Type O mortar is a low-strength mix, around 350 psi, suited to interior non-structural work and to repointing older, softer brick that a stronger mortar mix would damage.
Type O is a low-strength mortar, around 350 psi, with a higher lime content and softer bond than N, S, or M. It's too weak for exterior load-bearing brick or anywhere exposed to significant weather, but that same softness makes it the right match for interior non-structural work and for repointing older, softer brick that a stronger mix would damage. Matching mortar strength to the masonry unit it's bonding matters more than most people assume.
Type O mostly comes up on interior work for us: garden walls that see little weather exposure, indoor fireplace surrounds, and other non-structural masonry where a soft, workable joint is more useful than raw strength. It's not something we'd use on an exterior wall or chimney exposed to the weather up here, freeze-thaw winters need a mortar with more resistance to moisture cycling than Type O is built to provide.
More masonry vocabulary from the glossary.
The services where this term comes up.
Chelo's Brick & Stone serves Hillsville, Galax, Wytheville, Fancy Gap, and Independence, VA. Call for a straight answer and a free on-site estimate.
