Roof Flashing Done Right
If you’re collecting dripping water in a pot beneath your roof, you may want to inspect the roof flashing. Anywhere surfaces intersect on a roof is a prime spot for water seepage. Flashing provides the extra protection these spots need. These problem areas include the edges of skylights and chimneys, soil stacks, vent fans and roof valleys, as well as the intersection of the roof deck and dormer walls.
Most flashing is made of galvanized metal, but DIY’ers may prefer aluminum flashing because it’s easier to bend. Most flashing products are designed for easy installation, and if the material is installed correctly, then your roof shouldn’t leak. However, from time to time, the nails that fasten flashing work loose, or the flashing material pulls away from seams and joints and requires maintenance. Here’s a look at some general principles of roof flashing that will help guide you through a proper repair or replacement—or even new construction.
A Look at Roof Design
All roofing systems are made of a number of different components: roof sheathing, underlayment, roofing material, roof intersections, flashing details and ventilation. Each of these systems must be installed correctly for the system to work as designed.
Common pitched roof systems rely on the force of gravity and the surface friction of the roofing materials to direct the flow of water downward and outward. These systems rely on overlapping elements—roofing felts, shingles, tiles and flashing details—to redirect the rainfall. The pitch of the roof provides the gravity and the detailing provides the redirection.
However, in low-slope roofing systems it is easier for water to accumulate. The force of gravity drives the water into every imperfection in the waterproofing system. So, for low-slope roofs, water is kept outside the building envelope by providing a perfect waterproofing barrier over the entire roof system and around every penetration in that roof.
Roof sheathing is attached to the roof framing, trusses or rafters, and provides the nail base for the other components of the roof system.
Next, roofing underlayment, often made of building paper or “felt,” is installed as the first weatherproofing layer for a pitched roof. Underlayment should be installed from the bottom of the pitched roof to the top, such that each upper layer overlaps the lower layer. This method of installation channels the water out and down, away from the wood-panel sheathing below.
Finally, roofing material, the visible finished layer on a roof, provides the primary waterproof barrier for the structure. For pitched roofs, almost all roofing materials rely on some form of shingling to provide the weatherproof barrier. Like the underlayment, these roofs are installed from the bottom-up, with successive layers overlapping both vertically and horizontally to shed away water. Low-slope roofs can use many different proprietary and non-proprietary systems to form their waterproof barrier, from single to multiple-ply; adhered, mechanically anchored or ballusted; hot mopped or cold applied (solvent, urethane or epoxy-based).
Flashing works in conjunction with all of these elements to reinforce the waterproofing at roof intersections or penetrations.
Closer Look at Flashing
For pitched roofs regardless of the application or type of flashing used, the purpose of flashing is to direct the flow of water that leaks into the intersection down and away from the interior of the structure to the topside of the roofing material. In every case, the top edge of the flashing passes underneath the underlayment, the upper pieces of flashing pass over the lower pieces, and the lower edge of the flashing always passes over the top of the roofing material. In such a manner, the flashing never directs the flow of water to the bottom side of the underlayment, never putting it in contact with the wood structural panel sheathing.
Valley flashing protects the valleys where two roof planes meet. This material is available with a V- or W-shaped profile and is placed over the top of the building felt before the roof’s finishing material is installed.
Step flashing protects the joints between the roof deck and chimneys or dormers. Step flashing fits to each course of shingles and appears to “step” up the wall of the chimney.
Vent pipe flashing fits over flues or pipes. The shape of vent flashing is typically a cylinder with a wide flange at the base, which is lapped into the shingles as the roofing is installed.
Drip edges are strips of flashing material that run along roof eaves and rakes to prevent water from seeping under the finished roof along its edges.
The majority of roof leaks occur in locations where the plane of the roof is interrupted by a ridge, another roof intersecting at an angle, a wall or penetration. Even the simplest of rooflines has dozens of potential leaks sites due to chimneys, ridges, valleys, etc. Proper detailing around these areas, as detailed in the diagrams, is critical to prevent these leaks.
Metal flashing material is generally soldered or brazed. Similar towelding, the brazing process bonds two pieces of metal into one single piece. In many cases, flashing components have to wrap around corners or be spliced together, and in these cases they can be soldered or brazed to ensure a strong, durable joint.
Also, remember that both temperature and humidity can cause roofing materials (shingles, wood sheathing, flashing) to expand and contract with seasonal changes. The flashing materials will continue to be leak-proof if they can withstand this movement of the roofing materials. Well engineered and properly installed two-part flashing can handle this movement with no problem. Two-part flashing systems consist of a base flashing—often step-flashing—that is laced into the finished roof material. The base is then covered by another metal flashing piece lapped over it. The second piece should not be fastened to the base, so the two pieces can move against each other independently when the roofing materials shift with seasonal change.
Attention to Detail
Keep in mind that flashing details are a very technical aspect of roof construction, and this type of work is best left to experienced professionals. Soldering vertical surfaces requires skill, and you don’t want to gamble with the integrity of your roof. The best way to learn proper flashing techniques is to spend time with an experienced craftsman as he installs a complete roof.
That being said, here are a couple of flashing tips that even an inexperienced homeowner should make note of. Always check the roof flashing and the condition of the shingles whenever you clean your gutters. Look for loose nails and any damage to the seals at the edges of the flashing. Roofing cement can dry out and crumble away, exposing joints to water. Fasten loose nails and cover exposed nail heads with roofing cement. Renew flashing seals by chipping out old caulking and mortar along the edges of the flashing. Recaulk the joints between the roof and the flashing. It’s much easier to reseal the flashing than it is to patch a water-stained wall or ceiling.
Badly corroded flashing will need to be replaced. To do this, you will also have to remove several rows of shingles as well as the old flashing. Then replace and fasten the flashing, and reinstall the shingles with the proper overlapping procedure.
Also, new flashing can be exceptionally shiny, especially in contrast to the earthy brick walls of many homes. You may want to paint the flashing to match the roof. First use a stiff wire brush and solvent to clean the surface of any dirt, debris or corrosion (keep solvent away from asphalt shingles). Coat the flashing with a zinc-based primer followed by two or more coats of spray-on, rust-preventive metal paint.
Editor’s Note: This article appears courtesy of APA – The Engineered Wood Association. Check out more of APA’s building tutorials at www.apawood.org.
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