Although various new products and installation methods constantly are being introduced to the roofing industry, one important theme has been consistent during the years—how products and components are assembled and installed are key to their performances. Edge-metal systems are particularly important to roof system performance, and it is crucial you know how to properly install edge metal at two of the most common locations in low-slope roofing: parapets and perimeter edges.
Roofing professionals know small components of a roof system can cause big problems during the system's life cycle if not properly addressed during design and installation. Although edge-metal systems are small parts of roof systems in overall proportion, they are a roof system's first line of defense against wind. If a roof edge comes off, the integrity and watertightness of the roofing and insulation materials may be compromised.
The Roofing Industry Committee on Weather Issues (RICOWI) noted in its September 2007 report about Hurricane Katrina that nearly 95 percent of roof system failures resulting from the storm were caused by poor workmanship or substituted materials. (Editor's note: NRCA does not necessarily agree with RICOWI's findings. For more information about NRCA's findings, see "Hurricane Katrina: observations from the field," December 2005 issue, page 22.)
RICOWI's Hurricane Katrina Investigation Report repeatedly cites poor attachment of perimeter edge detail as a contributing factor of damage. This could be caused by a variety of oversights, including using the wrong fastener or using too few fasteners that were improperly placed.
In some cases, RICOWI says, a nailer was not sufficiently secured to a structure to resist the wind loads that should have been anticipated according to ASCE 7-02, "Minimum Design Loads for Buildings and Other Structures."
RICOWI's report notes that, as with Hurricanes Charley and Ivan, "edge failure propagation was one of the leading causes of significant roof damage."
The International Building Code requires roof edges to adhere to wind-load performance requirements with the inclusion of ANSI/SPRI ES-1, "Wind Design Standard for Edge Systems Used with Low Slope Roofing Systems."
Edge-metal systems must be tested to meet projects' calculated load requirements and properly installed as tested to avoid failure. Because ANSI/SPRI ES-1 is required by code, it is important you understand the testing protocols and familiarize yourself with the proper installation for tested products so the installation method will be replicated in the field.
NRCA, edge-metal manufacturers, the Sheet Metal and Air Conditioning Contractors' National Association, and some architectural specifications by design professionals provide installation procedures and considerations for the various edge-metal systems used in roof system construction.
ANSI/SPRI ES-1 is made up of three test protocols: RE-1, RE-2 and RE-3. RE-1 is for roof edges used with mechanically attached and ballasted single-ply roof membranes and tests an edge-metal system's ability to hold a membrane in place. RE-2 is an outward pull test for fascia and determines the edge-metal system's ability to withstand certain wind loads. RE-3 is a test for coping that simultaneously pulls upward and outward on the cap to simulate bidirectional wind loads that can occur on coping.
RE-2 and RE-3 are performed on full-length pieces of complete edge-metal systems with fasteners included. "Full length" is defined as the shipped length fabricated by a manufacturer or contractor (minimum of 8 feet). The tests are conducted with cycled loads to simulate wind gusts.
One of the first considerations for installing an edge-metal system is the substrate to which it will be attached. Different substrates, such as wood nailers, masonry walls, lightweight insulating concrete and gypsum decks, tilt-up concrete walls, steel decks and nonpenetrable walls, require slightly different installation practices.
FM 1-49 illustrates one way to detail wood nailer cant strip assemblies and shows examples of assemblies that are not recommended, such as attaching a wood nailer to brick with short nails. In many cases, installation will be dictated by a fastener's pullout or shear resistance (if applicable) to a substrate. A fastener should be documented and tested to be the right size and type for a given substrate to withstand the calculated load forces.
The NRCA Roofing Manual: Metal Panel and SPF Roof Systems—2008 charts common sheet-metal fasteners differentiated by substrate. If you use a premanufactured edge-metal system, its manufacturer should recommend the appropriate fasteners to be used for attachment to a particular substrate. Ring shank nails or hex head screws are common with premanufactured edge-metal systems, and some manufacturers provide these with edge-metal systems to ensure proper fasteners are installed. Moreover, ring shank nails typically will provide increased hold-down capacities compared with traditional smooth shank nails.
