How are bed joints tooled in exterior masonry using split-faced ribbed concrete masonry units?

It is difficult to tool horizontal mortar joints in ribbed concrete masonry. Two methods can be used. One is to form wide mortar joints (3/8 inch or slightly larger) so that the mortar can be tooled straight across without placing any mortar between the ribs of adjacent units. There are some problems with this approach, however. First, if the joints get too narrow, they are very difficult to tool. Second, the top surface of the ribs forms a ledge that may allow water to penetrate the masonry during rainstorms. Finally, the depth of the joint back from the surface of the rib makes it difficult to apply coatings that are needed to make some wall systems resistant to water penetration.

The second approach is to tool a mortar joint following the profile of the rib. This is difficult because a conventional jointer is designed to tool straight joints. In this approach, the surface of the joint will usually be struck flush with the rib using a margin trowel. Consequently, this joint is not compacted and is more susceptible to water penetration.

Whichever approach is taken, you must exercise considerable care during design and construction to create a wall that will resist wind-driven rain. Strong consideration should be given to using ribbed units only in cavity wall construction. An alternative to cavity wall construction is to use block units with integral water repellent admixtures that work as drainage walls by allowing water to drain within the cells of the unit. In low-rise buildings, large roof overhangs or other features can shield the walls, which will improve their performance during wind-driven rains.