The primary function of roof flashing is to shed or direct water away from roof edges and penetrations to prevent infiltration into the building. Where visible, it frequently is also an aesthetic component of the building. Flashing can be made from a variety of materials including both metallic and non-metallic ones. This chapter addresses only metal flashings, and the best practices to assure they remain secure and prevent water infiltration. The guidelines in this chapter are general in nature, and do not apply to every application.
All definitions in this chapter are referenced from the National Roofing Contractors Association (NRCA) Technical Glossary (www.nrca.net/Technical/Search), unless otherwise noted.
Cleat: a continuous metal strip, or angled piece, used to secure metal components
Clip: a noncontinuous metal component, or angled piece, used to secure two or more metal components together
Coping: the covering piece on top of a parapet wall exposed to the weather, commonly made of metal, and sloped to carry off water
Counter flashing: formed metal secured on or into a wall, curb, pipe, rooftop unit or other surface, to cover and protect the upper edge of a base flashing and its associated fasteners; (adapted from NRCA glossary)
Drip edge: a metal flashing or other overhanging component with an outward projecting lower edge, intended to control the direction of dripping water and help protect underlying building components
Drip—Kick out: the bottom portion of a vertical flashing component that angles away from the building; it prevents capillary action behind the flashing and frequently is hemmed to engage on a cleat or clip; (not included in NRCA glossary)
Drip—Hook: also called a “blind drip,” it is the bottom portion of a vertical flashing that angles back up to the building; it prevents capillary action behind the flashing and frequently engages on a cleat or clip; (not included in NRCA glossary)
Fascia: the vertical or steeply sloped roof or trim located at the perimeter of a roof; (adapted from NRCA glossary)
Fastener: any of a wide variety of mechanical securement devices, including nails, screws, bolts, and rivets; (adapted from NRCA glossary)
Gravel stop: a flanged device, commonly metal, designed to prevent loose aggregate from washing off the roof; (adapted from NRCA glossary)
Parapet wall: the part of a vertical wall that extends above the roof line; (adapted from NRCA glossary)
Reglet: a cut into a wall, typically masonry, or a surface-mounted sheet metal receiver into which flashing or counter flashing is inserted; (not included in NRCA glossary)
Roof flange: the horizontal or sloped section of flashing that is attached to the roof deck; (not included in NRCA glossary)
Valley flashing: a lining placed where two planes of a roof intersect at a downward or “V” angle; (not included in NRCA glossary)
3.3 Metal Flashing Dimensions
Flashings are required to be fabricated to dimensions specified by local building codes, and/or the roofing material manufacturers.
3.4 Metal Flashing Material Thickness
Metal flashings are required to be fabricated to thicknesses and dimensions specified by local building codes and/or the roof material manufacturers. The thickness of metal flashing is referenced by gauge, weight, inches, or millimeters depending on the material. Table 3.4-1 provides approximate equivalent thicknesses for commonly used flashing materials.
Table 3.4-1: Metal Thickness
|Thickness Approximate (inches)||Steel (gauge)||Copper (ounces/SF)||Aluminum (inches)||Zinc Alloy (mm)||Stainless Stl (gauge)||Lead (pounds/SF)|
3.5 Thermal Movement
Building materials expand when heated and contract when cooled, and metals tend to expand and contract more than other materials. When metal flashings move due to thermal expansion and contraction they can loosen or damage fasteners, disengage from cleats and clips, and allow water infiltration at joints. Therefore, it is important that flashings are designed and installed to accommodate thermal movement.
Table 3.5-1 shows the expansion expected in a 10-foot section of common building materials, due to a 100°F temperature change. Note that metal flashings expand much more than wood, masonry, and other substrate materials to which they are attached. Fabricating and installing flashing per the following guidelines is recommended to accommodate thermal movement.
- When attaching metal flashing or cleats to the substrate with fasteners, the fastener clearance holes should be oversized or slotted to enable the cleat to move without causing the fastener to move or become damaged.
- Flashing drips secured on cleats or clips should be able to slide and not be tightly crimped or fastened to the cleat or
- Joints should be lapped to allow the joining sections to move, or have gap wide enough to allow the expected movement with a splice plate to prevent water infiltration.
Table 3.5-1: Linear Expansion of Building Materials
|Increase in inches of a 10 foot length due to an increase of 100°F|
|Building Material||Coefficient of Thermal Expansion||1/64||1/32||3/64||1/16||5/64||3/32||7/64||1/8||9/64||5/32||11/64||3/16||13/64||7/32|
3.6 Galvanic Compatibility
When two dissimilar metals (e.g., copper and aluminum) come in contact in an electrolyte solution (e.g. salt water), galvanic corrosion will occur. Galvanic corrosion is an electrochemical process in which ions of one metal (the anode) migrate to another metal (the cathode), which results in corrosion of the anode. A galvanic series classifies and orders a group of metals based on their likelihood of losing ions in the process (see Table 3.6-1). The likelihood of galvanic corrosion occurring between two metals increases the further apart the metals are on the galvanic series (i.e., more than two spaces apart). Corrosion also increases with more conductive electrolytes. Therefore, greater attention to this matter is needed near coastal areas or in facilities that store or manufacture potentially corrosive chemicals.
