Flashing is a crucial component of any roofing system, designed to prevent water intrusion at vulnerable areas such as parapets, dormers, chimney sides, and where the roof abuts against a wall. Proper flashing installation ensures a watertight seal, protecting the structure from costly water damage. This blog post will detail the application of flashing in these critical areas to help ensure the longevity and integrity of your roof.
What is Flashing?
Flashing is material used to bridge adjoining structures and prevent water penetration. Flashing can be made from corrosion-resistant metals like aluminum, galvanized steel, or copper. Other materials include roll roofing, vinyl flashing, and plastic sleeves. Flashing is installed to overlap and shed water, working with gravity to direct water away from vulnerable areas. Effective drainage, achieved through corrosion-resistant drip edges and proper gutter and downspout placement, is also a key part of a good flashing system.
General Flashing Principles
- Base Flashing: This flashing is installed over or under the roof covering and extends up the vertical surface.
- Counterflashing (Cap Flashing): This is built into the vertical surface and bent down over the base flashing.
- Material Compatibility: Use flashing materials with a similar lifespan to the roofing material.
- Sealants: Use asphalt plastic cement or mastic to embed flashing and seal edges, but avoid excessive use to prevent blistering.
- Movement Accommodation: Allow for structural movements due to settling, expansion, and contraction by not fastening the cap or counter flashing to the roof deck or step flashing.
Flashing at Parapets
- Parapet walls often require long runs of flashing material, which can be made of rigid materials like 26-gauge galvanized sheet metal.
- Design the flashing to provide sufficient coverage and adequate drainage.
- Establish a windblock at longitudinal edges by grouting with portland cement mortar.
- Parapet rake flashings are often designed to float, even if the roof is fixed, to accommodate thermal movement differences between masonry and steel.
- For masonry parapets, it is better to make a saw cut and insert a counterflashing rather than using a surface-mounted counterflashing.
Flashing at Dormers
- The junction of a dormer with a sloping roof requires a base or apron flashing below the window sill.
- Step flashing is used where the dormer meets the side of the roof.
- The valley between a dormer roof and the main roof requires a special flashing procedure, using open valley flashing.
- Cut the lower edge of the valley flashing to conform to the eaves flashing strip.
- Extend the flashing from the main roof onto the dormer eaves, overlapping the shingles.
- Secure the metal flashing to the roof deck with metal cleats.
- Cut front flashing to extend around the corner where a vertical front wall meets a sidewall.
Flashing Against Vertical Walls
- Sidewall (Step) Flashing:
- Use rectangular metal flashing shingles, at least 10 inches long and wider than the exposed shingle surface. A 6 x 7 inch size is suitable for shingles laid 5 inches to the weather.
- Bend the flashing shingle so it extends over the roof deck and up the wall surface.
- Place each flashing shingle above the exposed edge of the overlapping shingle and secure it to the wall sheathing with one nail in the top corner.
- Bring the finish siding down over the flashing to act as counterflashing.
- Do not fasten the cap or counter flashing to the roof deck or to the step flashing, since they move independently of each other to accommodate any structural movement.
- Front Wall Flashing:
- Apply shingles until a course must be trimmed to fit at the base of the wall.
- The last course should be at least 8 inches wide.
- Apply a continuous piece of metal flashing over the last course, embedding it in asphalt plastic cement and nailing it to the roof.
- Bend the metal flashing to extend up the wall and onto the last shingle course.
- Apply an additional row of shingles over the metal flashing strip.
- Bring the siding down over the vertical flashing to serve as cap flashing, but do not nail siding into the vertical flashing.
Flashing Around Chimneys
- Chimneys require base flashings secured to the roof deck and counterflashings secured to the masonry.
- Construct a cricket or saddle between the back face of the chimney and the roof deck to prevent snow and ice accumulation and deflect water.
- Base Flashing:
- Apply base flashing to the front of the chimney, extending over the shingles and up the chimney face, setting both sections in plastic asphalt cement.
- Use metal step flashing for the sides, securing each piece to the masonry with plastic asphalt cement and to the deck with nails.
- Place the rear base flashing over the cricket and the back of the chimney.
- Counterflashing:
- Set metal counterflashing into the brickwork by raking out the mortar joint and inserting the bent edge of the flashing.
- Refill the joint with portland cement mortar.
- Bend the counterflashing down to cover the step flashing.
Other Penetrations
- Soil Stacks and Vent Pipes:
- Apply shingles up to the vent pipe, then cut a hole in a shingle to go over the pipe, setting the shingle in asphalt plastic roofing cement.
- Place a preformed flashing flange (or boot) over the pipe and shingle, setting it in asphalt plastic cement.
- Resume shingle application, cutting shingles to fit around the pipe and embedding them in asphalt plastic roofing cement.
Proper flashing installation is essential for a durable and leak-free roof. By paying close attention to the details of flashing application at parapets, dormers, vertical walls, chimneys, and other penetrations, you can ensure that your roof provides reliable protection against the elements for years to come. Always consult with roofing professionals and follow local building codes to achieve the best results.
