Restoration Guide: Roof Flashing
Editor's Note: This is article 4 of 13 in the Roofs Chapter of the Old House Web's Home Restoration Guide. This guide was developed and edited for old homes from original material in the U.S. Housing and Urban Development (HUD) Rehab Guide.
Section 1--Flashing: The Basics
Flashing don't get no respect. Long the butt of jokes about dirty old chimneys wearing nothing but overcoats and flashing, this system deserves loving attention as your roof's last line of defense against water intrusion.
Old house or new, flashing is used in many places:
- As the buffer between your roof and walls or chimneys
- As protection around holes in the roof, such as skylights, vents and exhausts
- As the connector in the valley where sections of roof meet
- As protection along the eaves and rakes (sloping edge of the roof)
And flashing must protect against devious water, which can penetrate your roof in three ways:
- Surface tension
- Wind pressure
Flashing takes abuse and must be durable. It must be resistant to the elements (including salt and acid), and it must be flexible and compatible with adjacent materials. And unless you like working on the roof or undertaking major home renovation, you want it to be low maintenance.
Flashing, when properly installed, is one of the most durable elements in construction. But even with modern membranes and sealants, it can fail.
The most common causes of failure include:
- Exposure to salt air
- Excessive heat
- Acid rain
- Excessive snow and ice
- Extreme wind
A small area of damaged flashing can often be fixed by simply reinstalling, or patched with like material. But if the damage is extensive, as might happen with a long-neglected old house or faulty installation, the roofing really needs to be removed and new flashing installed--a major home renovation.
Section 2--Flashing: Materials and Methods
There are two categories of flashing:
- Membrane, which includes ice and water barriers and roll roofing
- Sheet metal, typically aluminum, copper, lead-coated copper, lead, stainless steel, galvanized steel, zinc and Galvalume
Here is a rundown of those materials.
2.1: Copper and Lead-Coated Copper
Copper flashing can be used around chimneys; in valleys on tile, wood shake, or slate; as step flashing on roof-to-wall intersections; and along ridges and hips.
Durability: Copper flashing is a superstar of the sheet-metal flashings. It is extremely durable and resistant to corrosion, and it is pliant and easy to work with. It can be fabricated and soldered on the job to create rigid, continuous pieces, and it is available in standard 5"x7" step flashing.
Price: Although copper flashing is very expensive, some roofers argue that its extended durability makes up for the added expense.
Special notes: Copper is particularly effective with masonry because, unlike most other sheet-metal flashings, it does not react with mortar. The Copper Development Association recommends that cants be used under cedar shingles to prevent reaction between acid in the cedar and the copper. It also says the copper will not have problems in acid-rain areas if water is not allowed to stand.
Durability: Aluminum flashing is durable and easy to work with. Preformed aluminum drip edges make eave and rake flashing simple.
Price: It falls in the mid-price range.
Special notes: Aluminum flashing should be nailed with aluminum nails to avoid possible corrosive galvanic interaction between dissimilar metals. Aluminum does not have copper's benefit of easy soldering, and it should not come in contact with alkaline concrete, mortar or other cement materials.
2.3: Galvanized Steel
Durability: Galvanized steel is one of the less durable flashing materials. In harsh environments, it may start corroding within 15 years, and it is not considered cost-effective when used with long-lasting roofs such as slate or tile.
Price: Galvanized-steel flashing is the cheapest of the metal flashings, though it also is the least durable.
Special notes: The zinc coating on galvanized steel slows corrosion. It is fairly rigid and is preformed to fit many applications, making it quick and easy to use. Galvanized nails should be used to avoid galvanic action. It cannot be soldered, and joints are made by crimping and/or the use of high-quality, exterior sealants. It should not be in contact with green wood (moisture) or treated wood because of corrosion.
2.4: Galvalume Sheet Metal Flashing
Durability: Similar to galvanized steel, Galvalume flashing, developed by Bethlehem Steel, is hot-dipped into an alloy of aluminum (55 percent) and zinc (45 percent). It is a rigid material that lasts about twice as long as galvanized steel but at a similar cost.
Price: Galvalume is similar in price to galvanized steel and falls on the low end of the cost spectrum.
Special notes: As with aluminum flashing, it should not be in contact with concrete, masonry, or cement-board siding because of corrosion. Galvalume should also be kept out of contact with green lumber, treated lumber, copper or lead. It comes in rolls only, but can be shaped in a shop (because of its stiffness, field shaping is not practical).
2.5: Stainless Steel
Durability : Stainless steel is highly durable and is extremely resistant to corrosion.
Price: It costs about the same as copper, though it's more expensive to install.
Special notes: It comes in rolls and sheets, but is not preformed. Because of its stiffness, shaping at the work site is not practical and would need to be done at a shop. It can be used against concrete and masonry. Stainless-steel nails are recommended.
Durability: Rheinzink is similar to copper in many ways, including durability. Some manufacturers suggest Rheinzink can last up to 100 years if well-maintained.
Price: Rheinzink is similar to copper in price, making it one of the more expensive flashing materials.
Special notes: Rheinzink flashing is high-grade zinc with minute amounts of copper and titanium. It does not leach chemicals and stain. Though similar to copper in handling, it is much more finicky, needing underside ventilation. It should not touch cement/mortar, wood preservatives, and copper.
Durability: Lead flashing is both durable and pliable, and it is useful where unusual shapes are needed. It is inert and pretty much unaffected by salt and acid. Because it is so soft, it can tear and fatigue.
Price: The price of lead is on the rise as natural supplies of lead decrease.
Special notes: An old standby, lead flashing has fallen out of favor because of fears of lead poisoning. If you are using lead flashing, make sure to follow safety protocols to avoid ingesting lead fumes or dust.
2.8: Roll Roofing
Durability: Roll roofing has about the same lifetime as asphalt shingle and needs to be replaced with the roof, or about every 15 to 25 years.
Price: Roll roofing is both economical and easy to install.
Special notes: Roll roofing should only be used in the valleys of your roof where two sections meet. Caution is needed, as walking on a valley made with roll roofing can cause tears.
2.9: Ice and Water Barrier Membrane
Durability: These materials are a plastic and asphalt combination that goes down as a self-adhering sheet above the eaves to guard against ice dams and the water backup at the eaves. Ice and water barriers are easy to install and are most commonly used in conjunction with other materials as an additional line of defense rather than a stand-alone flashing material. Heat and ultraviolet light may cause damage to your barriers.
Price: The membrane is a relatively cheap protection against ice dams and is a must in cold climates.
Special notes: Some roofers now are recommending it to cover the entire roof deck, though it is meant to be applied above the eaves to at least 36" inside the exterior wall. Ventilation is an issue if the membrane is used for total coverage, as the membrane acts as a vapor barrier and can increase moisture buildup in the attic.
Learn more about protecting the roof of your old house from moisture in chapter 5, underlayments and moisture barriers.
Jim Mallery, a semi-retired journalist and onetime registered contractor, has extensive experience remodeling, repairing, and rebuilding homes.