Think Like a Raindrop - Primer on Water Intrusion Construction Defects Expert Article

“Think like a raindrop” is an old adage used by designers and builders of low rise residential buildings – think single family homes, townhouses, and two and three story apartment buildings. It’s also applicable to commercial buildings of the same height and light weight construction, i.e. motels and small office buildings. The concept refers to designing and constructing building exteriors that will reliably shed water and keep the underlying structure and building interiors dry.

In this article, Architect & Construction Expert, Thomas Pienciak, AIA gives a primer on water intrusion construction defect claims by walking through the means and methods of building construction.

Water intrusion construction defect expert

Expert Primer on Water Intrusion Construction Defects

This article is written assuming the basic building construction is wood framed with wood based sheathing. The concepts are still relevant for buildings constructed with structural metal studs with wood-based or alternative sheathings.

The basic physics dictates that, absent other forces, gravity will cause water to flow downhill. The exterior skins of low rise buildings are typically composed of multiple elements and materials in multiple planes and orientations. They generally have sloped roofs with elements like chimneys, plumbing vents, and dormers poking up through them. They have vertical walls, and things penetrating them, such as windows, doors, dryer vents, and hose bibs. In order to keep a building interior dry, any rainwater that falls onto the highest element of a building exterior should flow out onto, or fall away from, the face of lower elements without an opportunity to get behind them. This is commonly done by installing exterior materials “shingle style.”

The term “shingle style” derives from the manner that roof shingles are commonly installed, but it applies to other building materials as well. Common residential shingle roofing is installed from the bottom up, where each course of shingles naturally overlaps the course that is downhill from it. In this configuration, rain that falls onto the highest course of shingles flows out onto the face of the next lower course of shingles. The water path continues downhill from course to course until it reaches the eaves where it is commonly collected by a gutter and directed away from the building via downspouts/leaders. Any protrusion through the plane of the roof shingles, (plumbing vents, chimneys, skylights, etc.) must be installed and flashed in the same manner. If any element is reverse-lapped, i.e. the uphill edge is on top of the underlying material, it acts like a scoop and will allow water to enter onto the underlying materials. That intruding water has the potential to collect and cause damage.

Siding Material Types and Installation

Many siding materials are also installed shingle style. Traditional wood clapboards and popular cement-based substitutes for them are installed shingle style. Vinyl siding is installed shingle style, although the bottom edge of vinyl siding courses interlock with the course above instead of being “open” the way that the bottom edge of the wood clapboards that vinyl siding visually replicates is.

Other siding materials, like traditional and artificial stuccos, and masonry veneers, take a different approach. They are assumed to function as slightly leaky, larger planes of cladding. Consequently, they rely on an underlying layer of water resisting barrier, or WRB, to prevent intruding water from making its way to the underlying framing and sheathing materials. The WRB was historically asphalt impregnated building paper, referred to by laypeople as tarpaper. More contemporary WRBs are composed of a variety of synthetic materials. Regardless of the material that the WRB is composed of, it typically comes in rolls of various sizes, and is installed shingle style in horizontal courses.

Historically, stucco was installed over a single layer of WRB. More recent rules require two layers of WRB under stucco. Traditional stucco is installed by troweling multiple layers of cement-based materials over expanded metal lath that has been fastened to the building framing and sheathing directly over the WRB. In that system, the expanded metal lath-reinforced stucco is installed in contact with the WRB. Any water that makes its way through the stucco, through cracks or other avenues, is intended to travel down the face of the WRB without coming into contact with the (historically) wood based sheathing behind it. Some synthetic WRBs are produced with wrinkles or dimples to provide some space for the water to flow through.

Traditional masonry veneers, like 4 inch thick brick, were commonly installed with a 1-1/2 inch air space between them and the WRB-protected wood sheathing behind them. It was expected that the majority of the intruding moisture would travel down the back side of the bricks, without contacting the WRB. Any moisture that made its way across the air gap to the face of the WRB would flow down the face of the shingle-lapped WRB.

Contemporary developments in construction have combined traditional stucco with the look of full thickness masonry veneers into a product known as adhered concrete masonry veneer or ACMV; also referred to as cultured stone. The stone-look version of this product is cement based and the veneer portion is typically less than 2 inches thick. The brick-look version of this product can be as thin as ¼ inch thick and is sometimes called brick tile. Regardless of the final outer look, the system sheds water like a stucco system that has no masonry veneer facing. This means that any water that penetrates the outer veneer and the underlying “stucco” flows down the face of the underlying WRB if the system functions properly.

Flashing and Weeps

Regardless of whether the masonry veneer has an air gap behind it or is installed over stucco that is in contact with the WRB, any water that gets through the outer layers of the system should flow down the outer face of the WRB. The same is true for water that passes through “plain” stucco that has no adhered veneer on it. In a well designed and constructed system, water will get diverted back out to the face of the wall before it has a chance to collect and damage any of the underlying wood construction. The elements that divert the water flowing down the face of the concealed WRB back out to the outer face of the wall construction are called flashing.

Horizontal flashing components are required wherever the downward flow of water on the face of the WRB would be obstructed. The most common places for flashing are at windows and doors, and where the exterior wall cladding and underlying wood construction meets the (typically) concrete-based foundation walls.