Finally, chromated copper arsenate chemical compounds used as wood preservatives are being replaced with amine copper quat (ACQ) or copper azone in many pressure-treated wood products used as roof edge substrates. You need to verify the fasteners you use are compatible with the treated nailer being used. Some manufacturers and roofing contractors have simply made the decision to use only stainless-steel fasteners to eliminate possible fastener corrosion. Also, isolate all metal roof edge parts from ACQ-treated wood with the appropriate membrane material. NRCA issued a Special Report about this issue in February 2005; the report is available at www.nrca.net.
Cleat attachment is essential whether edge metal is on a parapet or perimeter edge. Cleats, either continuous or evenly spaced, anchor sheet-metal components. A cleat's gauge is critical to edge-metal system performance. Designers and installers should install what was tested. Generally, cleats should be one gauge heavier than the edge metal.
A cleat must be attached with the appropriate load-rated fastener that is compatible with the construction materials; one aspect in proper spacing of fasteners is keeping a system in place. ANSI/SPRI ES-1 testing helps ensure fastener size and spacing will withstand design loads.
Deviations of fastener size and/or location can affect the wind-load resistance to which edge-metal systems are classified. Manufacturer installation guides for perimeter edge products should reflect fastener size and location, especially if a roof system is to meet a performance rating such as an FM Global 1-90 rating.
Typical spacing patterns seen on tested systems are 6 inches on center for continuous cleats, 4 inches on center for nailed roof flanges, up to 12 inches on center for screwed cleats and 18 inches on center for fastened back legs of coping.
Cleat placement is just as important as attachment. If a cleat is not parallel to a substrate, it will not properly engage with the coping cap or fascia piece and will not provide a secure hold to keep the cover piece in place. The minimum cleat engagement should be between 3/8 of an inch and 1/2 of an inch. Visual checks should be performed to confirm cleat engagement with the drip or hooked edge of a coping or fascia cover piece.
Expansion and contraction
Failure to allow for expansion and contraction within an edge-metal system poses another potential problem with how an edge-metal system will perform. It is common knowledge metal expands and contracts with temperature changes. This expansion and contraction can be quite severe depending on the type of metal used and is compounded by the fact that a substrate typically expands and contracts at different rates.
By providing space at joints and slotted or oversized fastener holes to accommodate this movement, an edge-metal system will not distort and disengage from the cleat or loosen the fasteners. Limiting edge-metal lengths to 8 to 12 feet also helps avoid expansion and contraction problems.
Installing a 1/8-inch-wide gap between pieces (1/4 of an inch in cold weather) will allow for movement, and, in some cases, installing a fastener or fixed point will control expansion and limit the distance an edge-metal system can move in any one direction. If too much room is allowed between joints or a fixed point is not installed, sections may migrate away from one another, causing a wide joint, especially on long runs.
For joints located near corners, NRCA recommends a joint be located within 18 inches from each direction of corners measured on the interior side; this prevents a corner piece from pulling away from adjacent pieces during temperature-related expansion and contraction.
In a related recommendation regarding thermal expansion, NRCA suggests avoiding flashing details that embed or sandwich rigid metal flanges into roof membranes. In these details, metal movement may cause a membrane to split at the sheet-metal joint.
Because a membrane also can split at the joints between metal sections, fascia-cap edge metal was developed. Fascia-cap edge-metal systems are installed over either a wood cant that is typically a diagonally ripped 4x4 or a manufacturer-engineered prefabricated raised metal profile. This design accommodates different coefficients of expansion and contraction of metal and a membrane.
Pre-manufactured edge-metal system warranties require proper installation. Coping and/or fascia systems are warranted for periods that are similar to those provided for roof membranes, but a warranty is valid only when materials are designed and installed properly.
More important, proper installation is good roofing practice and helps ensure an edge-metal system will perform well on a building regardless of whether the edge metal is warranted.
Other factors that must be considered with installation are ambient moisture conditions and material degradation associated with dissimilar materials.
Building owners, architects and contractors often raise the concern of material degradation associated with dissimilar materials. Metal deterioration in commercial and residential buildings can occur if incompatible materials are used with each other, especially in buildings located in or near salt-spray environments.
