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12-30-2016, 11:18 AM
I'm finishing my 800sqft basement. Our intent is to install ceramic tile. Here’s the background/details:

•House is 60+ years old, in New York. No settlement issues. No bulk water issues.
•I’ve lived here for 10years. As extra insurance for the future, I personally waterproofed the exterior 5 years ago: (excavate, tar coating, waterproof liner, tile drain with gravel, drains to daylight).
•Basement floor currently painted concrete
•There are 2-3 cracks across the basement floor (Photos attached for reference).
•Over the past 10years, I've observed some efflorescence along them, but NEVER any bulk water. Cracks have not gotten worse. They run across the entire basement in 2 locations.

EDIT: cracks are level across.

I plan to rent a concrete grinder to mechanically scrape/roughen the floor and remove the paint (which I know would act like a bond-breaker if I left it).

I’ve been advised by others online to install a decoupling membrane on the entire floor prior to tiling. This, I’m told, is to prevent the cracks from affecting the tile work and potentially cracking or popping them off in the future. I’ve never used such a product before, so I’m a bit unfamiliar and have started to learn/watch videos.

1. Is there any other recommendation or practice that I should consider? For example, are there decoupling membranes that only cover the crack plus several feet on either side? Seems almost wasteful to cover the entire basement when the cracks are clearly defined and localized. (But I’m not trying to cheap-out. I want to do this once and right… not deal with popped tiles on an annual basis).

2. Regardless of a decoupling membrane or not, how should I treat those cracks? Fill with what?

3. Anything else I should watch out for?

Many thanks

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12-30-2016, 11:35 AM
FIrst, you need to determine if the two sides of the cracks are even...if one is raised from the other, you should not tile the floor!

Industry standards call for 'honoring' any cracks up and through the tiled surface. That means some sort of expansion joint, either mechanical (a profile) or elastomeric (a caulk).

If the cracks are level across them, depending on their width, there are crack isolation membranes that can be installed across them. These often are required to extend a couple of feet either side (specifics on the one you might choose). Something like Ditra is not a crack isolation membrane, but they do have a version of their banding tape that is elastic - Kerdi-Flex. That could help with dissipating any moisture that might come up from below since it has air channels that criss-cross the membrane.

From their product guide:
"Schluter®-KERDI-FLEX is a flexible polyethylene waterproofing strip used to seal movement joints over Schluter®-DITRA in specialty applications where large movements are expected, i.e. over expansion joints or construction joints."