Flashings and Copings

Introduction

Most modern construction materials are fairly resistant to moisture penetration. However, many joints between masonry units, panels, or architectural features are not. The effects of natural movement due to settlement, expansion, and contraction tend to compound the problems and may eventually lead to leaks. Flashing is used to prevent moisture from entering at such locations. It is also used to divert to the exterior moisture that has already entered various components of a structure.


HOMETIME, the nationally-syndicated DIY program on PBS, recently aired an episode demonstrating how to install a timber shed roof and explained the importance of using copper flashing. Host Dean Johnson goes through the steps of why and how copper flashing is laid down before installing the roof tiles. The segment also addresses copper’s high resistance to corrosion and moisture, as well as its use to extend the life of a roofing system

Moisture that penetrates into a building may cause serious damage to its interior. In freezing temperatures, it can also cause severe damage to the exterior of the building. Cracking, spalling, and disintegration can result. Over a long period of time, moisture can also weaken structural elements.

Copper is an excellent material for flashing because of its malleability, strength, and high resistance to the caustic effects of mortars and hostile environments. Flashing, in general, is expensive to replace if it fails. The long life copper flashing offers, is a major asset in this application.

General Requirements

  • Support Requirements:
    Copper wall flashing in new masonry is typically held in place by setting the upper edge into the mortar joint between the courses in the backing material. In a retrofit condition, the upper edge of the copper flashing can usually be inserted into a reglet or an existing joint that has been raked. If the backing is not masonry it must be a nailable material.

    Wood blocking must be provided for the attachment and support of copper copings, gravel stops, edge strips, roof sumps, scuppers, and other copper roof accessories. These are shown in the accompanying details.
  • Materials:
    Flashings and copings are fabricated from cold rolled copper in weights ranging between 16 and 20 ounces per square foot. The required weight depends on the application and is outlined in the discussion related to each detail.

    Wherever sealant is used that comes into contact with copper a rubber or synthetic-base sealant that is compatible with copper must be specified.
  • Description:
    The wide variety of uses for copper as a flashing material makes it impossible to discuss installations for all specific conditions. However, the principles behind good applications can be summarized.

    Copper movement must always be anticipated. It should be accommodated with proper details, but in circumstances where movement cannot be tolerated, it should be limited. The latter condition is often encountered with edge strips, gravel stops, and continuous cleats.

    The flow of water must be planned for and not impeded. Flashing should, typically, have at least an 8" vertical drop from its upper to its lower edge. The ends of discontinuous flashings, such as at window sills, must be dammed to prevent moisture from flowing into the wall cavity. Pea gravel should be used on throughwall flashing to help prevent construction debris from blocking flow. Where a sealant is used in conjunction with flashing, as at a shelf angle for example, it should be installed below the flashing so that moisture diverted by the flashing will not be trapped by the sealant.

    Contact between copper and other metals should be avoided. Where this is not possible, such as at a steel shelf angle, some method of material separation is required. This condition is most often solved with the use of a bituminous coating applied to the metals to prevent direct contact.

The details shown in this section illustrate these and other important points.