Adhering to the following guidelines will minimize the possibility of galvanic corrosion occurring:
- Assure all metals in contact with metal flashing, including cleats, fasteners, and substrate components, are the same or are as close together as possible on the galvanic series. If two dissimilar metals are in the presence of an electrolyte, or they are more than two spaces apart on the galvanic series in Table 3.6-1, accelerated galvanic corrosion will be a concern.
- When dissimilar metals are used, separate them with an electric isolator such as a rubber membrane or bituminous paint.
- Many pressure-treated wood solutions contain copper, and other additives, and should be treated as if it were copper; therefore, dissimilar metal flashings, cleats, and fasteners should be compatible with, or separated from, treated woods.
- Treated and some natural woods can contain chemicals or acid that can break down some coatings and corrode materials. Refer to the manufacturer of the treated or natural lumber for additional installation recommendations.
Table 3.6-1: Galvanic Series
|More Protected (Nobel)|
|304 Stainless Steel (passive)*|
|304 Stainless Steel (active)*|
|Zinc & Galvanized Steel|
*Stainless steel that is exposed to air and free from high concentrations of chlorides, as is most common in flashings, is typically passive.
Fasteners are a critical component of metal flashing as they are used to secure cleats, clips, and/or the flashing to the building substrate, or to connect separate components of a flashing system.
Fastener type, size, and spacing should be as required by local building codes and/or the flashing manufacturer or designer. The following guidelines are considered industry best practices:
- Required fastener embedment in wood, or engagement in metal, varies depending upon the fastener and the material to which it is secured. The designer should consult the fastener manufacturer’s specification data.
- Fastener heads should be sufficiently large, or have washers, to prevent withdrawal. The required fastener head or washer size will vary based on the material being secured and the size of any clearance hole or slot in that material. The designer should consult the fastener manufacturer’s published data.
- Fastener spacing can vary significantly based on the application, fastener type, expected loads, and substrate.
- Fasteners should be galvanically compatible with the materials with which they will be in contact. See Section 3.6.
3.8 Cleats and Clips
Cleats and clips are typically used to secure exposed componets of a flashing without the use of exposed fasteners. Cleats and clips typically engage in a hem, drip kick out, or drip hook, to secure the flashing and provide greater wind resistance. The use of cleats and clips should be as required by local building codes, the flashing manufacturer, the roof system manufacturer, or industry standard guidelines. Resources such as the NRCA Roofing Manual or the SMACNA Architectural Sheet Metal Manual can provide additional details for specific applications. The following guidelines are best practices regarding cleats and clips:
- Exposed flashing faces that are 4 inches or greater, and are not secured with a face fastener, should be secured with a cleat, or appropriately spaced clips.
- Cleats should be one gauge heavier than the flashing they are securing. See Section 3.4.
- Cleats should be positioned such that flashing hem or drip fully engages on the cleat.
- To accommodate thermal movement, cleat fasteners should be installed through oversized or slotted clearance holes, and flashing should not be fastened to, or tightly crimped on, the cleat. See Section 3.5.
- Cleats and clips should be galvanically compatible with the materials with which they will be in contact. See Section 3.6.
3.9 Joints, Seams, and Splices
Joints occur wherever two sections of flashing or flashing components meet. When that flashing component is intended to prevent water infiltration, the joint must be seamed. Seams are typically either lapped with one section of the flashing overlapping the next, or a separate splice plate is used either under or over the joint. Flashing, such as coping, are sometimes seamed with a standing seam, “S” lock, or drive cleat.
Joints and seams should be as required by local codes and the flashing manufacturer. The following guidelines are considered industry best practices:
- When a lap seam is used, the overlap should be a minimum of 2 inches for vertical faces, and a minimum of 6 inches for conditions where water must be shed such as valley flashing.
- Cleats, and sections of flashing utilizing a splice plate, should have a joint gap of 1/8–1/4 inches wide between sections to allow for thermal movement. See Section 3.5.
- Seams should be sealed with a flexible or non-curing sealant that allows for movement and prevents water infiltration, or the seam should be fabricated to otherwise allow for movement and control or direct water to prevent infiltration into the building, e.g., a coping splice plate with channels.
3.10 Specific Flashing Conditions
3.10.1 Copings—Parapet Walls
The tops of parapet walls need to be covered, or coped, in order to prevent water infiltration. Metal flashing, called coping, is one of the most common methods for capping the tops of parapet walls. Metal copings should meet the required dimensions (Section 3.3), and be well secured using cleats or clips per Section 3.8, and/or fasteners per Section 3.7. Refer to design requirements for specific metals.
