April 1995 Professional Roofing
Apron flashings used for repair, prevention
Apron flashings are an inexpensive solution to abrasion holes
By: Matt Millen
A common problem with older slate and tile roof systems is holes in the copper valleys or apron flashings immediately below where the water drips off of the slates or tiles (see reference 1). One way to repair abrasion holes is to install an apron flashing over the worn metal (see figure 1). This additional apron flashing also is known as a renewable overcloak or sacrificial apron.
Holes in valleys or apron flashings typically are the result of acidic precipitation. Rainwater picks up acid from mosses or lichens growing on the slates or tiles or from the atmosphere. Heavy condensation and dew drops are even more heavily charged with acid from air pollution that settles on the roof.
In addition, various lumber species, including oak, cedar and redwood, as well as asphalts, especially modified bitumens, can reduce the pH level of rainwater to the level where it will corrode copper and lead. (The acidity or alkalinity of a solution, or its pH, is rated on a scale of one to 14; a score between on and six is considered acidic.) The mechanical wear of constant dripping water also can be a factor in metal degradation.
Rainwater and condensation drops can be acidic enough to dissolve the copper oxides, copper sulfates and lead oxides that form the familiar brown, green and gray-white patinas. The persistent attack on the patina and reforming of the oxides continually thins the metal, and holes eventually develop (see reference 2). In some cases, holes can develop in 10 years; more typically, holes develop after 60 to 70 years on roof systems with expected service lives of 100 to 150 years.
When inspecting valleys and exposed flashings, the focus should be on the drip lines where the patina is worn away or where it never developed. Finger pressure will reveal how much metal is left. If the metal indents under moderate pressure, more than one-half of the metal probably is worn away. Moderate finger pressure will punch through metal that is foil-thin.
If the problems are worn areas or holes in the metal and the roof otherwise is functioning well as a watershedding system, an apron flashing can be inserted along the drip points to accept the wear (see reference 3) (see Figure 2). This method also can be used where the budget and/or remaining service life of the other system components do not warrant replacing the metal.
Installing an apron flashing can be less expensive than other repair methods. For example, replacing a copper valley (see “Repair, maintenance are keys to protecting an owner’s investment,” March issue, page R4) costs about $100 per linear foot (300 mm).
Following are the steps for repairing an old copper valley:
Installing more than one apron flashing at single location can be used as more than just a repair method.
For example, when new valleys are installed where acid and abrasion wear is expected, an additional apron flashing can be part of the new valley. If used, it should be secured to the outer edge of the new valley. This method also can be adapted to saddles, gutters, lower copper roofs and other areas where acidic rainwater drips and wears out copper, lead-coated copper and lead work. In addition, it can be used with any metal, though holes typically appear only with copper.
No matter what its use, the installation of multiple apron flashings is an easy and cost-effective solution to abrasion hole problems.
1. Install, Donald W., The Care of Old Buildings Today, Whitney Library of Design, 1972.
2. APT Bulletin: The Journal of Preservation Technology, Vol. 23, No. 4, 1991, Historic Structures in Contemporary Atmospheres, Association for Preservation Technology International.
3. Ashurst, John, Nicola Ashurst, Geoff Wallis and Dennis Toner, Practical Building Conservation, English Heritage Handbook, Vol., 4, 1988.