Though waterproofing is often considered within the realm of architects, waterproofing historic facades presents challenges both to the architect and structural engineer.

Structural engineer’s perspective
Cuono: Although architects and façade experts have devised creative ways of weatherproofing historic masonry facades, we must keep in mind load paths, too.

One example we have run into in our practice is for steel-framed buildings with exterior, un-reinforced, masonry infill walls consisting of 8-inch terracotta backup and a 4-inch brick face. We have found some wall systems like this where the exterior brick has headers into the backup terra cotta and some where the headers are fake with no extension or any ties at all to the backup. If — from the waterproofing point of view — the best solution is to “skin” the exterior brick face, add a waterproofing membrane in the collar joint, and rebuild with a veneer; then the wall will have to be analyzed since this could change the out-of-plane load capacity. In this scenario, the new exterior brick is now a veneer transferring wind load (through new ties) to the existing backup.

One could theoretically argue that historic masonry wall systems were originally designed and installed with no consideration for wind loading (such was the case for the New York City building code prior to the 1960s, which allowed wind design to be neglected in buildings that were less than 100 feet tall) or even that the brick was not contributing to the out-of-plane strength because of poor construction, weak collar joints, lack of headers, bonding, ties etc. However, the point is that any tampering with an existing system needs a thorough engineering analysis to ensure a continuous and adequate load path.

If, in the case of the wall described above, it was found that the backup was not adequate, then the remaining terra cotta may have to be reinforced. Some ideas include grouting vertical cells and adding rebar, cutting down the span with vertical reinforcement welded to and cantilevered from the spandrel, or using FRPs, among others.

Consideration must also be given to temporary loading conditions. The contractor may have to provide temporary shoring or bracing to maintain the strength of the wall until the permanent reinforcing is installed.

Having the engineer analyze all parameters early in the game, before an architect commits to a waterproofing system, will allow the team to focus on a solution that satisfies all requirement of good design.

Architect’s perspective
Bates: The use of proprietary terra cotta header blocks in bonded masonry wall construction became popular in the early 1920s. The system employed an “L”-shaped back-up tile course, which would receive the back ends of the face brick header courses, efficiently bonding the exterior veneer to the back-up tile. Depending on the porosity of the face brick and mortar, however, it has been found that this configuration may actually conduct absorbed rainwater further into the wall section, causing damage to interiors.

If probes and absorption tests reveal that the brick veneer requires wholesale replacement — whether for structural or waterproofing reasons — it is important to research the effects of proposed integral water control membranes. Also give consideration to any proposed structural improvements that would interrupt membranes or divert absorbed water, such as beam anchors, brick ties, or shelf angles.

Historically it was understood that bonded masonry walls would absorb moisture during rain events and gradually dry through vapor migration back to the exterior before reaching the relatively absorptive interior plaster finishes. Likewise, ambient vapor from relatively humid interiors would be released through similar paths. The integration of improper water control membranes behind the face brick at large scale wall areas may impede the transmission of vapor and lead to condensation at the interior surface of the membrane causing loss of adhesion and other various interior wall failures.

It is critical that the material layers within the wall section exhibit ever-increasing levels of vapor permeability from the interior face to the exterior face. The new water control system must perform the dual task of permitting the passage of vapor to the exterior while preventing the passage of liquid water to the interior. There are many such products (available in either sheet form or spray-applied liquid form) that have a relatively high permeability rating, which allows the passage of vapor.

Ciro Cuono, P.E., LEED AP, is an associate at Hage Engineering, in New York. His experience spans renovations and new construction projects, including historic renovations, cultural, civic, residential, and commercial projects. He can be reached at ccuono@hageengineering.com. Robert C. Bates, RA, AIA, is a principal at Walter B. Melvin Architects, LLC, in New York. His experience includes restoration and reconstruction of exterior envelopes on historic buildings in and around New York. He can be reached at rbates@wbmelvin.com.