4.3 Metal Roofing, Wood Shakes & Slate Systems

4.3 Metal Roofing, Wood Shakes & Slate Systems

[!NOTE] Arizona Registrar of Contractors (CR-42) Trade Focus: Steep-slope metal roofing, cedar shakes, and quarry slate represent specialized, high-performance architectural roofing sectors across Arizona. In our intense desert lowlands and high-altitude mountain environments (such as Flagstaff, Prescott, and Sedona), these systems are subjected to severe conditions: metal roofs experience violent diurnal thermal movement and intense UV exposure; wood shakes face extreme wildfire hazards and dry-rot cycles; and natural slate demands exacting structural framing and layout geometry. Roofing contractors must execute precise engineering for thermal movement, galvanic isolation, wildland-urban interface (WUI) fire retardancy, and fastener metallurgy in accordance with IBC Chapter 15 and IRC Chapter 9.

Unlike modular asphalt shingles, metal panels, wood shakes, and slate tiles function under distinct mechanical and structural principles. Metal roofs require engineered accommodation for thermal expansion and hydrostatic hydrostatic sealing; wood shakes require breathable interlayment to baffle wind-driven moisture; and natural slate requires non-corrosive copper fasteners that endure for a century.

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Metal Roofing Systems: Architectural vs. Structural vs. Exposed Fastener

Modern metal roofing panels are classified into three distinct structural categories based on their hydrostatic capability, seam geometry, and fastening method:

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|                      METAL ROOFING SYSTEM CLASSIFICATIONS                      |
+--------------------------------------------------------------------------------+
|  SYSTEM TYPE        | HYDROSTATIC NATURE | SUBSTRATE       | MINIMUM SLOPE     |
|---------------------+--------------------+-----------------+-------------------|
|  Architectural      | Hydrokinetic       | Continuous Wood | 3:12 (or 2:12     |
|  Standing Seam      | (Water-Shedding)   | Solid Deck      | with underlayment)|
|---------------------+--------------------+-----------------+-------------------|
|  Structural         | Hydrostatic        | Open Purlins or | 1/4:12 (with      |
|  Standing Seam      | (Water-Barrier)    | Solid Deck      | factory sealant)  |
|---------------------+--------------------+-----------------+-------------------|
|  Exposed Fastener   | Hydrokinetic       | Wood Deck or    | 3:12              |
|  (R-Panel / 5V)     | (Water-Shedding)   | Purlins         | (Screw Washer)    |
+--------------------------------------------------------------------------------+

1. Architectural Standing Seam Systems

Architectural standing seam panels are designed as hydrokinetic (water-shedding) roof assemblies:

• Seam Configuration: Panels feature vertical ribs (typically 1 to 1-1/2 inches high) joined by snap-lock profiles or mechanically seamed 90-degree single locks.
• Concealed Clips: Fasteners are concealed beneath the seam using engineered metal clips, eliminating exposed fastener penetrations.
• Substrate: Architectural panels possess low structural beam strength and require a continuous structural deck (minimum 15/32-inch plywood or 7/16-inch OSB) covered by an approved high-temperature underlayment.
• Minimum Slope: Prescriptive minimum slope is 3:12 per IBC Section 1507.4.2, though manufacturer listings permit reduction to 2:12 (or 1/2:12 when installed with continuous high-temperature self-adhering underlayment and factory-applied seam sealants).

2. Structural Standing Seam Systems

Structural standing seam panels are engineered as hydrostatic (water-barrier) roof assemblies:

• Seam Configuration: Panels feature tall, robust vertical ribs (2 to 3 inches high) containing continuous, factory-applied hot-melt butyl sealant inside the female rib. Panels are seamed using a motorized electrical seaming machine that crimps the ribs into a 180-degree double-lock (Pittsburgh lock).
• Open-Framing Spanning: Structural panels possess substantial structural section modulus, allowing them to span open framing purlins (structural steel Z-purlins spaced 4 to 5 feet on center) without a solid wood deck.
• Low Slope Capability: Under IBC Section 1507.4.2, structural standing seam metal roofs with double-locked, sealed seams are approved for slopes as low as one-fourth unit vertical in 12 units horizontal (1/4:12 or 2% slope).

