In masonry, what is the maximum size of a bed joint directly on the foundation?

• A. 1/2 inch
• B. 1/4 inch
• C. 3/4 inch
• D. 3/8 inch

Answer: (C)

The bed joint on the foundation is the initial mortar layer that seats the first course of masonry units. Its thickness is controlled because it sets how well the units bear on the foundation, stay level, and bond with the foundation.

Directly on the foundation, the maximum allowed bed joint is 3/4 inch. This limit accommodates small irregularities in the foundation surface, ensuring you can level and seat the first course without creating a thick, weak mortar bed. If you go thicker than 3/4 inch, you risk poor bond, increased settlement or cracking, and uneven bearing. Keeping the joint to 3/4 inch or less helps maintain a solid, consistent connection between the foundation and the masonry above.