Why? First of all, many of them are antiques. Over time, all buildings become antiques and require restoration to arrest—or reverse—their deterioration.

It’s also important to understand how building envelope components are interconnected. A problem with one assembly often affects other parts of the envelope. Roof leaks can damage adjoining wall assemblies, and compromised parapets can cause damage to roofs.

Water and Other Environmental Enemies

Water is the main enemy of the building envelope. Almost all building facades, roof, and vault problems can be traced to a breakdown of systems designed to protect against the physical action of water. If we understand how water behaves, then discovering, diagnosing, and repairing most building envelope problems follows.

NYC’s climate creates the perfect storm for water’s assault on the building envelope.

Freeze Thaw

NYC experiences numerous freeze thaw cycles each winter. Water trapped within wall crevices freezes, expands, and enlarges the fissure, allowing even more water to enter during the next cycle. This process is known as frost jacking. Incidentally, Superstructures monitors this effect through our seasonal Frost Jacking Tracker.

Salinity

In a saline environment like New York City’s coastal climate, the presence of salt water accelerates electrochemical reactions—namely, rusting of structural steel skeletons and concrete reinforcing steel.

The building envelope experiences tremendous abuse from these environmental factors—often in combination. Water infiltration, temperature variation, freeze-thaw cycles, salinity, acid rain, and ultraviolet radiation all conspire to damage building envelope components. Cracks develop and enlarge, sealant disintegrates, steel lintels rust, and curtain walls corrode.

A Dubious Distinction

With its density of older buildings and its unique combination of precipitation, freeze-thaw cycles, and ocean salinity, New York City is America’s capital of exterior restoration.

Case Study: The Parapet Wall

Sometimes a problem results when a critical issue has been overlooked in the original design of the building, whether antique or newly constructed. Occasionally, the culprit is sub-standard material or workmanship. These mechanisms can act individually or in concert, creating a vicious cycle that exponentially accelerates their damaging effects.

Eventually, this process results in serviceability failures such as energy loss, leaking roofs, flaking paint, or powdered plaster. When neglected, problems become exacerbated. If neglected long enough, unstable elements of the facade can threaten the life-safety of pedestrians below.

The parapet wall of a typical pre-war building provides a good case study of the deleterious action of water infiltration, the freeze-thaw cycle, and other environmental factors.

The parapet is the most vulnerable section of a building because it is the uppermost construction and is exposed on all sides.

The parapet system is a facade element that’s integral to the roof and includes the roof membrane, flashing, the masonry parapet, the concrete roof slab, and the steel beam (in this example) that supports the slab.

When all elements of the system are new, the roofing membrane and flashing protect the underlying components. Eventually, breaches in the membrane and flashing develop and water migrates below the parapet. The water freezes and expands with enormous pressure—a force far greater than the resistance offered by masonry or concrete.

The ice acts as a wedge beneath the parapet, forcing the masonry outward. Flashing between the parapet and roof is stretched further, creating more breaches, which permit the intrusion of still more water and eventually more ice. Water migrates down to the steel frame where it accelerates the rusting of the supporting steel beam.

Rust occupies more volume than the original steel. Rust, like ice, expands with great pressure, pushing aside the surrounding material, and pushing the parapet farther outward. There is now even more room for water to occupy the crevice below the parapet, where it freezes and expands.

Deterioration accelerates as the components of the system interact with one another.  As each component degenerates, it negatively impacts the other elements of the system. As time passes, the speed of this vicious cycle increases.

If the problem is discovered early, repair of the rusted steel beam simply involves surface preparation and coating. If it is discovered late, repair entails welding of supplemental steel plates to the deteriorated beam. If discovered still later, repair may entail removal and replacement of the entire beam. If not discovered at all, the problem may announce itself when a portion of the parapet wall lands in the street. Obviously, owners want to avoid such wholesale failure at all costs.

Copings

Copings protect the top surface of the parapet wall. They are the parapet’s first line of defense against water. On many buildings, copings are made of bluestone or terra cotta. Both materials are readily deteriorated by the abrasive action of water and acid rain.

Among the most obvious points of entry for water are the joints between coping stones, often referred to as "sky joints," or "cross joints." If not sealed adequately, they’re horizontal, open invitations to water.

Appurtenances

The principles that apply to copings and parapets apply equally to appurtenances such as chimneys, balconies, and water tower enclosures. If anything, these components are even more vulnerable, because they have even more exposed surface area.

Case Study: Glazed Brick Facade

Let’s look at a post-war example:

Building facades erected after World War II were designed mostly as engineered systems—a departure from the fortress mentality of pre-war buildings. Post-war buildings generally feature a reinforced concrete skeleton clad in masonry cavity-wall construction.

A typical post-war residential facade consists of a brick veneer (often glazed) and a back-up of concrete masonry units (CMUs), tied together laterally by metal anchors. The veneer is supported vertically by a steel relieving angle fastened to the concrete structural slab.

But there are inherent characteristics of the glazed brick veneer that render this type of construction vulnerable to water. While rainwater does not penetrate the waterproof glaze, it does penetrate the porous mortar joints between bricks. The moisture trapped against the inner surface of the glaze freezes, expands, and the glaze spalls off the "biscuit."

Design / Construction / Maintenance

Sometimes, as-built construction does not reflect the design architect's drawings. Some of the more common problems:

• Debris in the wall cavities, including mortar droppings, candy wrappers, and newspapers bearing the date of the building's construction
• Weepholes clogged with debris or omitted entirely
• Metal ties between the brick veneer and back-up concrete masonry irregularly spaced, missing, or not engaging the mortar joint

Because they are intrinsically lighter, post-war systems in both commercial and residential buildings offer less margin for error in design, construction, and maintenance than their pre-war counterparts.