3. Exposed Fastener Metal Roofing (Corrugated, R-Panel, 5V-Crimp)

Exposed fastener systems utilize overlapping profiled metal sheets secured by driving self-tapping hex-head screws directly through the face of the metal panel into the underlying wood decking or steel purlins:

• Fasteners: Screws are fitted with integral EPDM rubber sealing washers designed to compress against the panel face.
• Minimum Slope: Prescriptive minimum slope is 3:12 per IBC Section 1507.4.2.
• Vulnerabilities in Arizona: In Arizona's intense desert climate, exposed fastener systems suffer severe long-term vulnerabilities. Solar UV radiation oxidizes and embrittles EPDM washers within 7 to 12 years, causing them to crack, shrink, and leak. Concurrently, continuous daily thermal expansion and contraction cycles force screw fasteners to back out or elongate the penetrations into oval slots (fastener slotting), initiating chronic leaks.

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Metal Types, Alloys, and Manufacturing Gauges

Roofing metals must resist atmospheric corrosion, thermal fatigue, and chemical degradation:

1. Galvalume (ASTM A792)

Galvalume consists of cold-rolled sheet steel coated on both sides with an alloy comprising 55% aluminum, 43.4% zinc, and 1.6% silicon by weight. The aluminum provides an impervious barrier film, while the zinc provides galvanic sacrificial edge protection. Galvalume is available in 24-gauge (0.024-inch nominal thickness) and 26-gauge (0.018-inch). Galvalume offers extraordinary longevity in arid environments, lasting two to four times longer than standard galvanized steel. However, Galvalume must never be placed in direct contact with wet concrete, mortar, stucco, or pressure-treated lumber containing alkaline copper quaternary (ACQ) compounds, as strong alkalis and free copper rapidly destroy the protective alloy layer.

2. Galvanized Steel (ASTM A653)

Carbon steel sheet coated with pure zinc. Commercial architectural roofing requires a G90 coating weight (0.90 ounces of zinc per square foot total both sides). Standard gauges are 24-gauge (heavy commercial) and 26-gauge (residential).

3. Aluminum (ASTM B209)

Fabricated from aluminum-manganese alloys (such as 3003-H14 or 3105-H14) in nominal thicknesses of 0.032 inch (light commercial) and 0.040 inch (heavy-duty architectural). Aluminum is naturally immune to red iron rust, making it ideal for high-humidity or corrosive industrial zones. However, aluminum has a low tensile yield strength and expands thermally at roughly twice the rate of steel.

4. Copper (ASTM B370)

A noble architectural metal specified by weight rather than gauge. Standard roofing weights are 16 ounces per square foot (0.0216-inch thickness) and 20 ounces per square foot (0.0270-inch thickness) in cold-rolled temper. Exposed to the atmosphere, copper oxidizes rapidly from shiny salmon-pink to dark brown cuprous oxide, eventually developing a permanent protective green patina (basic copper carbonate). Copper provides an extraordinary service life of 100+ years without painting.

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Thermal Expansion and Contraction Accommodation

Metal expands when heated by solar radiation and contracts when cooling at night. In Arizona, rooftop temperatures fluctuate from 35°F on winter mornings to 175°F+ on summer afternoons—a temperature swing ($\Delta T$) of 140°F:

Thermal Movement Physics

The total linear change in panel length ($\Delta L$) is calculated using the formula: $\Delta L = L \cdot \alpha \cdot \Delta T$ where $L$ is panel length in inches, $\alpha$ is the material's coefficient of thermal linear expansion, and $\Delta T$ is the temperature differential in °F.

