NASCLA Accredited Commercial General ContractorFinishesHard
A contractor is installing a large-format natural stone tile floor in a commercial lobby. To minimize the risk of staining and efflorescence migrating up from the setting materials or subfloor, what is the most effective preventative measure to apply to the back of the stone tiles before installation?
- AApply a cement-based backer board.
- BInstall a decoupling membrane.
- CSeal the back of the tiles with a penetrating sealer.
- DUse a rapid-setting thin-set mortar.
Show answer & explanationAnswer & explanation
Correct answer: C. Seal the back of the tiles with a penetrating sealer.
Sealing the back of porous natural stone tiles (back-buttering with sealer) prevents moisture and minerals from the setting bed or subfloor from wicking into the stone, which causes efflorescence and staining. This is distinct from surface sealing.
Why the other options are wrong
- A. Backer board provides a stable substrate but does not inherently prevent migration into porous stone.
- B. A decoupling membrane addresses substrate movement and cracking, not efflorescence or staining from below.
- D. Rapid-setting mortar reduces the time moisture is present but doesn't prevent its absorption into porous stone.
Back-Sealing Natural Stone
The application of a penetrating sealer to the unexposed sides (typically the back and edges) of porous natural stone tiles prior to installation.
- Prevents moisture and minerals (efflorescence) from wicking into the stone from the setting bed.
- Reduces the risk of staining caused by impurities in mortar or substrate.
- Different from sealing the finished surface of the stone.
Memory trick: SEAL the BACK of the stone, to block the rise of salts.