Costs vary widely based on roof size, materials, complexity, and current condition. Residential roof replacements in the Orange County and LA area typically range from $3.50 to $14 per square foot. Commercial flat roof replacements range from $5 to $10+ per square foot depending on the system type, tear-off requirements, and insulation needs. Factors that affect price include whether you need structural repairs, the number of existing layers, accessibility, permits, and disposal costs. Get detailed written estimates from licensed contractors to compare accurately.