• Carbon Steel / Galvalume: $\alpha \approx 6.7 \times 10^{-6} \text{ in/in/°F}$
• Aluminum: $\alpha \approx 12.8 \times 10^{-6} \text{ in/in/°F}$
• Copper: $\alpha \approx 9.3 \times 10^{-6} \text{ in/in/°F}$

Calculation Example

For a 50-foot panel run (600 inches) subjected to a 140°F temperature swing:

• Steel Panel Movement: $\Delta L = 600 \times (6.7 \times 10^{-6}) \times 140 \approx 0.56\text{ inches}$ (over 1/2 inch).
• Aluminum Panel Movement: $\Delta L = 600 \times (12.8 \times 10^{-6}) \times 140 \approx 1.08\text{ inches}$ (over 1 full inch).

Fixed vs. Floating / Sliding Clips

If a 50-foot metal panel is fastened rigidly at both ends, this massive thermal force will buckle the panel, sheer fasteners, or tear seams:

• Fixed Point: The panel is anchored solidly to the deck at only one location along its run (typically at the ridge or eave) using fixed clips or through-fasteners, establishing a permanent anchor point.
• Sliding / Floating Clips: Along the rest of the panel run, the standing seams are secured using two-piece sliding (floating) clips. The base bracket is screwed into the deck, while an upper movable tab engages the panel rib. The upper tab slides freely within a slotted base track (typically permitting 1 to 2 inches of longitudinal travel), allowing the metal panel to expand and contract silently without structural stress.

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Galvanic Corrosion and Dissimilar Metals Isolation

When two electrochemically dissimilar metals are placed in physical contact in the presence of an electrolyte (rainwater, dew, or atmospheric humidity), an electrochemical galvanic reaction occurs:

+--------------------------------------------------------------------------------+
|                       THE GALVANIC SERIES OF METALS                            |
+--------------------------------------------------------------------------------+
|  ANODIC (Active / Sacrificial)                                                 |
|  [ Magnesium ] -> [ Zinc ] -> [ Aluminum ] -> [ Carbon Steel ]                 |
|                                                                                |
|  CATHODIC (Noble / Protected)                                                  |
|  [ Lead ] -> [ Tin ] -> [ Stainless Steel (304/316) ] -> [ Copper ]            |
+--------------------------------------------------------------------------------+

The Galvanic Corrosion Mechanism

The more active (anodic) metal experiences accelerated corrosion, sacrificing its mass to protect the more noble (cathodic) metal. The rate of galvanic destruction depends on the electrochemical potential difference between the metals and their relative surface area ratio.

Critical Incompatibilities in Roofing

• Copper Runoff onto Steel or Aluminum: Water flowing over copper picks up microscopic copper ions. When this acidic, copper-laden water washes onto downstream Galvalume, galvanized steel, or aluminum panels, a severe galvanic reaction is triggered. The copper ions plate onto the active metal, creating microscopic galvanic cells that pit and perforate the steel panel within a few seasons. Copper must NEVER be installed upstream of or in contact with aluminum, zinc, or steel.
• Fastener Metallurgy: Never install electro-galvanized or steel fasteners into copper flashing or panels. Solid copper, brass, or compatible 300-series stainless steel fasteners must be used exclusively.
• Isolation Protocols: Where dissimilar metals must intersect (such as aluminum coping meeting structural steel framing), they must be completely isolated using non-conductive barriers: EPDM or neoprene gaskets, heavy polyethylene separator films, or self-adhering modified bitumen membranes.

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Oil-Canning: Causes and Prevention

Oil-canning is defined as visible waviness, rippling, or localized buckling in the broad, flat pan areas of standing seam metal roofing panels:

• Nature of Oil-Canning: Under national trade standards (Metal Construction Association [MCA]), oil-canning is classified as an inherent aesthetic characteristic of light-gauge cold-rolled sheet metal, NOT a structural defect or waterproofing failure.
• Primary Root Causes:
  1. Uneven, misaligned roof framing or non-planar deck sheathing (forcing panels to twist across high spots).
  2. Over-driving clip fasteners, pinching panel ribs tight against the deck.
  3. Restrained thermal expansion (improper fixed clips preventing panel movement).
  4. Wide, flat panel pans roll-formed from thin-gauge coils with uneven internal coil stresses.
• Field Prevention Strategies:
  • Maintain structural framing alignment within a strict planar tolerance of not more than 1/4 inch in 20 feet.
  • Utilize two-piece floating clips to ensure unrestrained thermal movement.
  • Specify narrower panel widths (e.g., 12 to 16 inches instead of 18 to 20+ inches).
  • Specify surface visual stiffeners roll-formed into the pan: pencil ribs, V-grooves, striations, or clip-relief beads that break up flat reflections.
  • Utilize heavier metal gauges (24-gauge steel instead of 26-gauge; 0.040-inch aluminum instead of 0.032-inch).

