Signs your house, roof or deck needs cleaning, and signs it should not be pressure washed
Updated · KeystoneProGuide editorial team
What are the signs a house needs washing?
Most signs are visible growth or grime, and the more useful question is what is causing it. The U.S. Environmental Protection Agency (EPA) says mold damages what it grows on and that the longer it grows, the more damage it can cause. It also says no mold will grow without water or moisture. So a stain on siding or a roof is a reason to clean and a reason to look for the wet spot behind it.
The signs below are grouped by surface. Each section ends with the conditions in which the sources say to stop and think before anyone picks up a wand.
Dark streaks on the roof
Brown to black streaks on a shingle roof may be algae. The shingle makers' trade association, ARMA, says the discoloration may be uniform or streaked, is most visible on light-colored roofs, and is often mistaken for soot, dirt, moss or tree droppings. It names one species, Gloeocapsa magma, as probably the most common, and says the algae travel through the air. Moss and tree droppings, by contrast, usually leave only local patches.
ARMA offers only a temporary fix. It says a household bleach solution may lighten the stain, that the effect does not last, and that the discoloration may come back.
Hold off if the plan is to pressure wash the shingles. ARMA says not to use a power washer, or any brush or broom, on algae, because high-pressure systems are likely to damage asphalt roofing and scrubbing may loosen granules.
Green, gray or black spots on siding
Mildew spots on siding can often be handled gently. The vinyl siding industry's association, PEPA, treats them as routine: for larger areas of mold and mildew it suggests a solution of 30% vinegar and 70% water, and it says a rinse with a garden hose is often enough for everyday dirt.
The place where growth keeps returning is the clue. The EPA lists cleaning and repairing gutters regularly among ways to keep moisture problems away, so a streak below an overflowing gutter calls for the gutter to be fixed as well as the wall to be washed.
Hold off if you plan to paint soon. The EPA says paint applied over moldy surfaces is likely to peel, so the mold has to be cleaned and the surface dried first.
A gray, worn deck
The time to clean a wood deck is when its finish is due to be renewed. A chemist at the U.S. Forest Service's Forest Products Laboratory said a deck must be cleaned before it is refinished, and the laboratory's release gave typical lives for the finishes: about one year for clear water repellents, about two more years for tinted ones, and about four to six years for semitransparent stains. When the finish has worn through, a cleaning is the first step back.
Hold off if the cleaner or the pressure is aggressive. The same chemist warned that some cleaners are strong enough to pulp the wood surface, that laundry bleach tends to do this, particularly on cedar, and that a power sprayer can damage the surface. He gave a sign to watch for: lots of loose fibers washing away as you rinse.
Peeling or chipping paint
Peeling paint is a sign the surface needs work, but on an older house it is also a warning about how that work is done. The EPA says paint that is not maintained can peel, chip or crack and become a hazard, producing chips and dust. A home built before 1978 may have lead-based paint, and the older it is, the more likely it has lead-based paint.
Hold off if the house is pre-1978 and the company is not lead-safe certified. The EPA requires anyone paid to disturb painted surfaces on such a home to be certified, and says that if you cannot find out when the house was built, the safest course is to assume the paint contains lead.
Soft or crumbling brick, stone or mortar
Grime on brick is easy to see; the condition under it is the part that matters. Before cleaning historic masonry, the National Park Service asks whether the surface is soft, powdery or sugaring, and says some historic materials can be damaged even at low pressure. It describes low-fired historic brick as having a soft, porous interior beneath a protective fire skin, and lists lime mortars among sensitive materials.
Hold off if the brick or mortar is soft or powdery, or if the company plans high pressure. The Park Service says excessive pressure can remove surface material and scar historic brick, stone or concrete, and recommends a test patch before any large-scale cleaning.
Signs a wash went wrong
- Loose fibers on a wood deck. The Forest Products Laboratory chemist called this the sign of excessive pulping.
- Loose granules after a roof cleaning. ARMA warns that friction may loosen and remove them.
- Water inside after a siding wash. PEPA says aiming a washer upward can force water behind the panels.
- An injury that looks minor. The Centers for Disease Control and Prevention says spray wounds can be serious even when they first appear minor, and urges prompt medical attention.
How often to wash covers timing, and whether to do it yourself covers which of these jobs suit a homeowner. Companies offering the service are on the pressure washing directory page.
Sources
- Algae Discoloration of Roofs, Asphalt Roofing Manufacturers Association (ARMA)
- How to Clean Vinyl Siding, Polymeric Exterior Products Association (PEPA)
- Mold Cleanup in Your Home, U.S. Environmental Protection Agency
- A Brief Guide to Mold, Moisture and Your Home, U.S. Environmental Protection Agency
- Springtime Means Deck Maintenance for Many Homeowners, USDA Forest Service, Forest Products Laboratory (news release via Newswise)
- Lead Renovation, Repair and Painting Program, U.S. Environmental Protection Agency
- Lead-Safe Renovations for DIYers, U.S. Environmental Protection Agency
- Preservation Matters: Maintenance - Graffiti Removal from Historic Features, National Park Service
- Protecting Historic Structures from Oil Contamination, National Park Service
- Pressure Washer Safety, Centers for Disease Control and Prevention