Tooling Mortar Joints

JNX-clusives

Chips Off the Old Block

and other ruminations about masonry, by

Larry D. Jenks

Tooling of Mortar Joints

Aesthetically, "tooling" produces the joint profile, which is made when the jointer or “tool” is struck against the surface of the mortar before the mortar has set. This profile is usually concave but it can have other configurations as well, depending on the profile of the jointer or tool. And from a functional point of view, proper tooling increases the water penetration resistance of the wall. The tool compresses the mortar against the unit helping seal any slight separation cracks that might form at that interface. The action of joint tool on the unset mortar brings the cement fines in the mortar to the surface. This creates an exterior skin on the joint that is more resistant to water infiltration. This re-emphasizes the need for a high-lime mortar, as I discussed last time, in order for the mortar to have sufficient workability to enable the cement fines to move to the surface.

In order for the joints to have the best moisture intrusion resistance properties, tooling must be done at the correct time, as this affects both their effectiveness and appearance. Too early and the joint won’t seal, and often result in rough joints. If tooling is delayed too long, the surface of the joint cannot be properly compressed and sealed to the adjacent brick. Tooling should be done when the joint is “thumbprint” hard—when some of the moisture has been absorbed into the unit but there is still enough available to keep the joint plastic and the fines can be brought to the surface (pressing the thumb into the mortar leaves an indentation, but no mortar is transferred to the thumb). 

In order to maintain a consistency of color throughout the project, tool all the joints at approximately the same level of moisture throughout a job. Early tooling often results in joints of a lighter color. Later tooling results in darker shades. 

All mortar joints should be tooled concave with a rounded or “V” jointer. The forming of these concave tooled joints densifies the mortar at the surface and seals the joints. Concave and “V” joints best resist water penetration in exterior masonry. These joints produce a more dense and weathertight surface, as the mortar is pressed against the brick. Raked, flush, struck, beaded, or extruded joints are not recommended as they do not compact the mortar and/or create ledges that intercept water running down the face of the wall. Head and bed joints need to be the full thickness of the face shells for optimum water-tightness.

Figure B: Concave mortar joint

Figure C: V-shaped mortar joint

Mortar joints should be double struck. Concave and “V” joints direct water away from the building interior, unlike rake joints that create a ledge for water collection. Brush or flush mortar joints, which are common when laying split ribbed concrete block, do not provide the same degree of weather protection as a tooled joint.