The potential for incompatibility between two metals can be estimated by knowing the placement of metals in the galvanic series. Some galvanically compatible metal pairs include aluminum and galvanized steel, aluminum and stainless steel, and copper and stainless steel. Copper should not be used with steel, zinc or aluminum, and copper fasteners should be used exclusively with copper.
A factor that sometimes is overlooked occurs after a roofing crew has finished an installation. Potential risk can come from other trades when appliance attachments such as lightning protection, antennae or signs typically not installed by roofing crews are put in place. These appliances can compromise edge-metal systems' effectiveness and typically will void any wind performance warranty.
Because appliances can affect an edge-metal system's wind performance, ANSI/SPRI ES-1 states they should not be applied to a tested system. Appliances also can penetrate the water seal, induce a galvanic reaction, and prevent proper expansion and contraction of an edge-metal system.
Coordination among trades will help eliminate problems such as appliance attachment to perimeter roof edge. Until then, take steps to protect edge-metal systems by isolating them from appliances. Designers should design and coordinate the construction process so edge-metal systems' watertightness is not compromised.
Performance is not the only goal of proper edge-metal installation. The aesthetics of edge metal and how it will look over time also depend on the considerations taken with the initial installation. Using the appropriate metal thicknesses and accommodating for expansion and contraction allows for flatness and can help prevent oil canning.
As its exposed face dimension increases, metal's thickness should increase to provide adequate stiffness. This also applies to the dimension at the top of a parapet wall. If a wall width increases in dimension, it will require a thicker metal to help prevent oil canning and provide strength to the edge-metal system.
Fastener selection and installation also are critical to an edge-metal system's final appearance. Fastener heads that are too large or not fully installed can telegraph or read through the face of edge metal. On the other hand, over-torquing fasteners during installation can distort metal, prevent proper thermal movement and possibly contribute to oil canning.
Initial consideration of a metal's finish will ensure an edge-metal system ages well and avoid the need for future field painting and touch-ups. For example, mill finish aluminum will form white corrosion in the absence of oxygen. It is important to always remove the material from its packaging, keep it dry and provide adequate ventilation.
Field-painted metal must be properly prepared for long-term paint adhesion. Baked enamel finishes are less prevalent as the industry has shifted to PVDF (Kynar® or Hylar®) coatings. PVDF coatings have superior weatherability that allows for gloss and color retention with less chalking and fading.
Anodizing is another finish where an aluminum oxide film becomes part of the sheet through electrolytic process. It is important to note that with an anodized finish, shade variations from piece to piece can be noticeable. Also, when formed, hairline cracks can occur in anodized coating. Sending formed pieces through the anodizing process after they have been bent can help eliminate cracking potential. It should be noted that an edge-metal manufacturer may not warranty anodized finishes because of durability and job-site storage concerns.
Fascia extenders often are added to an edge-metal system for enhanced aesthetics and typically are used when the face dimension is larger than has been tested on the edge-metal system or greater than is recommended for the gauge used. An extender piece's profile typically has a drip edge and should be installed with the same considerations used for installing the fascia. The appropriate fastener should be used with a specific fastening pattern; alignment should be consistent at joints. Roof edges, when installed properly, provide a clean edge and attractive finish on a building's facade.
During the past two decades, the roofing industry has gone through many changes with the advent of new technologies, products, tests and methods. Roof systems still depend on attention to detail within the tiniest component for success when it comes to wind performance and watertightness, and edge-metal systems are vital to keeping a roof system intact and water out of a building.
If you review installation requirements and implement them into everyday practice in the field, copings and edge-metal flashings will perform well during the long term even under the intense stresses of significant wind events.
Lisa McIlvoy is special projects manager for W.P. Hickman Co., Asheville, N.C.
The galvanic series
Corrosive potential can be roughly predicted by the placement of metals in the galvanic series. The farther apart two metals are in location in the series, the greater the potential for corrosion. Metals that are adjacent to one another have little potential for corrosion.
Appliance attachments, such as lightning rods, signs or antennae that penetrate the water seal, induce a galvanic reaction or otherwise compromise an edge-metal system's effectiveness and should be eliminated or isolated to prevent problems.
Another table with more specific information about anodic indexes can be found at www.engineersedge.com/galvanic_capatability.htm.
Anodic or least noble (corroded end)