Which is stronger: a wide joint or a narrow one? Is it better to have a 3/4 inch joint or a 1/4 inch joint if it varies from the specified 3/8 inch joint?

I would like to design a wall with ¾-inch wide joints to achieve a different appearance. Will this size significantly impact the wall’s performance?

Narrower joints are stronger than wider joints in both compression and flexure. A study performed by the Structural Clay Products Institute, reported in Research Report No. 9, "Compressive, Transverse and Racking Strength Tests of Four-Inch Brick Walls," examined the impact of bed joint thickness on compressive strength. This study found that walls built with 1/4 inch joints had more than twice the compressive strength as walls built with 3/4 inch joints. Walls built with 1/4 inch joints were 12% stronger in compression than walls built with 3/8 inch joints. Walls built with 3/4 inch joints had approximately half the compressive strength of walls built with 3/8 inch joints.

The study also showed that variations in joint width had an even greater influence on bond strength. Wallettes measuring 16 inches x 16 inches built with 1/4 inch joints were approximately 2.4 times stronger than 3/4 inch joints. Wallettes built with 1/4 inch joints had 23% higher bond strength than those built with 3/8 inch joints. Those built with 3/4 inch joints had approximately half the strength of those built with 3/8 inch joints. Wide mortar joints likely have more problems than narrower joints with shrinkage. Excessive mortar shrinkage, which results in cracking and joint separations, can significantly increase water penetration into masonry walls.

The standard thickness of 3/8 inch joints is large enough to easily fill with mortar and to accommodate variations in brick unit sizes without significantly affecting the appearance of the wall, yet small enough to achieve the desired strength. Walls built with significantly wider joints have reduced strength; therefore, wider joints should generally not be used unless the impact of reduced strength and shrinkage are carefully considered.