The top of the coping should be sloped (typically toward the roof) to drain water, and because they have a large area exposed to rain and snow, special attention needs to be given to joints to assure they prevent or control water infiltration. It is also a good practice to install an underlayment material under a metal coping to prevent condensation or leaking water from infiltrating the parapet wall construction. Figure 3.10.1-1 shows some common coping details.
Figure 3.10.1-1a. Example of a continuous front cleat with exposed rear fasteners.
Figure 3.10.1-1b. Example of a continuous front and rear cleats with hem/drip crimped on cleat.
Figure 3.10.1-1c. Example of a snap-on coping with intermittent chairs (cleats/clips).
Copings should be manufactured and installed as required by local building code, industry standard recommendations, and/or the coping manufacturer. International Building Code (IBC) specifies the following related to coping:
- Section 1503.3 requires that all parapet walls be coped with a noncombustible weatherproof material of a width no less than the thickness of the parapet wall.
3.10.2 Fascia/Drip Edge—Exterior Perimeter of Roof
At the perimeter of most roofs, the horizontal or sloped roof plane is transitioned to a lower vertical or nearly vertical plane. At this transition a flashing (fascia, drip edge, or gravel stop) is used to terminate the roof, and prevent water infiltration. These flashings typically direct water away from the roof edge either into a gutter, onto a water-impermeable surface below, or to the ground. The roof flange of these flashings should meet the guidelines of the dimensions in Section 3.3, and be well secured using cleats or clips per Section 3.8, and/or fasteners per Section 3.7. Figure 3.10.2-12 shows some common fascia details.
Figure 3.10.2-1a. Example of an un-cleated drip edge into a gutter.
Figure 3.10.2-1b. Example of a cleated drip edge at a metal roof.
Figure 3.10.2-1c. Example of a cleated gravel stop at a low-slope roof edge.
Figure 3.10.2-1d. Example of a snap-on fascia at a low-slope roof edge.
Fascia, drip edge, gravel stop, and similar metal flashings should be manufactured and installed as required by local building code, industry standard recommendations, and/or the flashing manufacturer.
3.10.3 Counterflashing—Interior Roof to Higher Wall and Roof Penetrations
Counterflashing is used to prevent water infiltration where the horizontal or sloped roof plane is transitioned to a higher vertical or nearly vertical plane, such as at parapet or interior wall. Counterflashing is also used around penetrations, which extend above the plane of the roof, such as chimneys and vent stacks. Counterflashing is typically the top component of a two-piece flashing assembly, as it is intended to flash over a base flashing, which could be another metal flashing, a flexible flashing, or a roofing membrane. The base flashing should be secured to the wall or penetration independently of the counterflashing. The counterflashing should be attached to the wall or penetration in a manner that keeps it secure and prevents water intrusion. Prevention of water infiltration is typically accomplished by use of sealants, solder, or gaskets, and/or by installing part of the counterflashing in or into the wall, and/or by installing part of the counterflashing under the wall cladding. Figure 3.10.3-1 shows some common wall counter flashing details.
Figure 3.10.3-1a. Examples of counterflashing details.
Figure 3.10.3-1b. Example of counterflashing at a chimney roof penetration.
Counter flashings should be manufactured and installed as required by local building code, industry standard recommendations, and/or the flashing manufacturer.
3.10.4 Valley Flashing—Low Side Intersection of Sloped Roofs
Valley flashing is used to prevent water infiltration where two sloping planes of a roof intersect at a downward or “V-shaped” angle creating a “valley.” Metal valley flashing may be used with any roof material, but is most commonly used with more rigid roofing materials, such as metal, tile, slate, and wood shakes. Valley flashings should meet the guidelines of the dimensions in Section 3.3, and be well secured using cleats or clips per Section 3.8, and/or fasteners per Section 3.7. Note that some valleys may contain one or more raised ribs to control the flow of runoff within the valley, and to keep it from flowing onto adjacent roof surfaces. Figure 3.10.4-1 shows some common metal valley flashing details.
Figure 3.10.4-1a. Common metal valley flashing details for tile roofs.
Figure 3.10.4-1b. Common metal valley flashing details for standing seam roofs.
Figure 3.10.4-1c. Common metal valley flashing details for asphalt shingle roofs.
Valley flashings should be manufactured and installed as required by local building code, industry standard recommendations, and/or the flashing manufacturer.
NOTE: The information stated in this chapter is not intended to supersede local building codes; check with the authority having jurisdiction. Specification and installation shall be in accordance with local building codes and the manufacturer’s instructions. Refer to roofing material specialists for project specific design details, as some details may not be required for some applications.