You probably agree that a leaky roof is a bad thing. Good thing we have flashing to keep that from happening.
Metal roof flashing is both a method and a device for keeping water from entering a structure at a roof penetration or joint. Flashing can be made of several kinds of materials, but the most flexible and durable are made of metal.
Flashing is your last, best line of defense against the enemy of interiors, water.
So, what is flashing?
Let’s take a closer look at flashing.
Flashing forms the intersections and terminations of roofing systems and surfaces. As mentioned earlier, it is designed to keep water from penetrating those areas where there is a discontinuity of the roof.
It typically comes as thin pieces of an impervious material, metal, in this case, installed at strategic junctures of a roofing system to create a more weather resistant envelope.
Flashing is typically found around the base of chimneys and skylights, places where you put a hole in the roof on purpose to install a feature. It can be difficult to seal the entire hole up, and the intersections tend to expand and contract when the temperature and humidity changes. Flashing keeps things sealed while allowing for this movement.
There are three ways flashing keeps water out of roofing:
- Surface tension
- Wind pressure
Flashing can be installed like shingles, with one piece lapping another, or it can be sealed to function as one continuous surface. Either way, flashing can also be reconfigured with a non-continuous profile to prevent water from entering via surface tension.
Beyond chimneys, skylights, and vents, flashing is also installed in:
- Roof valleys
- Roof to wall intersections
Flashing is vital!
Historically, humans have tried a number of ways to keep water from entering their abodes and to keep air movement in check. After all, a drafty structure isn’t very energy efficient.
We have tried angling the roof shingles away from the joint, placing the chimneys at the roof ridge, and even building steps into the sides of the chimney to help throw off water.
It's safe to assume these methods were not very successful, so we came up with flashing instead. Chimneys, vent pipes, walls abutting roofs, and window and door openings all benefited from having flashing installed.
One final point: if water can’t get in, then mold growth will be inhibited as well. Flashing is another piece of the puzzle for mold inhibition and structural durability. If the flashing becomes damaged, a leak will be the result.
If it’s installed incorrectly, flashing can actually direct water into a structure instead of away from it.
Flashing can be made of several different types of materials, from plastic and rubberized asphalt to a variety of metals. It can be exposed or concealed. Metal flashing is typically exposed but can be installed under the shingles or outer covering.
Flashing can come as roll roofing or as a membrane; each has its own best use. Membranes are one piece and can simplify installation around roof projections.
Metals typically used for flashing include:
- Lead-coated copper
- Malleable metals such as aluminum, zinc, and stainless steel
Plastic and other softer materials can be used but must be able to withstand direct sunlight and weathering. Plumbing vents and air ducts often come wrapped in stainless steel or other flashing material.
Types of flashing are usually named for the location of use or the shape it comes in.
Roof flashing is used around projections or discontinuities of, what else? The roof. Its main job is to deflect water away from the seams or joints and in roof valleys where water runoff is concentrated. Roof penetration flashing is for water-proofing places where pipes, supports, cables, and other roof protrusions are built.
Channel flashing is specialized roof flashing shaped like a “U” or a channel to catch water at the edges of a tile roof where it meets the wall. A drip edge performs a similar purpose. Another design for roof edges is called kick-out flashing. It is installed at the bottom of a roof and wall intersection, with the lowermost portion formed to deflect water away from the wall.
Valley flashing is to protect the valley where two roof planes intersect, rather like channel flashing.
Wall flashing serves the same purpose as roof flashing and can be directly embedded into a wall to direct any water that gets in right back outside. It’s used at windows, areas of structural support, and other interruptions in the wall. It operates by directing water toward weep holes.
Through-wall flashing spans the thickness of the wall. A specialized type of wall flashing is sill flashing or a sill pan; it is concealed under windows or door thresholds to prevent moisture leakage. Cap flashing, also known as a drip cap is installed above the windows and doors.
Pipe flashing is also called a pipe boot or vent boot. It’s pretty self-explanatory where this is used. Chimney flashing is another specialized flashing material.
Step flashing is also known as soaker or base flashing. This is a method of installation where pieces of flashing overlap each other in “steps.” Kick-out flashing uses this technique.
Counter flashing (cap flashing) covers base or step flashing.
Design and installation
When you select flashing material, keep in mind what the flashing will be above or touching in case there is chemical reactivity with adjoining materials.
This is called galvanic corrosion, which causes premature failure of the flashing and roof. For instance, do not place copper and lead adjacent to each other, although you can use lead coated copper without the corrosion issue.
Some flashing or roof materials can react and cause stains.
Flashing should be provided with expansion joints on long runs so it won’t deform during expansion and contraction of the roof or walls.
Installing flashing using traditional materials and methods have built some of the longest lasting structures erected. However, this requires experience and is a time-consuming process.
Newer membrane materials and modern sealants can complement traditional techniques and streamline the job.
Some areas need special consideration:
- Open valleys where shingles or slate must overlap the flashing by at least six inches
- Equal slopes with unequal water flow require 1 ½ inch baffles to prevent higher velocity water from forcing its was past the opposite edge of valley flashing
- Closed valleys where the intersection roof must have the same slope so shingle-butts lines up at the intersection
- Unequal slopes also require a baffle
Common failure modes
The environment plays the main role in flashing and roof failure. Exposure to salt air and acid rain can cause corrosion while excessive heat, heavy snows, and scouring winds can physically change the shape of the flashing.
In coastal areas, stainless steel is preferred because it is so resistant to salt corrosion.
Any time there is an interruption in the building envelope you invite water entry. Water and mold damage is the most commonly seen problem in any structure.
Select the right type of flashing, install it correctly, and the roof should last for ages.