Typical flashing materials are constructed of metal, plastic, or rubber. When they are used in a masonry veneer system that incorporates an air gap they are commonly called “through wall flashing.” When they are used in a wall cladding system where each layer is essentially in contact with the layers in front of and behind it, they are simply called flashing. Flashing above windows and doors is called head flashing. Flashing below windows and doors is called sill flashing.

Horizontal flashing components utilize the “shingle style” concept of shedding water, but stretch it out horizontally. This is typically done with some form of “Z” flashing, as shown in the diagram below.

Z flashing expert diagram

The upward pointing leg typically gets inserted into the wall construction tight to the wall sheathing. The bottom edge of the WRB laps over this leg shingle style. The horizontal portion has a slight downward slope towards the outside face of the wall. The downward pointing leg overlaps whatever material is lower than it, shingle style.

Some form of gap or series of weep holes is provided on top of the horizontal leg. Now, any water that makes its way to the WRB flows down its face until it reaches the horizontal leg. It then flows across the downward sloped horizontal leg, through the provided gap or weep holes to the outer face of the wall cladding, down the downward pointing vertical leg, and off its bottom edge.

A properly detailed and installed downward leg bends out slightly away from the face of the building so that ejected water can drip off without running down the face of the wall below. This aspect of the detail is called the drip edge. Missing, clogged or badly constructed weep holes will inhibit the expulsion of water from the wall. The trapped water then collects within the wall system where it can cause damage.

Water Intrusion Caused by Wind-Driven Rain

Roofs, clapboards and similar cement based sidings are regularly subjected to wind driven rain without leaking. Part of the reason for this is the size of the overlaps. It takes a fair amount of wind for rain to be driven up and over the typical 7 inch overlap of a 12 inch roof shingle course. Shingles, clapboards, and other materials that are installed shingle style are all installed over a layer of WRB, and each has one or more routes for water to escape from the system in the event that it reaches the underlying WRB. The vertical wall systems, with or without an air cavity behind the cladding, will drain wind driven rain out the same as non-wind driven rain.

Construction Sequencing

Despite the simple physics behind shingle style detailing, some buildings still leak, resulting in construction defect lawsuits. Typically, this is partly due to construction sequencing, and partly due to splits in the responsibilities of various subcontractors. In a leaky building, it’s rare that the sloppy work of a single contractor would be to blame. Leakage will more often occur where the work of two or more subcontractors comes together and interacts.

Ignoring all other practical limitations, the ideal way to construct a building would be for a single contractor to start at the bottom and work their way to the top. In that scenario, as work progressed each upper layer could naturally and easily overlap the lower layers shingle style. But for a variety of practical reasons that isn’t the way that residential construction is typically built.

Example Scenario

In this example, we’ll look at a two-story condo with a shingle roof, vinyl siding on most of the exterior walls, and ACMV on the lowest three feet of the first floor exterior walls. There’s an accent wall where the ACMV is one story tall. Ideally, construction of the exterior finishes would start at the bottom and work its way up. The ACMV would be installed first, followed by the vinyl siding, followed by the roofing. However, for a variety of reasons, construction is typically done in the opposite order.

  1. The roofing and roof edge trim is installed first. This keeps the building interior mostly dry so that other trades (plumber, electrician, etc.) can work there. At this point the gutters and leaders are not yet installed, because the siding hasn’t been installed so the leaders can’t be fastened to the building.
  2. Next the windows and exterior doors get installed; frequently by the rough framing subcontractor. If installed properly, their edge details can accommodate integration with the WRB that will commonly be installed by a different subcontractor.
  3. Next the WRB and vinyl siding gets installed by a siding subcontractor. The builder doesn’t want to install the ACMV yet because water flowing off the gutterless roof will be splashing mud up onto the lower portion of the exterior walls.
  4. Finally the ACMV can be installed, promptly followed by gutters and leaders to avoid having it spoiled by splashing mud.

Essentially, the exterior finish materials get installed from the top down by different subcontractors. Each subcontractor has to install the bottom of their work in a manner that allows the following subcontractor to properly tuck their work under the preceding construction. That facilitates the flow of water down the WRB, and properly exiting to the exterior of the wall construction. When the work of two trades meets the details and the sequencing of their work must be looked at carefully. In the event of a construction defect claim due to water intrusion, a forensic investigation would typically involve an assessment of these areas.

Water Intrusion Construction Defect Investigations

Investigating a construction defect claim related to water intrusion typically starts with the review and analysis of the various construction documents. It then proceeds with an analysis of the as-built construction, evaluating how the sequencing of work resulted in conditions where water intrusion and damage could occur. When defects are identified, it’s typically because somebody failed to think like a raindrop.

For more information, contact the author of this article or submit an inquiry.

Featured Expert

Thomas Pienciak, Architecture, Construction & Premises Safety Expert

Thomas Pienciak, AIA

Architecture, Construction & Premises Safety Expert
Tom is a Licensed Architect with over 40 years of experience in building design, document preparation, code compliance, and construction administration procedures. He has a diversified background that… read more.


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