Flashing is a hot-button topic amongst residential designers, builders, and claims adjusters alike. In fact, currently one of the most prominent construction defect claims that we see involves moisture intrusion around improperly flashed window openings in wood-framed structures. Other trouble spots include the roof penetrations at chimneys and vent pipes.
The design intent of flashing is to allow moisture to move out, down, and away from a structure. Water will remain on the outside of a building where it belongs if the right methods and materials are used. Water is a shrewd opponent. It can be driven by wind or sucked into crevices by negative air pressure inside the structure, after which it can actually flow uphill between construction layers. Once inside, water will subject building materials to alternating spells of soaking and drying. until decay of the wood framing members finally results.
We all know that if there are any ways for water to get in, it will find those entry points. Unfortunately, if water seeps in around a window, chimney, or vent pipe, the damage it can do to the wooden structural components such as plywood sheathing, wall studs, floor joists, and roof rafters usually occurs before the leak is detected. This is the case because these components are hidden by their coverings and finishes. Therefore, often the damage is done before it becomes too late to avoid invasive, destructive, and costly repairs.
First, I will examine flashing around window openings. Most modern windows are made of wood, have metal nailing flanges around their perimeter, and are clad with either aluminum or vinyl on their exterior to protect the wood from the weather. Flashing typically consists of bituminous tape, i.e., bituthane, applied at the head, sill, and jambs of the window. How the housewrap moisture-resistive barrier (MRP) interfaces with this flashing is of primary importance. When the cut edge or “flap” of the housewrap is run behind the tops of the windows, as shown in the picture below, the opening cannot be flashed properly. The housewrap top flap should be pulled up on the exterior of the building at the top of the window, the corners taped, and the flashing applied over it. As always, the specific window manufacturer’s written instructions for flashing should always take precedent. When brick masonry veneer cladding is used, additional “thru-wall” flashing through the brickwork is required to ensure that the structure is properly sealed against any moisture which may end up behind the brickwork.
Next, I will look at the installation of flashing around roof penetrations at chimneys. This flashing is typically made of metal, such as copper, aluminum, or galvanized steel. EPDM rubber may also be used. The goal is to achieve a leakproof chimney penetration assembly. Roofers are usually responsible for flashing chimneys, but this can be one of the toughest tasks of a roofing job, and requires the most experience and skill. Because of this, on many re-roofing projects, the roofers may even skip the installation of new flashing altogether and rely on the old, original flashing to keep the new roof dry. As one can imagine, this seldom works out well for the homeowner. The pictures below is are examples of botched chimney flashing jobs, which not only have the appearance of shoddy workmanship, but probably leak as well.
Finally, I will examine a method for installing flashing around roof vent pipe penetrations. This flashing is available in premolded or prefabricated “boots”, which are properly sized for the specific vent pipe, and fit snugly over the pipe. The boot is set down onto a bed of fresh roofing cement/caulk. Then, the shingles are installed, with one shingle notched to fit around the flashing boot, as shown in the graphic below. This method is very basic and straightforward, and when done correctly, will ensure a leakproof assembly.
Remember: The goal is a perfectly dry structure. Properly installed flashing can completely weatherproof a home and make it impervious to stormwater and condensation, which can cause major problems if they find their way to the wrong places. If you have a water intrusion claim, always investigate possible issues with flashing.
George Sanford, PE, holds a Bachelor of Science in Mechanical Engineering from North Carolina State University in Raleigh, North Carolina. George has more than 20 years of applied structural engineering experience specializing in residential, commercial, and industrial structures and foundations. Throughout his career, George has designed and analyzed structures, supervised engineers, prepared construction documents (drawings and specifications). He has an in-depth knowledge of many building codes, standards, rules, and regulations including the agencies that govern and provide guidance to building designers such as the International Code Council (ICC) American Society of Civil Engineers (ASCE), Steel Joist Institute (SJI) and the American Iron and Steel Institute (AISI).