Filling Mortar Joints

JNX-clusives

Chips Off the Old Block

and other ruminations about masonry, by

Larry D. Jenks

Filling Mortar Joints

A uniform bed of mortar should be spread over only a few brick, and furrowed only lightly, if at all. Filled joints result when plenty of mortar is placed on the end of the brick to be laid and it is shoved into place so that mortar is squeezed out of the top of the head joint. 

All block walls should be laid with full mortar coverage on horizontal and vertical face shells. Block should be laid using a “double butter” technique for spreading mortar head joints. This practice provides for a mortar to mortar contact as two block are laid together in the wall. Hairline cracking in the head joint can generally be avoided. 

All exterior walls exposed to wind-driven rain should be solid grouted, filling all the cells with grout. Severity of exposure varies considerably due to factors such as roof overhang, site location, orientation, etc. 

Tests conducted by the National Bureau of Standards (1929) proved that to obtain good masonry construction, all mortar joints must be completely filled. Deeply furrowed bed joints were observed to reduce masonry compressive strength by 33 percent. The effects on bending resistance are even more pronounced: tests conducted in Great Britain by Dr. R. C. de Vekey PhD showed that, for each 1/4-inch of mortar bed joint left unfilled, masonry flexural strength was reduced by 16%.

Unfilled head joints had a minimal effect on masonry strength, yet they provide a major avenue for water leakage. Most masonry problems and disputes are· related to water penetrating through the wall, and full, well-compacted mortar joints -- particularly head joints -- are crucial for proper wall performance. Mortar joints provide the first line of defense against moisture penetration, and it is well-known that partially  filled bed and head joints will result in leaking walls. Excessive moisture penetration leads to saturated walls which, in turn, increases corrosion of reinforcement and ties, intensifies freeze-thaw damage, and is damaging to interior finishes. Solid masonry walls will not be weathertight unless all the mortar joints are filled.

Bed Joints

A bed joint is the horizontal layer of mortar on which a brick is laid. The length of time between placing the bed joint mortar and laying the succeeding brick impacts the resulting bond. If too long a time elapses, poor extent of bond will result. Masonry units should be laid within 1 minute or so after the mortar is placed. For solid brick (brick like the ones pictured below, with holes in them, are considered solid brick as long as the holes make up less than 25% of the bedding surface) are, bed joints should be constructed without deep furrowing of the mortar, as full bed joints (covering the entire bedding surface) are an inherent requirement for water-resistant brick masonry construction. For hollow brick and concrete masonry units, bed joints may be laid with face shell bedding (mortar placed only on the front and back face shells). Both face shells must be completely covered with mortar.

Figure D: Concrete block with face shell bedding

Figure E: A Typical Hollow Brick

Head Joints

A head joint is the vertical mortar joint between two masonry units. For both solid and hollow brick, it is important that head joints be completely filled. The best head joints are formed by completely buttering the ends of the brick with mortar and shoving the brick into place against previously laid brick.

Figure F: Drawing showing the right and wrong ways to butter the ends of a brick

“Slushing” (“throwing” mortar into the joint with the edge of a trowel) does not adequately fill joints or compact the mortar, resulting in joints that are less resistant to water penetration.