Mortar Joint Repair

Chips Off the Old Block

and other ruminations about masonry by

Larry D. Jenks

Mortar Joint Repair

The properties of masonry systems, and especially concrete masonry systems, are such that they have a propensity to crack and leak, even when they are constructed in the most competent, good, and workmanlike manner. 

Even though concrete block, and especially ground-face or split-faced block, is a porous  material that readily absorbs water, water leaking to the inside of the wall usually is not attributable to moisture migration through the block units themselves. This is because moisture moves through the block faces and webs by capillary action, gradually and serially working its way into small voids. This is an  exceedingly slow process. 

When water is discovered inside the wall, it is usually prudent to study other possible sources of the moisture, beyond the moisture-absorbing properties of the block itself. For example, leaking roof and flashing details can allow water to migrate into the wall system, and this kind of leakage usually occurs at a much faster than water migrating through the block faces. Copings at the tops of the parapet walls may have open, or poorly constructed/poorly maintained joints, creating a direct path for water to enter the wall. Failed sealants around windows and doors can allow seepage around these openings which, without closer scrutiny, may be blamed on the wall itself. Water will find the path of least resistance, usually in the form of open joints or cracks. It is far more common, and far more likely that leaks in masonry walls occur at cracks between the masonry units and the mortar that is intended to bind the units together (figure 2), as opposed to leaking through the masonry units. As mentioned above, there are many reasons why masonry walls crack, and it is likely that you will discover cracks when you do your periodic inspections.

Figure 2: Debonded/cracked head joint

Fortunately, repair of cracked or  mortar joints is very effective in reducing the amount of water that penetrates exterior  masonry. Cracks in masonry that are more than a few millimeters in width or that are suspected to have been caused by settlement or other structural problems (for example, cracks that continue through multiple brick units and mortar joints, or follow a stepped or diagonal pattern along mortar joint) are beyond the scope of this article. These cracks often require professional investigation to determine the cause and appropriate method of repair.