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Wood Shingles vs. Wood Shakes: Cedar Shake & Shingle Bureau (CSSB)

Wood roof coverings provide natural insulation and rustic aesthetics, categorized under Cedar Shake & Shingle Bureau (CSSB) standards:

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|                      WOOD SHINGLES VS. WOOD SHAKES                             |
+--------------------------------------------------------------------------------+
|  CRITERIA          | WOOD SHINGLES (Certigrade)   | WOOD SHAKES (Certi-split)  |
|--------------------+------------------------------+----------------------------|
|  Manufacturing     | Sawn smooth on both faces    | Split face, sawn back      |
|  Surface Texture   | Smooth, uniform taper        | Rustic, irregular, rough   |
|  Butt Thickness    | Thin (approx. 3/8" to 7/16") | Heavy (1/2" to 3/4"+)      |
|  Minimum Slope     | 3:12 (Reduced Exposure) / 4:12| 4:12 Minimum              |
|  Underlayment      | Continuous underlayment      | Mandatory Interlayment     |
+--------------------------------------------------------------------------------+

Wood Shingles (Certigrade)

Wood shingles are sawn on both the face and back, producing a smooth surface with an even, regular taper. Standard lengths are 16 inches (Fivex), 18 inches (Perfection), and 24 inches (Royal). Certified under CSSB Certigrade standards, the premier grade is Number 1 Blue Label (100% heartwood, 100% clear, 100% edge-grain). Edge-grain wood ensures uniform dimensional stability and minimizes splitting during seasonal moisture cycling.

Wood Shakes (Certi-split)

Wood shakes are manufactured by splitting cedar blocks along the natural grain. The most common type is handsplit and resawn (split on the exterior face and sawn smooth on the back). Shakes exhibit a rugged, deeply textured face with heavy butts ranging from 1/2 inch (medium shakes) to 3/4 inch or thicker (heavy shakes). Shakes are graded under CSSB Certi-split standards.

Wood Species and Natural Durability

The overwhelming wood species specified for roofing is Western Red Cedar (Thuja plicata). Western Red Cedar contains high natural concentrations of extractive chemicals called thujaplicins, which provide exceptional natural resistance to wood-destroying decay fungi and insect attack.

Wildfire Hazards and Fire-Retardant Pressure Treatments

Untreated cedar shakes and shingles are combustible and strictly prohibited across most Arizona municipalities and Wildland-Urban Interface (WUI) fire districts. To comply with IBC Section 1505 and local fire codes, wood shakes must be factory pressure-impregnated with fire-retardant polymers complying with ASTM E108 / UL 790:

• Class C Roof Assembly: Pressure-treated shakes installed over standard underlayment.
• Class B Roof Assembly: Pressure-treated shakes installed over specialized fire-resistant cap sheets.
• Class A Roof Assembly: Pressure-treated shakes installed over an approved fire barrier system (such as 1/2-inch Type X gypsum board covered by mineral-surfaced cap sheets).

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Mandatory Interlayment Requirements for Wood Shakes

Unlike wood shingles (which lie flat and tight against adjacent plies), handsplit wood shakes are thick and irregular, leaving open cavities along their underside. Wind-driven rain and blowing snow easily penetrate beneath shake butts. Therefore, model building codes enforce a strict interlayment requirement:

