A.Sean Gerolimatos, technical services manager for Schluter Systems, responds: With a few exceptions, unmodified thinset mortar is required for setting ceramic and stone tile over Kerdi membranes (800/472-4588, www.schluter.com ), as well as for bonding overlapped seams and connections to Kerdi drains. A blend of portland cement, sand, and water-retention agents, this type of mortar — sometimes referred to as dry-set mortar — is simply mixed with water (look for the ANSI A118.1 standard on the packaging).

Modified thinset mortars are similar but include additional polymers — such as latex — which are introduced in either liquid or powder form. For the liquid form, an unmodified or dry-set mortar powder is mixed with a liquid polymer additive instead of with water. For the powder form, the manufacturer adds powdered polymers to the dry blend, and...

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