The one-line rule
If you would not want a firehose pointed at it, it needs soft washing. Concrete can take pressure. Your roof, your stucco and your siding cannot.
Surface-by-surface
- Concrete driveway, sidewalk, curb: pressure washing with a surface cleaner
- Pavers: reduced pressure, then re-sand joints
- Asphalt shingle or tile roof: soft wash only - pressure strips granules
- Stucco: soft wash - pressure forces water into the wall system
- Vinyl and painted siding: soft wash - pressure chalks and pits it
- Lanai screens and pool cages: soft wash at very low pressure
- Wood decks and fences: low to moderate pressure, with the grain
Why soft washing lasts longer
Pressure removes what you can see. Soft washing kills the algae and mildew colony at the root, so regrowth is far slower. On siding, soft wash results commonly hold for two to three years versus about one year for pressure alone.
Damage we get called to fix
- Zebra striping on driveways from wand-only cleaning without a surface cleaner
- Granule loss and bare spots on shingle roofs after pressure washing
- Water intrusion behind stucco and around window frames
- Chalked, pitted vinyl siding that cannot be restored
- Torn lanai screens from excessive pressure
Every one of those is permanent or expensive. Matching the method to the material is the entire job.