NASCLA Accredited Commercial General ContractorMasonryMedium
A masonry restoration specialist is preparing to repair deteriorated mortar joints in an existing historic brick wall. Which type of mortar is generally preferred for repointing historic masonry to ensure compatibility and prevent damage to the historic brick units?
- AType N mortar for general-purpose use.
- BType S mortar for good bond strength.
- CType O mortar for softer, more permeable joints.
- DType M mortar for maximum durability.
Show answer & explanationAnswer & explanation
Correct answer: C. Type O mortar for softer, more permeable joints.
Type O mortar, with its lower compressive strength and higher permeability, is generally preferred for repointing historic masonry. This ensures the mortar is softer than the historic brick, preventing the mortar from damaging the brick as it expands and contracts, and allows moisture to escape through the joint.
Why the other options are wrong
- A. While Type N is a general-purpose mortar, Type O is typically a better match for the softer, more permeable characteristics of historic mortars and bricks.
- B. Type S mortar is also generally too strong for most historic brick, posing a similar risk of damage.
- D. Type M mortar is too hard and strong for historic brick, potentially causing the brick units to spall or crack.
Historic Mortar Repointing
The process of removing deteriorated mortar joints and replacing them with new mortar, typically using a softer, more permeable mortar (like Type O) to protect historic brick units.
- New mortar must be softer than original brick.
- Type O mortar is often preferred.
- Allows moisture to escape through joints.
- Prevents damage to historic brick.
Memory trick: Old structures need 'O'ld-school soft mortar.