+--------------------------------------------------------------------------------+
|                 WOOD SHAKE FELT INTERLAYMENT GEOMETRY                          |
+--------------------------------------------------------------------------------+
|                                                                                |
|   WOOD SHEATHING DECK                                                          |
|   ========================================================================     |
|                          ^                                                     |
|                          | 18" Felt Interlayment extends up onto deck          |
|   [ 18" #30 FELT ]       |                                                     |
|   +----------------------+-----------------------------------------------+     |
|   |                                                                      |     |
|   | Bottom edge of felt placed at TWICE the weather exposure (e.g. 20")  |     |
|   | above lower shake butt line                                          |     |
|   +----------------------------------------------------------------------+     |
|                                                                                |
|   [ PRECEDING SHAKE COURSE ]                                                   |
|   ==============================================> [ BUTT LINE (0") ]           |
+--------------------------------------------------------------------------------+

Interlayment Specifications (IBC 1507.9.5 / IRC R905.9.4)

• Material: Interlayment must consist of minimum 18-inch wide rolls of #30 asphalt-saturated organic felt (ASTM D226 Type II).
• Placement Geometry: An 18-inch wide strip of felt interlayment must be installed between every single course of shakes.
• The Twice-Exposure Rule: The bottom edge of the 18-inch felt interlayment must be positioned above the butt edge of the overlying shake course by a distance equal to twice the weather exposure:
  • Example: If 24-inch shakes are installed at a 10-inch weather exposure, the bottom edge of the 18-inch felt interlayment must be positioned exactly 20 inches above the butt line of the preceding shake course.
• Critical Installation Rule: The felt interlayment extends 18 inches upward onto the structural deck. The felt interlayment must NEVER be installed flush with or extending below the shake butt line. Exposing organic asphalt felt to direct sunlight accelerates UV degradation and creates an unsightly fire hazard, while placing it too high fails to baffle wind-driven water.

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Natural Roofing Slate Assemblies: ASTM C406 & Geometry

Natural roofing slate is a fine-grained metamorphic rock quarried from natural stone deposits. It is completely noncombustible (Class A fire rating), chemically inert, and provides the longest service life of any steep-slope roofing material.

ASTM C406 Slate Classifications

Under ASTM C406, natural roofing slate is classified into three structural grades based on water absorption, flexural breaking load, and acid resistance:

• Grade S-1: Maximum water absorption $\le 0.25%$, depth of softening $\le 0.002\text{ inch}$. Expected service life: 75 to 100+ years.
• Grade S-2: Maximum water absorption $\le 0.36%$, depth of softening $\le 0.009\text{ inch}$. Expected service life: 40 to 75 years.
• Grade S-3: Maximum water absorption $\le 0.45%$, depth of softening $\le 0.014\text{ inch}$. Expected service life: 20 to 40 years.

Structural Fasteners for Slate

Slate is brittle and heavy (700 to 1,500 lbs per square). Fasteners must match the century-long lifespan of the stone:

• Metallurgy: Fasteners must be solid copper or 300-series stainless steel slating nails (minimum 10-gauge or 11-gauge with large 3/8-inch flat heads). Electro-galvanized nails are strictly prohibited; they corrode away within 15 to 25 years, destroying the entire assembly.
• Fastener Driving Depth: Nails are driven into pre-punched, countersunk nail holes. Nails must never be driven tight against the slate. Over-driving pinches the stone and fractures the countersunk hole. Under-driving leaves protruding heads that punch through the overlapping slate. The nail head must sit perfectly flush within the countersink, allowing the slate to hang freely.

Slate Headlap Formula and Exposure Geometry

Under IBC Section 1507.7.2, natural slate requires a minimum slope of 4:12 and a standard 3-inch headlap (the overlap of the third course over the first course):

$\text{Weather Exposure} = \frac{\text{Slate Length} - \text{Headlap}}{2}$

• Calculation Example: For a standard 18-inch long slate installed with a 3-inch headlap: $\text{Exposure} = \frac{18\text{ inches} - 3\text{ inches}}{2} = \frac{15}{2} = 7.5\text{ inches}$
• If the roof slope is between 4:12 and 8:12 in severe wind-driven rain zones, the headlap is typically increased to 4 inches, reducing the weather exposure of an 18-inch slate to 7.0 inches.

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Technical Comparison: Metal, Wood & Slate Systems