Codes, Standards, and Regulatory Compliance
The rulebook hierarchy
Know the hierarchy: model codes (e.g., International Residential Code, International Building Code) are adopted by jurisdictions, often with local amendments. Manufacturer instructions are typically required to be followed by code. Standards organizations - ASTM, UL, ANSI, NRCA - publish test and installation standards referenced by code.
Know the key code topics for roofing: minimum slope, underlayment requirements, ice barrier in cold climates, ventilation (1/150 or 1/300 net free area rules), fire classification of roof coverings, wind uplift requirements, and drip edge requirements. Know that permits and inspections are required for reroofs in most jurisdictions, and that re-roofing over existing material is limited by code (often a maximum of two layers).
Traps: assuming a model code applies as written (local amendments may change it); confusing code minimum with manufacturer requirement (the stricter governs); and forgetting that ventilation is a code requirement, not an option.
Worked example: a 1,500 sq ft attic with a 1/150 ventilation ratio needs 10 sq ft of net free area, split between intake and exhaust. Using 1/300 (with a vapor barrier) halves that to 5 sq ft. Verify the adopted code.
Rule of thumb: the stricter of code or manufacturer governs - and always check local amendments.