Are metal copings effective in protecting the tops of masonry walls? How do you prevent them from leaking at splice joints? If they are used, how far should the vertical outside edge of a metal coping extend down over the top of brick masonry walls? Should sealant be placed between the drip of the metal coping and the brick wall to prevent wind-driven rain from being driven up behind the overlap?
I have seen many different details for metal copings, most of which have problems. I am curious to hear your thoughts.
If not properly designed and detailed, sheet metal coping can be the source of water leakage and often contribute to efflorescence problems and freeze-thaw deterioration near the tops of masonry, especially at parapets. In many cases, the cause of these problems is the failure to install coping in a manner that prevents water from bypassing the coping and saturating the top of the masonry walls below.
In colder climates, snow and ice collect on the copings. If the copings are not designed properly, melting snow and ice during warmer winter days flows into the masonry walls and refreezes, causing freeze-thaw problems over time.
There are many factors to consider when designing a sheet metal coping on top of a masonry wall.
First, most leakage problems associated with metal copings occur at splice joints. For this reason, many architects recommend using a continuous flashing membrane beneath the metal coping. This membrane covers the top of the masonry walls and roof base flashing so any water leaking at splice joints in the metal coping is prevented from reaching the top of the masonry wall below. This flashing or roofing membrane can be fairly thin because it is protected from the elements by the metal coping. Splice joints in the flashing, however, must be sealed watertight.
When a flashing membrane is used beneath the metal coping, the critical overlap is that of the membrane beneath the coping. If the plastic membrane beneath the coping is loose laid, it must overlap the top of the masonry wall sufficiently so wind-driven rain cannot be blown up behind it. The amount of overlap is less critical in cases where the plastic flashing beneath the coping is fully adhered to the masonry.
There are many recommendations on the subject published by different trade associations. The Brick Industry Association (BIA) in Technical Note 36A, “Brick Masonry Details, Caps, and Copings, Corbeling, and Racking,” states on page 6, “Metal caps and copings require an extension down the face of the wall, 4-inches (100 mm) minimum, and a sealant between the metal and wall to prevent wind uplift and water penetration.” This technical note also shows an approximate 4 ½-inch extension in Figure 2, and a smaller extension about one brick high, or 2 1/4 inches, in Figures 5 and 6. Sealant is shown between the metal and the masonry behind in all three figures.
The Sheet Metal and Air Conditioning National Contractors Association (SMACNA) manual, 4th edition, shows a minimum 1-inch overlap in Plate 155 for poured metal coping, and a 2-inch overlap in Plate 91. Both Figures A and B show single ply roofing caps.
In the National Roofing Contractors Association (NRCA) “Roofing and Waterproofing Manual,” 5th edition, the protection below the blocking is shown to be a1-inch minimum for the metal edge flashing on the flat roof, which is a similar condition to that of the metal coping. Neither NRCA or SMACNA show any sealant between the metal and the wall.
There are obviously several variations in these details and the lack of consistent recommendations is confusing. I typically recommend extending the metal coping and the flashing membrane beneath it down 2-inches below the top of the masonry wall (approximately one brick course with modular units) without installing any sealant. Placing sealant between the metal coping and the brick masonry prevents water reaching the flexible membrane beneath the coping from exiting the wall. The 2-inch overlap is sufficient to prevent water from being driven up and under the metal coping in most conditions.
Providing a drip on the edge of the metal coping is also important to prevent water from being driven up underneath this vertical leg. Smaller legs can be effective in many cases; however, they provide less of a margin of error in the case of construction tolerance variations.