Q: In many buildings that have rows of closely spaced windows, it is almost impossible to avoid having vertical expansion joints against the window frame. In these cases, should the expansion joint line up with the jamb of the window or should it jog around the end of the lintel angle?
A: When windows are spaced at least 8 feet apart, I typically recommend placing expansion joints in the center of the masonry between windows. When windows are spaced closer than that, it is best to line up the expansion joint at one of the window jambs, rather than shifting the joint beyond the end of the lintel.
This approach is discussed in the Brick Industry Association (BIA) Technical Note 18A, on page 5. BIA recommends installing a flashing membrane below the angle, in addition to the flashing above the angle, in order to create a slip plane to allow the masonry to slide around the end of this lintel angle. A thin plastic bearing pad beneath the full bearing area can also be used for this purpose.
In either case, it is important to hold the front edge of the slip plane material far enough back to avoid interfering with the bond surface for the sealant joint that will be installed in front of the lintel angle at the bearing points. It is also important to keep a space between the mortar and the edge of the lintel by using a compressible material, such as a backer rod, in this location. If this is not done, as the brick tries to expand, it will push against the end of the lintel which will push against the masonry on the other side, thereby bridging the expansion joint.
BIA cautions that when you use this procedure, it influences the way the masonry bears on the angle. Therefore, instead of structurally sizing the lintel to support a triangle of masonry above the opening as is typically done (based on the assumption that the masonry will arch across the opening), the lintel angle should be sized to carry the full height of the masonry from the lintel to the next shelf angle above.
As is the case with all lintels, the through-wall flashing above the lintel should extend to the first head joint beyond the end of the lintel and should contain an end dam to prevent water from reaching the end of the lintel angle. Figures 1 and 2 illustrate the design of expansion joints adjacent to lintels.
Some designers place the expansion joint at the end of the lintel, rather than lining the expansion joint with the window jamb. However, this alternate approach does not eliminate the need to install a slip plane beneath the lintel angle or the compressible material at the end of the lintel angle since the angle would still bridge the expansion joints if it were not able to slide.
Also, the steel angle still should be designed to carry the full height of the masonry from the lintel to the next shelf angle above, since there is not enough masonry between the jamb and the end of the lintel to resist the lateral trust from arching.
Therefore, I typically recommend placing the expansion joint to line up with the window jamb.