How Often Should You Wash the Exterior of a Boston Home?

What's Growing on Boston Home Exteriors

The dark green, black, or gray growth on Boston home exteriors is primarily biological: Cladosporium mildew on siding and painted surfaces, Gloeocapsa Magma algae washing down from roofing, and various moss species on horizontal surfaces and north-facing areas. Boston's combination of high humidity (annual average 72%), 43 inches of annual precipitation, and extensive urban tree canopy creates near-ideal conditions for biological growth on exterior surfaces. This isn't a failure of the house — it's just Boston. NWS Boston climate data →

Exposure-Based Frequency Guide

Full sun exposure with good air circulation in a suburban Newton or Needham yard: cleaning every 3–4 years before significant biological growth establishes. Partial shade with moderate tree cover in Cambridge or Somerville: cleaning every 2–3 years. Heavy shade, north-facing exposure, close building spacing in Allston or Brighton: annual cleaning or every 18 months to prevent established growth from staining and degrading the surface finish. The practical indicator: run your hand along the siding — sliminess or visible green discoloration means it's time. PWNA exterior cleaning standards →

Pre-Paint Washing: Always Required, No Exceptions

Regardless of when the house was last washed, washing is required immediately before any exterior painting or staining. Painting over mildew — even mildew not visibly obvious — is the most common cause of premature exterior paint failure. Mildew inhibits paint adhesion and continues growing through the new paint film. We wash every exterior surface before painting, apply a mildewcide rinse on north-facing and shade-exposed surfaces, and allow full drying (48–72 hours) before any primer or paint application. This step adds to the project timeline but is non-negotiable for durability. Benjamin Moore exterior prep guidelines →

Annual Spring Inspection for Boston Homeowners

The most practical approach: inspect your home's exterior every spring after snow melt. Boston's winter creates conditions for mildew to establish on protected surfaces (under eaves, on sheltered north walls) that aren't exposed to cleaning rain. A spring inspection identifies growth that's begun establishing before it becomes entrenched. Washing early-stage mildew is far easier than addressing heavy established growth. Annual spring inspection with washing when needed — combined with keeping gutters clean and vegetation trimmed away from the house — keeps Boston home exteriors in good condition between paint cycles. EPA safer cleaning products →