A Closer Look-Flashing
With sloping roofs irregardless of the application or type of flashing used, the use of flashing is to aim 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 beneath the underlayment, the uppermost pieces of flashing step over the lower pieces, and the lower edge of the flashing in every instance passes over the top of the roofing material. In such a fashion, the flashing at no time directs the flow of water to the bottom side of the underlayment, never putting it in proximity with the wood structural panel sheathing.
A roofing contractor should be a flashing expert as well.
Valley flashing protects the valleys where two roof planes adjoin. This material is accessible with a V- or W-shaped profile and is deployed over the top of the building felt before all 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 and every course of shingles and appears to “step” up the wall of the chimney.
Vent pipe flashing fits over flues or pipes. The profile 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 avert water from seeping under the finished roof along its edges.
The bulk of roof leaks appear 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 conceivable leaks sites due to chimneys, ridges, valleys, etc. Proper detailing about these areas, as detailed in the diagrams, is essential to block these leaks.
Metal flashing material is commonly soldered or brazed. akin to welding, the brazing approach bonds two pieces of metal into one single piece. In numerous cases, flashing elements have to wrap around corners or be spliced all together, and in these cases they may be soldered or brazed to certify a durable, dependable joint.
Also, bear in mind that both temperature and humidity can compel roofing materials (shingles, wood sheathing, flashing) to expand and contract with seasonal changes. The flashing materials will go on to be leak-proof if they can bear up against this movement of the roofing materials. Well engineered and accurately installed two-part flashing can contend with this movement with no burden. 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 anchored to the base, so the two pieces can move against each other alone when the roofing materials change with seasonal difference.
VISIT ROOF DOCTOR
VISIT ROOF DOCTOR
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
