Using raked mortar joints

Masonry Construction, March, 2004 by Norbert V. Krogstad

Q Many references strongly recommend using tooled mortar joints rather than raked joints. On many projects, however, raked joints are needed to match the historic fabric of the area. If this mortar has worked effectively in the past, what is the problem with using raked mortar joints in new buildings?

A Tooled joints have a compacted surface. During proper tooling, the mortar is forced against the bond surfaces of the brick above and below the joint in the case of bed joints, and on either side of the joint in the case of head joints.

In raked joints, the mortar is not compact against the surfaces of the joint. Instead, the front portion of the mortar joint is scraped out. Raking the mortar out can open up voids in the head and bed joints that increase water penetration. Also, a ledge is formed on the top surface of the brick unit at the base of the bed joints. When raining, water flowing down the face of the wall collects on this ledge and readily penetrates the masonry at any separations or voids that occur on the bottom edge of the joint.

A commonly overlooked problem with masonry walls using raked joints is that these walls are often aggressively cleaned. When mortar is raked out of the joints, it is smeared on the edges of the brick at the recesses. To remove the mortar, contractors often clean the walls multiple times with acid solutions or moderate-to-high pressure water. These aggressive cleaning techniques can open up additional voids and separations, which increases the likelihood of water penetration.

Whenever possible, I recommend using tooled joints to provide a hard compact surface that is more durable and resistant to water penetration.

Norbert V. Krogstad is a consultant at Wiss. Janney, Elstner Associates Inc., Northbrook, Ill.