Collar joints are one of the most misunderstood components in a masonry wall. Architects typically specify collar joints to act as a water barrier; in contrast, engineers specify them exclusively for structural purposes. The ACI 530/ASCE 5/TMS 102 Building Code Requirements for Masonry Structures (MSJC Code) defines a collar joint as the vertical longitudinal space between wythes of masonry or between an outer masonry wythe and another backup system. Often this space is specified to be filled solid with mortar or grout, but sometimes collar-joint treatment is left unspecified. A common detail for masonry walls containing a collar joint consists of a 4-inch nominal outer wythe of masonry and either a 4- or an 8-inch nominal inner wythe of masonry. The overall wall thickness is 8 or 12 inches, and the remaining space is a 3/4-inch-wide collar joint. However, it would be better for the collar joint to be 1« to 2 inches wide, so it could be filled solid more easily. Cracks and voids in collar joints filled with mortar are common and can be expected for a number of reasons. Separation cracks sometimes develop between the interface of the masonry units and the collar joint. This can be due to the different absorption rates of each masonry wythe and the fact that mortar-filled collar joints aren't compacted. In addition, thermal movement can cause the outer wythe of masonry to separate from the collar joint. Moreover, given the absorptiveness of both wythes, mortar used in a collar joint doesn't contain enough water to fully hydrate the cement; consequently, shrinkage cracks occur and low strength develops. Voids in a mortar-filled collar joint inevitably occur. This is particularly true for a 3/4-inch-wide joint, which can be reduced to « inch when allowable construction tolerances are considered. Because mortar isn't fluid, a 3/4-inch-wide joint is simply too difficult to completely fill with mortar. The only way to be assured that collar joints are filled 100% solid is to grout them. The MSJC Code allows the grout space in a composite wall to be a minimum of 1 inch. However, to create a functional barrier wall, this space should be sufficiently wide--1« to 2 inches--so that the mason can keep it clear of all mortar obstructions. Filling the space with high-slump grout and consolidating that grout will produce a 100% solid collar joint that provides the wall system with composite action and a water barrier.