Pressure Washing glossary

PSI (pounds per square inch) - the force of the water stream. Higher is not better. Fixr's figures span roughly 500 PSI for brick and softwoods to 3,500 for concrete.

GPM (gallons per minute) - the flow rate. NPS lists gallons per minute alongside pressure and nozzle distance as variables that significantly affect the outcome of a water cleaning process.

Soft washing - applying a diluted cleaning solution at low pressure and letting chemistry do the work, then rinsing gently. The correct method for roofs, most siding, wood and painted surfaces. Fixr prices house soft washing at $0.25 to $0.35 per square foot.

Power washing - pressure washing with heated water. Fixr prices it slightly above cold washing on fences, at $0.35 to $0.45 versus $0.30 to $0.40 per square foot, and notes heat can cause minor damage to older wooden fences.

Dwell time - how long a cleaning solution is left on the surface before rinsing. ARMA's roof method uses 15 to 20 minutes; Today's Homeowner's chlorine mix uses 25 to 30 minutes and its bleach mix 15 to 30 minutes.

Sodium hypochlorite - the active ingredient in chlorine bleach and the basis of most soft wash solutions. ARMA's recommended roof mix is 50:50 laundry-strength liquid chlorine bleach and water.

Granule loss - the shedding of the protective mineral surface from asphalt shingles. ARMA warns pressure washing causes it and very likely premature roof failure; Malarkey describes granules as a shingle's primary defense against damage and UV aging.

Surface cleaner - a rotating attachment that spreads the spray evenly over flatwork like driveways, giving a consistent finish without the striping you get from freehand wanding.

Etching - permanent abrasion of a hard surface by too much pressure. NPS warns that high-pressure water can easily etch marble, other soft stones and some types of brick.

Effluent - the runoff from cleaning. NPS's practical test: if it contains grains of stone or sand, the pressure is too high.