Which type of mortar is generally preferred for outdoor tile installations?

Modified thin-set mortar is preferred for outdoor tile installations primarily due to its enhanced bonding capabilities and flexibility. This type of mortar includes added polymers that improve adhesion and make it more resistant to moisture and temperature fluctuations, which are common in outdoor environments. Additionally, modified thin-set mortar has a higher bond strength, which helps to accommodate the stresses caused by outdoor conditions, such as freeze-thaw cycles.

While thin-set mortar is suitable for many indoor applications, it may not provide the same level of performance outside, particularly when dealing with variable weather conditions. Sanded grout is used for filling joints between tiles rather than as an adhesive, making it unsuitable for the initial tile adhesion required in outdoor settings. Cement backer board mortar is typically used to secure backer boards but does not serve the purpose of bonding tiles to the surface. Therefore, modified thin-set mortar is the optimal choice for ensuring a durable and lasting outdoor tile installation.