The commonly quoted “One Percent Rule” states that 99 percent of the sources of rainwater damage are found at 1 percent of the building envelope. Most building professionals would agree that leakage problems at exterior walls typically are found at changes in plane and changes in material, such as door and window perimeters and wall intersections with decks.

In other words, rainwater infiltration usually occurs at the transitions between the work carried out by different trades. In most cases, successful long-term weatherproofing of these transitions requires careful design and installation of flashing crafted from corrosion-resistant metal or flexible waterproof membranes.

The 2003 International Building Code (IBC) includes the following guidance for flashing exterior walls:

  • “Flashing shall be installed at the perimeters of exterior door and window assemblies, exterior wall intersections with roofs, chimneys, porches, decks, balconies and similar projects and at built-in gutters and similar locations where moisture could enter the wall.”
  • “Flashing with projecting flanges shall be installed on both sides and the ends of copings, under sills and continuously above projecting trim.”
  • “Flashing shall be installed in such a manner so as to prevent moisture from entering the wall or to redirect it to the exterior.”

These instructions are an example of performance language. The IBC authors do not prescribe any specific flashing design or material or any standard installation practice; instead, the IBC simply mandates that a project’s designer and builders have a shared responsibility to design, craft and install all flashing necessary to keep the exterior walls dry.

Example of a Flashing No-No

A majority of construction defects litigation cases arise from simple lack of attention to the One Percent Rule. Consider the photograph to the right, which depicts a transition between hardboard lap siding and white-painted wood trimboards that form an aesthetic “bellyband” at the floor line between the first and second stories of this multifamily residential building. Note the metal Z-flashing (also painted white) that was intended to weatherproof this change in plane (and change in material).

Is this metal Z-flashing “installed in such a manner so as to prevent moisture from entering the wall or to redirect it to the exterior?” The answer, of course, is “No.”

The Z-flashing is sloped toward the building interior, causing water to collect against the water-sensitive hardboard siding, which is improperly installed in direct contact with the metal flashing. Further, the corner overlap joint between the two pieces of Z-metal is not sealed, allowing water infiltration into the wall assembly.

Then, in an unsuccessful attempt to correct this leakage problem, an unknown party later smeared a bead of caulk along the base of the hardboard siding. The caulk, however, also blocks the intended exit route for any incidental moisture (from a source located higher up the wall) that may have reached the building paper installed behind the siding. Further, the topical application of caulk at the top of the metal flashing joint quickly failed, allowing continued leakage at the corner.

The Blame Game

In practice, virtually every insured party involved in the design and construction of a failed building gets blamed, sooner or later, during the ensuing construction defects litigation process. Sometimes, such widespread apportionment of shared responsibility for a construction defect is appropriate; however, it also can simply represent an unprincipled effort by one or more parties to hide their culpability behind a smokescreen of half-truths and outright misrepresentations. For example, in this case the sheetmetal installer may argue any or all of the following positions:

  • "I installed the metal flashing with proper outward slope but the flashing later was pushed flat by the siding installer. (It’s the sider’s fault!)"
  • "The hardboard siding soaked up water like a sponge, damaging the building paper and causing mold growth on the gypsum sheathing. (It’s partly the manufacturer’s fault!)"
  • "The framer should have provided outward slope in the horizontal wood trimboard under the flashing. (It’s the framer’s fault!)"
  • "The architect did not supply a satisfactory detail for how to flash this transition. (It’s the designer’s fault!)"
  • "It was the project’s maintenance personnel who applied the caulk that trapped water within the wall. (It’s partly the owner’s fault!)"
  • "I just did exactly what the general contractor paid me to do. (It’s the GC’s fault!)"
  • "Everyone who ever touched this wall is at fault, except me!"

Without debating the merits or legitimacy of any of these claims, they do help explain the most serious problem plaguing designers and builders throughout North America: skyrocketing insurance premiums due to the high costs of prosecuting and defending a growing number of mold and moisture damage claims that in most cases still can be traced back to an insufficient focus on the One Percent Rule.

The ultimate solution to this insurance crisis is education. To that end, the many industry organizations and manufacturers who, in the past five years, have established free websites that detail proper flashing practice should be commended. An excellent example is the Build a Better Home® program established by APA—The Engineered Wood Association, which provides an extensive series of flashing details and basic construction guidelines for foundations, walls and roofs.

Until all participants in a building’s construction—from the owner to the designer to the builder to the superintendent to the foreman to the skilled laborer—truly recognize the importance of proper flashing of the building envelope, increasingly costly mold and moisture damage claims will continue to threaten the viability and diversity of our homebuilding industry.

March 2007
Issue Three, Volume Two

This article by Colin Murphy and Lonnie Haughton.

Colin Murphy
Murphy is a founder and managing partner of Trinity | ERD.

Lonnie Haughton
Haughton is a construction codes and standards consultant with Richard Avelar & Associates.

Is this metal flashing installed in such a manner so as to prevent moisture from entering the wall or to redirect it to the exterior?

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"Pushing the Envelope: A Monthly Journal of Issues Concerning Building Design, Science, and Litigation" is a monthly publication of Trinity | ERD. This newsletter is intended as a thoughtful look into the issues of building construction.

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