7.4 Wood Shingles & Wood Shakes (IBC 1507.8–1507.9): Grades, Slopes, Sheathing, Exposure, Spacing & Fasteners

Why Wood Roofs Are on a Louisiana Exam

Wood roofs are less common in Louisiana than shingles or metal, but they appear on historic homes, custom houses, and porches. Both the IBC and the NRCA steep-slope manual treat them separately, and repairs and reroofs over wood are common on older houses. The exam's open-book references (IBC 1507.8–1507.9, NRCA's wood shake and shingle section, and Atcheson) cover:

• the difference between shingles and shakes;
• minimum slopes and sheathing;
• maximum exposure tables;
• spacing (keyways) and fasteners.

Shingles vs. Shakes

Grades and Labels

Wood shingles and shakes must be graded by an approved agency and each bundle labeled (IBC 1507.8.9 and 1507.9.10). The Cedar Shake and Shingle Bureau (CSSB) grades most cedar products.

• Shingles of naturally durable wood: Grades No. 1, No. 2, or No. 3 (Table 1507.8.5). No. 1 is all heartwood, edge grain, and clear.
• Shakes of naturally durable wood: Grade No. 1. Taper-sawn shakes: No. 1 or No. 2 (Table 1507.9.6).
• Preservative-treated taper-sawn shakes of Southern pine are graded under Texas Forest Service rules and treated per AWPA U1.

Deck: Solid or Spaced Sheathing

Wood shingles and shakes may go on solid or spaced sheathing (IBC 1507.8.1 and 1507.9.1):

• Spaced boards are at least 1 × 4 inches nominal, spaced on centers equal to the weather exposure so every course's fasteners land on a board.
• For shakes, where 1 × 4 spaced sheathing is installed at 10 inches on center, add boards between the sheathing boards.
• Solid sheathing is required where the January average daily temperature is 25°F or less or ice may form along the eaves. That is rarely the case in Louisiana, but many designers still use solid sheathing for wind, fire, or FORTIFIED reasons.

Spaced sheathing lets the wood dry from both sides. That helps durability in Louisiana's humidity, where wood that stays wet decays.

Maximum Weather Exposure

Exposure is how much of each course shows to the weather. Longer pieces and steeper roofs allow more exposure.

Wood shingles, naturally durable wood (IBC Table 1507.8.7), in inches:

Wood shakes at 4:12 or steeper (IBC Table 1507.9.8), in inches:

Worked example: No. 1 18-inch cedar shingles on a 5:12 roof may be laid at up to 5.5 inches exposure. On a 3.5:12 porch roof, the same shingle is limited to 4.25 inches.

Application Rules (IBC 1507.8.7 and 1507.9.8; Table 1507.8)

• Two fasteners per shingle or shake.
• Side lap: at least 1-1/2 inches between joints in adjacent courses. For shingles, no two joints in any three adjacent courses may line up.
• Keyways (spacing): 1/4 to 3/8 inch for shingles; 3/8 to 5/8 inch for naturally durable shakes; 1/4 to 3/8 inch for preservative-treated taper-sawn shakes. The gaps let the wood swell when wet without buckling.
• Fasteners: corrosion resistant, penetrating at least 3/4 inch into the sheathing, or through sheathing less than 1/2 inch thick. Table 1507.8 names hot-dipped galvanized or Type 304 stainless steel, with Type 316 stainless for coastal areas. That matters on the Louisiana coast, where salt air and the extractives in cedar corrode ordinary fasteners.
• Placement: put fasteners where the next course will cover them, near each edge. The CSSB and the NRCA manual show the exact distances.
• Shake interlayment: shakes are laid with strips of ASTM D226 Type I felt interlaid between courses (IBC 1507.9.5). The felt is placed so it is covered by the next course and does not show at the butt line.
• Underlayment: per IBC 1507.1.1. Below 140 mph, D226 Type I or II or D4869 Types I to IV; at 140 mph or more, D226 Type II or D4869 Type IV. Wood shingles laid from 3:12 up to 4:12 get the two-layer application.

Flashing and Valleys (IBC 1507.8.8 and 1507.9.9)

• Wall flashing and counterflashing per the manufacturer; metal at least 0.019 inch (No. 26 galvanized).
• Valley flashing extends at least 11 inches from the centerline each way with a splash diverter rib at least 1 inch high, and sections lap at least 4 inches.
• For slopes of 3:12 and over, a 36-inch-wide underlayment goes under the valley: one layer of Type I underlayment the full length of the valley, or an ASTM D1970 sheet, in addition to the other underlayment.

Fire Classification

Untreated wood shingles and shakes are usually nonclassified. IBC Table 1505.1 allows nonclassified roofing on Group R-3 and U buildings with at least 6 feet of fire separation. It also allows No. 1 cedar or redwood shakes and No. 1 shingles on buildings of two stories or less and 6,000 square feet or less of projected roof area, with 10 feet of separation on all sides. Fire-retardant-treated shakes and shingles are pressure-treated per AWPA C1, tested for weathering per ASTM D2898, and labeled with their class (IBC 1505.6). Special-purpose wood roofs add a 5/8-inch Type X gypsum layer under the sheathing (IBC 1505.7).

Reroofing Over Wood

The code does not ban recovering wood roofs. Metal panels, metal shingles, and clay or concrete tile may be installed over existing wood shakes (IBC 1512.2.1). Where the new covering over wood shingles or shakes creates a combustible concealed space, the whole existing surface must first be covered with gypsum board, mineral fiber, glass fiber, or another approved material (IBC 1512.3). Most asphalt-shingle manufacturers require wood roofs to be removed first.