Q. When installing stone tiles over Schluter's Kerdi waterproofing membrane, is it okay to use modified thinset mortar? I know that Schluter recommends unmodified thinset mortar, but my supplier suggests that the large and heavy tiles will be easier to install with a polymer-modified no-sag mortar.

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 this is mixed with water (look for ANSI A118.4 and ANSI A118.11 standards on the packaging).

Modified thinset mortars aren't recommended for use with Kerdi because they must dry out to gain strength, whereas the membrane inhibits drying, which can lead to unpredictable results. For example, if a shower is used before the mortar has cured properly, leaks and polymer leaching can occur. Unmodified thinset mortars, on the other hand, depend on the presence of moisture to gain strength, and therefore they are well-suited for these curing conditions.

Schluter's recommendation is to use unmodified thinset mortar when installing Kerdi over most shower substrates. In principle, modified mortars also could be used over water-absorbing substrates such as drywall, cement backerboard, or dry-pack mortar — but seams and connections to the Kerdi drain would still need to be constructed using unmodified thinset mortar. Frankly, working with more than one type of mortar seems unnecessarily complicated.