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.

Common Problems

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:

  1. The roofing professional should inspect the valley to confirm that the only problem is abrasion or wear holes and that an apron flashing is an adequate solution.
  2. Next, an apron flashing about 4 inches (100 mm) wide should be fabricated.  Typically, workable lengths of 4 to 8 feet (1.2 to 2.4 m) are used, depending on job conditions.  In addition, 16 or 20 ounce (4.87 or 6.09-kg/m2) copper usually is used.  A hem on the lower edge stiffens the metal and protects the installer’s hands.  The metal should be crimped about 2 inches (50 mm) from the top edge so that it is held in place with three-point compression between the slate or tile shingles and the old copper.
  3. The space between the slates or tiles and the hidden valley metal should be cleaned because there may be roofing cement holding some of the shingles to the metal.
  4. All holes and very thin spots should be patched.  The standard soldered patch can be used, though preparing the old copper for soldering may be difficult where it is too thin (i.e., more than half of the metal is gone) or too close to the shingle edges.
  5. Sometimes a copper patch can be pop-riveted and set in sealant, similar to the construction of the metal skin on aircraft or semi-trailers.  Polyurethane sealants stick better to clean, bright copper than to a copper patina, so the contractor should clean the metal before using this repair method.  Sometimes, thin metal or pinholes are covered only with a protective coating, such as a polyurethane sealant.
  6. The Contractor should snap a chalk line on the valley metal to keep the new apron flashing straight.
  7. About 2 inches (50 mm) of the apron flashing should be inserted under the slates or tiles.  The roofing professional should insert one corner under the slates or tiles, then slide it to its proper location while working the back end of the apron flashing into place.  This method is easier than trying to insert the entire edge of the apron flashing under the slates or tiles at one time.
  8. All side laps should be riveted; side laps of 3 or 4 inches (75 to 100 mm) are adequate. If the old metal overhangs at the base, the apron flashing should be riveted to the valley’s base.  At the ridge, the new apron flashing can be tucked under the ridge slate or tile.  A rivet, soldered over or otherwise sealed, can be used at the top of a valley where there is no accumulated water flow.  Copper hold-down cleats also can be soldered to the existing copper valley and used to fasten the new apron.
  9. In addition to the riveting or copper hold-down cleat process, dabs of sealant can be placed under the new apron every 16 to 24 inches (400 to 600 mm).  By dabbing the sealant instead of using a solid strip, water blockage can be avoided.

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.