Top Mistakes by Metal Roof Installers

Basic Preparation

One of the first things to ensure a successful metal roof installation is to measure the roof length and slope correctly and precisely. Each panel should be cut to the exact size, or there could be imprecise cuts and rough edges exposed to the elements leading to rust and roof coating failure. Metal roof panels come in a wide variety of thicknesses, shapes and profiles. So “measure twice, cut once” to attain panels that mold to roof contours and provide a weathertight fit.

“Take all penetrations, curbs and skylights into account,” says Bill Lawson, vice president– roofing operations, Chamberlin Roofing and Waterproofing, Houston. “Confirm all field dimensions before ordering materials, and be aware of lead time to help the project start and stay on schedule. Double-checking dimensions before fabricating and installing metal panels and flashings is a good idea. Check specifications for weathertightness warranty and engineering requirements; follow manufacturer standard details for proper installation. Planning and coordination prior to job start and clear communication between project management and crew members during construction is key to any successful project.”

”It’s crucial to start off on the right foot, which means reviewing drawings and design in detail and working with the manufacturer for proper detailing and to ensure the installation meets warranty compliance,” says Chad McLeish, vice president of estimating, Kalkreuth Roofing and Sheet Metal, Wheeling, W.Va. Mark James, marketing consultant at Roof Hugger, Lutz, Fla., believes some of the top mistakes metal roofing contractors make are not having the roof attachment properly designed by the manufacturer and not selecting the proper roof panel system to begin with. Also, a metal roof is only as good as the base it sits on. Before installation, check for dry rot and signs of water damage on the roof deck before installation. Otherwise, the roof will break loose or create gaps for moisture as it expands and contracts.

Fasteners and Clips

Leaking is probably the most common problem with metal roofing. But, metal doesn’t leak; penetrations in metal leak. One of the single largest reasons for metal roof leaks and prolonged roof maintenance is poorly installed roofing fasteners and faulty sealing washers. Errors here can be the result of:

• Over-tightened fasteners

• Under-tightened fasteners

• Off-center fasteners

During installation, always check the washer to ensure screws are not over- or under-tightened. Tighten the fastener so the rubber washer is completely compressed, but as a rule of thumb if the washer is splayed out and visible, then the fastener is too tight.

Fasteners should be screwed in easily with little resistance. Use the correct nut driver to do this. Installing the screw perpendicular to the metal surface is critical. This allows the fastener to penetrate the steel with less effort and allow the washer to seal correctly.

Also, roof fasteners should be driven into appropriate clips and pre-drilled purlins, not the wood furring strip. Wood moves at a different rate than metal and this movement mismatching will can cause fasteners to move out of the roof over time, break their coated sealant and produce more elongated holes in the panels. Furthermore, not correctly sealing the seam and fastener can cause visibly rusty fasteners that can stain panels.

Travis Lord, product manager–metal, The Garland Co. Inc., Cleveland, says installing a two-piece clip in the wrong position is an all too-common error. “Many metal systems are designed with two-piece clips, which allow limited thermal expansion and contraction. During installation, every single clip has to be installed in the same position, causing the roofer to have to guess whether the roof will need to expand or contract. Clips that are installed in the wrong position can cause binding, oil canning and sealant deterioration in the seams.”

James advises when retrofitting over an existing standing seam roof determine if the existing roof system has stand-off clips with or without thermal blocks. “If not, a standard retrofit sub-framing system will not allow a positive attachment to the existing roof purlins.”

Flashing, Seams and Sealants

Flashing is a transitional material providing a watertight seal between roofing materials, roof sections (roofline transitions, valleys, dormers) and other “non-roofing” material like vents, chimneys and skylights. A metal roof’s seams must be completely sealed and panels lapped correctly.

Incorrect attachment and improper sealing of the flashing will allow water to enter between the roofing systems and the roof structure. As a worst-case scenario, poorly installed flashing could cause roof sections to blow off in a heavy windstorm. Sealants and butyl tape seal out water, dirt and other debris that could compromise a metal roof. If these are damaged, missing or applied incorrectly, leaks can occur. A sealant should be applied to the underside of the panel or trim where it is shielded from the elements—exposed sealants disintegrate over time.

Corrosion

Galvalume or galvanized steel metal roof can run the risk of rust and corrosion. This happens because of water and water-borne pollutants, but protective coatings can prevent this. However, any damage to the coating increases the risk of rust. If metal roofing material is cut during installation, it leaves an open edge of the metal exposed, which wicks in moisture. This will begin the rusting process immediately.

To prevent scuffing and scratching during installation:

• Carriers must handle the coils and panels with proper lifting devices

• When rollforming, ensure the coil does not scrape up against a part of the machine

• Tools or other sharp objects must not rub against or be dropped on a panel

• Avoid walking on panels

Scratches can easily be fixed with a paint or touch-up pen designed to match the color of the roof. But if there is a gouge that goes down to the core of the steel, that piece of metal panel or flashing should be replaced immediately.

When certain dissimilar metals are put together, there could be a negative interaction that causes galvanic corrosion that would not occur otherwise. If the fastener and the proprietary substrate are dissimilar metals, it’s important to know how they will react when used together. Some metals can be very corrosive in these situations, severely compromising a structure’s durability. There’s a galvanic corrosion test to determine the reactions of dissimilar metals.

Oil Canning

Oil canning in metal roofing is an observed waviness or buckling across the flat areas of sheet metal panels that does not normally affect the roof’s structural integrity. While sometimes caused by inferior metal or too-low-gauge thickness, too often, it is caused by under- or over-tightening of roof fasteners that causes the metal to stretch, pull and dimple in various directions. An over-tightened fastener will pull down the cladding locally and can create deformations. Other causes include:

• Lack of tension leveling of product

• Poorly adjusted or worn rollforming equipment

• Uneven substrate

• Width and spacing of seam

Lawson stresses, “Clarify the spacing of clips and don’t start the installation of the panels out of square. [Also] leave room for thermal expansion at the eave of the roof panels to lessen oil canning.” Most panels accommodate transverse thermal expansion by flexing of webs and by “take-up” at sidelaps. When panels are over-tightened, these relief features are hindered or eliminated, particularly for flat panels without corrugations.

The substrate is often the source of oil canning. The substrate must be made of a material, or set of materials, which will not adhere to the underside of the metal and restrict the normal thermal movements of the metal. Deck deviation, bows, ridges and camber all induce stress in the finished panel installation. The deck substrate needs to be in a level plane. “Make sure the substrate has no defections and shim the panels when needed,” Lawson notes. Also, “slip sheets” between the metal and underlayment can prevent oil canning. Some underlayments have a surface that acts as a slip sheet.

Get Help

Your best defense against metal roof installation errors is a thoroughly trained installation crew. Resources such as the Metal Construction Association, Metal Building Contractors & Erectors Association and National Roofing Contractors Association can provide information on how to install error-free metal roof projects. Also, all of the major metal roofing manufacturers have installation guides that offer by-the-book advice on how to put their roofs together.

James recommends getting help from and communicating with other building teams, contributors and suppliers. “Too many times a contractor does not coordinate roof penetrations with other trades, which can include pipe penetrations, heating and air conditioning equipment, lightning prevention, snow prevention and anything else that will attach to the new metal roof. This is not to forget the importance of instructing other trades on how to actually walk on a metal roof to ensure the roof surface is not damaged.”