7.4 Metal Roof Flashing, Underlayments & Thermal Movement

7.4 Metal Roof Flashing, Underlayments & Thermal Movement

A metal roof's weatherproofing integrity depends on proper underlayment selection, galvanic isolation of dissimilar metals, and precise flashing fabrication. In Florida's high-temperature, high-wind, and coastal salt-spray environment, flashing detailing around valleys, eaves, ridges, sidewalls, and penetrations requires rigorous compliance with the Florida Building Code (FBC) and Sheet Metal and Air Conditioning Contractors' National Association (SMACNA) standards.

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High-Temperature Underlayments & Slip-Sheet Engineering

Underlayments installed beneath metal roofs are subject to extreme thermal conditions. Sunlit metal roof panels frequently transfer surface temperatures of 200°F to 250°F directly to the underlayment below.

  METAL ROOF UNDERLAYMENT SYSTEM LAYERS:
  
  ===============================================================
  STANDING SEAM METAL ROOF PANEL
  ---------------------------------------------------------------
  SLIP-SHEETS (Rosin Paper / Non-Asphaltic Synthetic Layer)
  ---------------------------------------------------------------
  HIGH-TEMPERATURE (HT) SELF-ADHERED MEMBRANE (250°F+ Rated)
  ===============================================================
  STRUCTURAL PLYWOOD ROOF DECK

1. High-Temperature (HT) Self-Adhered Underlayments

• Standard vs. High-Temp Self-Adhered: Standard organic asphalt self-adhered ice-and-water shields soften and liquefy at temperatures above 160°F–180°F. Under hot metal panels, standard asphalt bleeds out at seams and drips into soffits.
• HT Specification: Metal roofs require High-Temperature Polymer-Modified Bitumen or butyl-based self-adhered underlayments rated for continuous heat exposure up to 250°F to 260°F. HT underlayments feature a non-abrasive poly-film top surface and self-sealing characteristics around underlayment fasteners.

2. Slip-Sheet Functionality

• When metal roof panels undergo daily thermal expansion and contraction, the underside of the panel rubs against the underlayment.
• If a metal panel rests directly against a rubberized asphalt self-adhered membrane, heat will cause the metal to stick to the asphalt backing. Subsequent thermal movement will tear the underlayment or cause loud pop-and-groan thermal noises.
• Solution: Contractors install a slip-sheet—traditionally unsaturated 5-lb Rosin building paper or a smooth, non-asphaltic synthetic slip layer—directly over the HT self-adhered membrane before panel layout. The slip-sheet acts as a smooth friction-reducing barrier.

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Metallurgy & Galvanic Corrosion Prevention

Galvanic corrosion occurs when two electrochemically dissimilar metals are in contact in the presence of an electrolyte (moisture, rainwater, or salt spray). The less noble metal (anode) corrodes rapidly to protect the more noble metal (cathode).

  GALVANIC SCALE (Least Noble / Anodic to Most Noble / Cathodic):
  
  [ANODIC - Corrodes First]
  1. Zinc
  2. Galvanized Steel
  3. Aluminum
  4. Stainless Steel (Passive)
  5. Copper
  [CATHODIC - Protected]

Critical Galvanic Rules for Roofing Contractors

1. Copper Isolation: Copper is highly cathodic (noble). Copper flashings must NEVER touch aluminum, galvanized steel, or Galvalume panels. Runoff water carrying copper ions from a copper chimney flashing down onto a steel panel will cause rapid pitting and rust holes in the steel sheet.
2. Fastener Compatibility: Fasteners must match or be more noble than the metal panel being attached:
   • Use 300-series stainless steel fasteners for aluminum panels and coastal Galvalume applications.
   • Use copper or brass fasteners for copper flashings.
   • Never use zinc-plated carbon steel screws on aluminum or copper roofs.
3. Separation Barriers: Where dissimilar metals must intersect, contractors must isolate them using heavy Bituminous Paint coating, EPDM rubber isolation gaskets, or HT self-adhered underlayment tape.

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Valley Flashing Engineering & Construction Details

Valleys receive the highest volume of concentrated water flow on a roof structure. Florida Building Code requires valley underlayments to extend at least 18 inches on each side of the valley center line (36 inches total width).

  W-PROFILE VALLEY FLASHING DETAIL:
  
        Panel Hem Hooked over Cleat      Center Rib (Deflects Water)
        |---[Metal Panel]--|              / \
     ...|==================|=============|   |=============|==================|...
                           |---Cleat----/     \----Cleat---| 
                           |---1" Hem                 1" Hem---|
  =================================================================================
  HIGH-TEMPERATURE SELF-ADHERED VALLEY UNDERLAYMENT (36" Wide)

W-Profile Ribbed Valley Construction

• Center Water Diverter Rib: Valley metal flashings should incorporate a W-style splash diverter rib (minimum 1 inch to 1.5 inches tall) down the center line. This rib prevents high-velocity water rushing down one roof slope from washing up underneath the panel hem on the opposing slope.
• Cleat Edge Attachment: Panel edges must NOT be screwed through the bottom of the valley metal. The valley metal edges are folded into a 1-inch hem, which hooks onto continuous metal offset cleats fastened outside the water channel.
• Seam Sealant: Butyl tape sealant is placed between the panel hem and the valley metal hook to block wind-driven rain.

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Eave, Ridge & Wall Flashing Details

Eave Drip Edge & Starter Cleat Integration

• Standing seam panels must engage a continuous metal eave starter cleat (drip edge) installed along the eave line.
• The bottom edge of the metal panel pan is field-folded with a 180-degree hem, which hooks tightly over the lower leg of the starter cleat.
• This conceals eave fasteners completely and prevents wind uplift from lifting panel ends during hurricanes.

  EAVE STARTER CLEAT HOOK DETAIL:
  
  Metal Roof Panel Pan
  =======================================
                                        |
  Continuous Starter Cleat             /  <--- 180° Panel Hem Hooks Over Cleat
  +-----------------------------------+  |
  | Wood Blocking / Eave Board        +--+
  +--------------------------------------+

Vented & Non-Vented Ridge Flashing Assemblies

• Z-Closure Profile: Metal roof panels feature open standing seam vertical legs at ridge cutouts. To seal these gaps, contractors install continuous metal Z-closures (metal channels shaped like a "Z" cut to fit between panel ribs) set in heavy beads of butyl sealant tape.
• Vented Ridge: Accommodates attic exhaust ventilation by substituting solid Z-closures with perforated metal Z-closures lined with a continuous mesh insect/weather filter, covered by an overlapping metal ridge cap flashing.

Sidewall & Headwall Counterflashing

Where a metal roof terminates against a vertical masonry wall:

1. An apron / sidewall flashing extends up the wall at least 6 inches and covers the panel flat at least 4 inches.
2. A secondary reglet counterflashing is let into a 1-inch deep saw-cut mortar joint in the masonry, folded downward over the top leg of the apron flashing, and sealed with high-grade polyurethane sealant.

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Flashing & Underlayment Performance Specifications