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Wood construction of multi-family, high-density housing became a preferred method of building during the housing boom. Unfortunately, in wet climates insufficient measures were taken to protect the building envelope from weather intrusion, resulting in a high rate of buildings damaged by water intrusion.

THE BUILDING ENVELOPE

Since the dawn of man and at its most basic level, homes are intended to provide shelter and protect us from the effects of weather.  To do that, they depend on the proper protective performance of the Building Envelope.  The Building Envelope exists to keep weather out and typically consists of the roof, the exterior walls and the window and door openings penetrating through those exterior walls.

When we think of protection from the weather, we typically think of the roof.  It covers, shades and protects us from the weather.  Most weather protection systems consist of at least two layers: a primary protection layer and a secondary protection layer.  Roofing tiles or shingles provide primary protection in that they form the first line of defense against weather; most rainwater is diverted through sheet flow as it hits the roofing.

But underneath the roofing there is a secondary line of defense, typically in the form of underlayment paper, a tar treated paper that protects the wood framing from any moisture that might get past the first line of defense.

Although we are historically more familiar with roof leaks, the exterior walls also provide protection from weather, either against moisture flowing off the roof or wind-driven rain.  Exterior weather protection systems such as stucco also provide primary and secondary weather protection systems.  Water mostly sheets off the stucco surface, but the installation of stucco plaster requires the installation of a secondary weather protection barrier underneath the plaster in the form of underlayment paper, similar to that used on the roofs. 

WHEN LEAKS OCCUR

Water intrusion in most roofs occurs at surface transitions, i.e., where the roof surfaces meet other surfaces.  Roof valleys (where two roof surfaces meet), or the intersection of a roof and an adjoining wall, are examples of places where leaks potentially start.   

Projections through the wall system (such as windows and doors) create large openings through the exterior wall building envelope and require special treatment in installation to assure protection against weather intrusion.  Openings in the wall system created for the installation of doors and windows require that the perimeters of those doors and windows be “flashed”; a term of art describing the installation of weatherproofing materials at the wall, door or window interface.  Flashing can be made of galvanized sheet metal products or self-healing “peel-and-stick” taped products.  Windows may also come with fins around their perimeters to create physical barriers against water intrusion.

Leaks through wall systems can occur when the primary or secondary systems are not properly installed.  Window and door leaks can occur due to problems with the installation of flashing around the perimeter, or problems with the window/door product itself, which may be defective and not weather-proof.

ROT DAMAGE AND STRUCTURAL COLLAPSE

Problems with insufficient protection from the building envelope due to improper construction can easily become extensive in high-density residential construction.  Production housing is, by economic necessity, assembly-line repetitive in nature.  Mistakes or misunderstandings in installation of weather protection systems become systemic when they are repeated over and over across a construction site.  In this manner, a simple installation mistake can multiply its effect when extrapolated on a construction site with hundreds of residential units.

The effects of such a problem become particularly acute in wood construction, where sustained exposure to small amounts of moisture results in the formation of molds and destructive bacterial growth within a relatively short time.  This bacterial growth weakens structural support members and causes cracking of exterior and interior finishes, localized structural failures, and in extreme cases, complete structural collapse.

CONCLUSION

The costs to repair damaged systems depend largely on the extent of damage, but can exceed the original construction costs.  Not surprisingly, water intrusion damage forms the basis of multi-million dollar lawsuits against builders and their insurance companies.  During the housing boom, in wet climate construction where constant exposure to weather would seem to require a higher level of care, many builders responded with a lower threshold of protection against the elements.  Similarly, overworked building officials failed to provide a proper level of protection for the future tenants of those buildings.  The result, though predictable, weighs heavy on unsuspecting tenants of increasingly damaged buildings.