Doing a cedar shake roof for the first time.

Interlayment (Felt Underlayment): Per IRC R905.7.4 and CSSB standards, you must install 18" wide strips of No. 30 asphalt-saturated felt (ASTM D226 Type II or D4869 Type IV) between each course of shakes.

• Placement: Position the felt so its bottom edge sits 1" above the butt (thick end) of the shake course below. The felt laps over the shake course below the one you are currently installing.
• Purpose: It sheds wind-driven rain/snow that penetrates the shake joints. It is not a primary water barrier; the shakes are.
• Do not use synthetic underlayment as interlayment—it doesn't breathe, trapping moisture and causing rot. Run full-width No. 30 felt (or synthetic per mfr) over the deck first as your base underlayment.

Minimum Slope: 4:12 (18.5°) per IRC R905.7.3 and CSSB.

• 3:12 to 4:12: Allowed only with a double-course starter and interlayment felt extended 2" above the butt (CSSB "Low Slope" method). Expect shorter life.
• < 3:12: Prohibited for shakes. Use shingles (min 2:12 with special methods) or membrane roofing.

Critical First-Course Details:

1. Starter Course: Use a double layer of shakes (or shims + shakes) at the eave. Bottom layer butts flush with drip edge; top layer covers joints.
2. Overhang: 1.5" max past fascia/drip edge; 1" max past rake.
3. Spacing: 1/4" to 3/8" gap between shakes (swell room). Offset joints min 1.5" from joints in adjacent courses.
4. Fasteners: 2 corrosion-resistant nails (hot-dip galv, SS, or alum) per shake. 3/4" from edges, 1" above the interlayment line (so next course covers heads). Penetrate deck 3/4" min.

Ventilation: Cedar must dry. Use spaced sheathing (skip sheathing) 1x4/1x6 @ 4"-6" OC, or solid deck with a ventilation matrix (Cedar Breather, etc.) per CSSB. Solid deck + felt only = premature failure.

Check local amendments. Steep slopes (>18:12) need special fastening.

Interlayment (also called underlayment or felt) in a cedar shake roof is a water-resistant barrier installed between the roof deck and the shakes. It’s not the same as standard asphalt shingle underlayment — cedar shakes require a breathable, permeable interlayment (often #30 asphalt-saturated felt or synthetic underlayment) to allow moisture to escape from the wood shakes while still providing secondary weather protection.

Key points:

• Purpose: Prevents wind-driven rain from penetrating the roof deck, allows shakes to dry (critical for cedar longevity), and meets code requirements for secondary drainage.
• Material: Use #30 asphalt-saturated felt (ASTM D226 Type II) or approved synthetic underlayment labeled for wood shakes/shingles. Avoid impermeable barriers like polyethylene — they trap moisture and cause rot.
• Installation: Laid horizontally, starting at the eaves, with 2” headlap and 6” sidelap (per IRC R905.7.4). Fasten with corrosion-resistant staples or nails, not adhesive.

Minimum slope: Cedar shake roofs require a minimum 4:12 pitch (4 inches rise per 12 inches run) per the International Residential Code (IRC R905.7.2). Lower slopes don’t shed water adequately and increase leak risk — shakes rely on gravity and overlap, not a continuous membrane.

Important: Always check local amendments — some jurisdictions require steeper slopes or specific underlayments. If your roof is below 4:12, cedar shakes are not code-compliant; consider alternatives like metal or synthetic shake.

Never install cedar shakes over solid sheathing without ventilation or interlayment — moisture buildup will destroy the wood in years. If unsure, consult a licensed roofer or building official — improper installation voids warranties and risks structural damage.