Short Answer
Roof-cleaning methods depend heavily on roofing material and manufacturer recommendations. Improper pressure washing can damage roofing materials.
Detailed Explanation
Pressure washing a roof is generally not recommended. High-pressure water can strip granules from asphalt shingles (significantly reducing their life), force water under shingles (causing leaks), crack or break shingles, and damage flashing and sealants. Some roofing materials (like metal or tile) may tolerate low-pressure washing better, but even then, caution is required. For algae and moss growth on asphalt shingles, the recommended cleaning method is typically a low-pressure chemical treatment (soft wash) that kills the algae without damaging the shingles. Manufacturer installation instructions and warranty documents may specify approved cleaning methods — using unapproved methods can void the warranty. Before cleaning a roof, check the manufacturer's recommendations and consider hiring a professional who specializes in roof cleaning.
Why It Matters
Pressure washing can severely damage asphalt shingles and void the warranty. Understanding the risks prevents homeowners from destroying their roof while trying to clean it.