Metal roof flashing best practices:

This article describes metal roof flashing and sealing products and installation details for exposed-fastener metal roof systems: barn roofing, or agricultural building roofing systems.

This article series discusses best practices in the selection and installation of residential roofing.

Flashings and Accessories for Exposed-Fastener Metal Roofs

Adapted/paraphrased with permission from Best Practices Guide to Residential Construction (Steve Bliss, J Wiley & Sons) , chapter on BEST ROOFING PRACTICES:

Most manufacturers supply preformed flashings, drip edges, rake moldings, and ridge caps color-matched to their roofing panels, as well as color-matched coil stock for fabricating custom pieces onsite.

They also provide rubber closure strips or expandable foam tapes to seal panel ends against water and insect intrusion at eaves, valleys, ridges, and other terminations.

Pay particular attention to panel ends at metal roof valleys. Some manufacturers supply special closures for the angled cuts through ribs, but closures may need to be fashioned by cutting up standard closure strips.

Some manufacturers also provide an expandable foam sealant tape that conforms to the rib pattern for a tight seal up the valley.

Depending on the panel profile, the end treatment will vary, but ends should be fully sealed. Remember to place screws in flat sections and to use extra screws up the valley

(Figure 2-38).

 

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If you want to see the original installation details for the plumbing vent being installed at above right, take a look at the photos and text at the bottom of our article

on PLUMBING VENT DEFECTS & NOISES.

To see what happened later when snow and ice began shoving at the vent stack pipe,

see PLUMBING VENT REPAIR.

For a metal roof vented ridge, place short sections of a matrix-type ridge vent between the ribs and secure with a preformed metal cap (Figure 2-39).

Also see ROOF VENTILATION SPECIFICATIONS

For plumbing vents, most manufacturers recommend a moldable aluminum jack bent to conform to the profile of the roofing (Figure 2-40).

Rectangular openings, such as skylights and chimneys, typically require both base and counterflashing so roof panels are free to move with changes in temperature.

Depending on the panel profile, either use a pan flashing or an L-flashing sealed to the top surface of the roofing panel with sheet metal screws and butyl tape.

On large openings in metal roofs, a cricket is needed on the up slope to divert water around the penetration. Custom-made, one-piece curbs with built-in diverters simplify this type of installation. All flashing joints should be sealed with butyl tape or a manufacturer- recommended sealant.

Sealing Details for Exposed-Fastener Metal Roofs

For the watertight performance required on homes (as opposed to barns), metal roofs need careful sealing around all penetrations, side laps, and end laps. On side seams and lap joints, the sealant should always go on the uphill, or “dry,” side of any fasteners (Figure 2-41).

Sealant should also be used at ridge caps, valleys, and wherever flashings lap over or under the metal roofing.

The preferred sealant for most concealed seams in metal roof panels is butyl tape, which absorbs movement and will not shrink. Gunnable terpolymer butyl or urethane caulk can also be used, as specified by the manufacturer.

But never use acid-cure silicone caulking (the common type with vinegar odor) or asphalt roofing cement, as they will damage most metal coatings.

How to Allow for Panel Movement in Exposed Fastener Metal Roof Systems

Metal roof panels were originally designed for installation on purlins that can absorb the normal movement as the panels expand and contract from temperature changes. The thermal movement of a long panel installed over solid plywood, however, can cause problems.

Typically, either the hole in the metal roofing elongates—creating a potential leak—or the screw becomes loosened, making the roof vulnerable to blow-off. The problems are greatest with aluminum, which has 70% more thermal movement than steel and less tensile strength. To avoid problems, experts recommend the following:

• With exposed-fastener panels, avoid lengths over 40 feet for steel or 16 feet for aluminum—less for climates subject to wide temperature swings. Break the run into two panels.
• On long runs of painted roofing, choose lighter shades, preferably white.
• Use screws in the flat part of the panel, not on the ribs. Screws should penetrate the sheathing fully, plus 1/4 to 1/2 inch.
• Where leak-free performance is critical, fasten the roofing to Z-shaped metal purlins screwed horizontally across the plywood sheathing. Or switch to a concealed fastener system.

Also see

• Smith, Thomas L., AIA, CRC, METAL ROOFING: 'FIXING' for THERMAL MOVEMENT [PDF] NRCA, National Roofing Contractors Association

Wavy Metal Panels or Oil-Canning Problems on Metal Roofs: Thermal Expansion

Thermal expansion in light-gauge metal panels can cause a wavy appearance called “oil-canning” in the flat areas. In general, this does not signal a performance problem, but it may be visually objectionable.

Oilcanning tends to be most visible in bright light from a close distance, and it is generally more noticeable on shiny metals, such as Galvalume®, than on colored metal panels.

Oilcanning on metal roofs is primarily a problem in profiles with few ridges to stiffen the panels. To reduce the metal roof rippling or oilcanning effect, some manufacturers provide self-adhesive foam strips that are attached lengthwise to the bottom of metal panels.

- - Adapted with permission from Best Practices Guide to Residential Construction (Steve Bliss, J Wiley & Sons) .

Metal Roofing Manufacturers

• Classic Products www.classicroof.com Modular metal shingle panels and standing seam panels
• Decra Roofing Systems www.decra.com Modular metal shingle, tile, and shake panels
• Dura-Lok Roofing Systems www.duraloc.com Modular metal roofing shingles with granular coating
• Fabral www.fabral.com Exposed fastener and concealed clip metal roofing panels
• Gerard Roofing Technologies www.gerardusa.com Modular metal shake and tile panels with granular coating
• Met-Tile www.met-tile.com Modular metal roof-tile panels
• Atas International www.atas.com Modular metal shingle, tile, and standing-seam panels
• Custom-Bilt Metals www.custombiltmetals.com Modular metal shakes and standing seam panels
• Zappone Manufacturing, website www.zappone.com/
  Zappone Manufacturing, 2928 North Pittsburg St. Spokane, WA 99207 1-800-285-2677, Washington State Copper Roofing Supplier of Copper scallop shingles, copper shingles, copper bay windows, vertical walls, aluminum roof shingles

Venting Underlayments

• Benjamin Obdyke www.benjaminobdyke.com Cedar Breather, a 3/8 -in.-thick matrix-type underlayment designed to provide ventilation and drainage space under wood roofing

More Information about Roofing Materials, Methods, Standards

• Metal Roofing Alliance www.metalroofing.com

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Continue reading at METAL ROOF COATINGS & PAINTS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• CHIMNEY FLASHING MISTAKES & LEAKS
• DIVERTER KICK-OUT FLASHING
• FLASHING on BUILDINGS - home
• METAL ROOFING - home
• SNOW GUARDS on METAL ROOFS

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FLASHING for METAL ROOFS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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INDEX to RELATED ARTICLES: ARTICLE INDEX to BUILDING ROOFING

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