8.2 Wood Shingles, Wood Shakes & Synthetic Slate Materials

Wood Shingles, Wood Shakes & Synthetic Slate Materials

Natural wood roof coverings—comprising wood shingles and wood shakes—offer distinct architectural beauty and natural insulation. However, because natural timber is organic and hygroscopic, successful performance in Florida requires precise adherence to grading, interlayment, and exposure standards governed by FBC Building Section 1507.8 (Wood Shingles) and Section 1507.9 (Wood Shakes). Concurrently, modern polymer technology has birthed synthetic composite slate and shake alternatives that replicate natural aesthetics while offering superior fire and hurricane impact performance.

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Wood Shingles vs. Wood Shakes: Manufacturing & Physical Profiles

Although both wood shingles and wood shakes are manufactured primarily from Western Red Cedar (Thuja plicata), Alaskan Yellow Cedar, or treated Southern Yellow Pine, their structural geometry and surface textures differ fundamentally:

 FEATURE               WOOD SHINGLES                          WOOD SHAKES
─────────────────────────────────────────────────────────────────────────────────────────────
 Manufacturing         Sawn on both faces                     Hand-split/resawn or taper-split
 Surface Texture       Smooth, uniform sawn face              Rough split face, sawn back
 Butt Thickness        Thin, uniform (1/4 to 3/8 in)          Thick, dimensional (1/2 to 3/4+ in)
 Interlayment          Not required                           Mandatory #30 felt interlayment
 Minimum Slope         3:12 (with reduced exposure)           4:12

• Wood Shingles: Machine-sawn on both top and bottom faces, producing a smooth, uniformly tapered profile from head to butt. Their uniform thickness allows tight side-to-side nesting.
• Wood Shakes: Manufactured by splitting timber blocks along the natural grain line. Handsplit and Resawn Shakes feature a rough split face combined with a sawn back created by sawing the split block diagonally. Taper-Split Shakes are split from both sides without sawing. Shakes feature heavy, thick butt ends that create deep shadow lines on the roof deck.

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Cedar Grading & Certi-Split Quality Standards

The Florida Building Code incorporates by reference the strict grading and quality control standards established by the Cedar Shake & Shingle Bureau (CSSB).

No. 1 Grade Certi-Split Baseline

For residential and commercial roof coverings in Florida, code mandates the use of No. 1 Grade (Certi-Split / Certigrade Blue Label) wood materials. To achieve No. 1 Grade certification, wood units must satisfy three non-negotiable anatomical criteria:

1. 100% Heartwood: Wood must be harvested entirely from the dense inner core (heartwood) of the tree trunk. Heartwood contains natural extractives (thujaplicins) that confer high resistance to fungal decay and wood-boring insects. Soft, perishable sapwood is prohibited.
2. 100% Clear: Wood units must be completely free of knots, bark pockets, decay, cross-grain, or physical defects across their entire exposed length.
3. 100% Edge Grain (Vertical Grain): The annual growth rings must form an angle of 45 degrees to 90 degrees relative to the surface plane of the shingle. Edge-grain wood exhibits minimal volumetric expansion and contraction when wet, resisting cupping, curling, and splitting over time. Flat-grain wood (growth rings < 45 degrees) is prone to severe warping under Florida's intense humidity cycles and is prohibited in No. 1 Grade roofing.

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Slope, Exposure & Interlayment Requirements

Correct weather exposure (the length of shingle/shake exposed to the elements) is vital to maintaining water-tightness.

