pack mortar bed method? Wouldn’t poured concrete be much more water resistant? Or is that not the intention? I was working in a home yesterday that had a shower with a dry pack mortar bed but the bed wasalready spalling and if i used a very soft bristle broom the curb would sweep away as if it was concrete with no water added. How in the world is that supposed to support tile?
Blueshound
Blue Dog Flooing Solutions LLC
Edited 12/15/2006 12:13 pm ET by blueshound
Replies
Guessing here ... concrete actually requires a lot less water to cure than is actually used. Most of the water in concrete is there to make it easy to spread around the forms. The dry-pack is wet enough to cure, but dry enough that it can be easily shaped to the required slope, and it won't try to self-level before getting hard. And it's just a setting bed to support the thinset and tile. It's not supposed to be waterproof.
"When asked if you can do something, tell'em "Why certainly I can", then get busy and find a way to do it." T. Roosevelt
Let me add to what Fast Eddie just posted. Not only is a drypack mortar floor not supposed to be waterproof, but it is actually supposed to be porous so the water finds the subdrain under it.
Properly done drypack is plenty strong for the application, and has the advantage that it will not suffer shrinkage cracks like wet concrete will. Rebar would rust out, since the bed is always wet.
Personally, I prefer to use drypack with Kerdi. This way there is no subdrain, no mortar sponge full of who knows what microorganisms. I figured this out after building my own shower with a subdrain. The Kerdi floor has very little capacity to hold water, so it can often dry between uses. I figure my floor would take a month to dry out if I didn't use it for that time.
Bill
I've done several large drypack bases for tile in houses I've lived in. I never gave the porosity thought but it makes sense.
I never had loose material after the base cured though. It looked like concrete to me. Perhaps I put too much water in? Dry pack isn't really dry after all. We put enough water in to make the material ball up like a snowball.
blue
Blue,You were a tad too wet, but still lots drier than typical site-mixed concrete. Even done quite dry (barely stays in a ball when squeezed) the end result is plenty strong for the task it does.Here in CA I can buy prebagged deck mud mix. It has lower Portland cement content than sand mix. This is not to save money, but for these 2 reasons: 1. Less shrinkage, and 2. It maintains porosity so it can drain to the subdrain. Typical sand mixes have enough Portland to fill all the gaps between the sand, so they are way less porous and can shrink a lot, too.Bill