Recommended Weather Exposure Limits

 MATERIAL UNIT        UNIT LENGTH    SLOPE 3:12 TO < 4:12    SLOPE 4:12 AND GREATER
─────────────────────────────────────────────────────────────────────────────────────────────
 Wood Shingle (Fivex)    16 Inches      3-3/4 Inches           5 Inches
 Wood Shingle (Perfection) 18 Inches    4-1/4 Inches           5-1/2 Inches
 Wood Shingle (Royal)    24 Inches      5-3/4 Inches           7-1/2 Inches
 Wood Shake (Handsplit)  18 Inches      Prohibited             7-1/2 Inches
 Wood Shake (Handsplit)  24 Inches      Prohibited             10 Inches

Interlayment Paper for Wood Shakes

Because wood shakes possess rough, split surface faces, wind-driven rain can potentially migrate laterally beneath overlapping butts. To prevent water penetration, FBC Section 1507.9.6 mandates the inclusion of an interlayment felt paper between every course of wood shakes:

• Interlayment Specifications: Must be an 18-inch wide strip of #30 asphalt-saturated organic felt (ASTM D226 Type II).
• Placement Geometry: The lower edge of the 18-inch interlayment strip must be positioned above the butt line of the shake course being covered by a distance equal to twice the weather exposure.
• Example: For a 24-inch shake installed with a 10-inch exposure, the lower edge of the interlayment strip is placed 20 inches above the shake butt. This creates a multi-layered weather barrier where felt strips overlap shaking courses, shedding water from course to course without trapping moisture under shake butts.

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Fastening & Moisture Expansion Gaps

Wood is a dynamic material that expands significantly upon absorbing moisture. Fastening and joint spacing must accommodate volumetric timber movement.

Fastener Specifications & Placement

• Hardware Standards: Fasteners must be corrosion-resistant hot-dipped galvanized box nails (complying with ASTM A153 Class D), stainless steel (Type 304 or 316), or silicon bronze. Stainless steel nails are mandatory within 3,000 feet of saltwater coastal environments.
• Fastener Count: Exactly two fasteners are installed per shingle or shake, regardless of width.
• Fastener Placement: Driven 3/4 inch to 1 inch in from each side edge, and 1-1/2 inches above the weather exposure line (so the nail head is covered by the subsequent overlapping course).

 FASTENER PLACEMENT GEOMETRY
 ┌──────────────────────────────────────────────┐
 │  [Nail] 1 in from edge        [Nail] 1 in    │  <── 1-1/2 in Above Exposure Line
 │  ──────────────────────────────────────────  │
 │                                              │
 │                                              │
 │             EXPOSED WEATHER FACE             │
 │                                              │
 │                                              │
 └──────────────────────────────────────────────┘
  ▲                                            ▲
 Butt Line                                  Butt Line

Joint Spacing (Moisture Expansion Gaps)

Wood units absorb rainwater and swell horizontally across their width. If units are installed butt-to-butt without expansion gaps, lateral swelling forces will cause the wood to buckle, cup, and split off the roof deck:

• Wood Shingles: Must be installed with a side-to-side joint gap of 1/4 inch to 3/8 inch.
• Wood Shakes: Must be installed with a side-to-side joint gap of 3/8 inch to 5/8 inch.
• Side Lap Offset: Side joints in adjacent courses must be offset by a minimum of 1-1/2 inches to prevent continuous vertical seams from conducting water directly to the underlayment.

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Synthetic Composite Slate & Shake Materials

Modern synthetic composite roofing products replicate the authentic appearance of natural slate tiles or hand-split cedar shakes while eliminating organic rot, high weight, and brittleness.

Polymer Formulation & Engineering

Synthetic slates and shakes are manufactured using advanced compression-molded or injection-molded formulations containing:

• Engineered polyolefin polymers (polypropylene and polyethylene) mixed with recycled rubber or virgin synthetic resins.
• Mineral fillers (such as ground limestone) for dimensional stability and fire resistance.
• Carbon black and specialized UV-inhibitor packages to resist long-term thermal degradation under intense solar radiation.

Code Testing & Performance Ratings

Under FBC Building Section 1507.13, synthetic composite tiles must hold valid Florida Product Approvals based on rigorous physical testing:

Thermal Expansion & Installation Protocols

Unlike wood, synthetic polymers undergo dimensional movement due to temperature changes rather than moisture absorption. Synthetic composite tiles feature molded spacer ribs or tabs on their sides that automatically maintain a 3/8-inch thermal expansion gap between tiles during installation. Fastening requires ring-shank stainless steel or hot-dipped galvanized roofing nails driven through molded fastener target rings.