Oh, the Pressure

Start cleaning "at the back of the deck and walk your way across, blowing the dirty water over the dirty [portion of the] deck," Beckman said. Use a wide-angle tip. Moving the nozzle in a side-to-side sweeping motion should lessen uneven cleaning or scarring. Using an approved cleaner will help lift dirt and eradicate mildew spores. After rinsing, wait for the deck to completely dry -- about 48 hours in warm, dry weather -- before staining or sealing it.

Cantor had additional advice. "Use a high dilution of water with whatever chemical you're using and constantly keep [the wood] wet. If you let it dry, it's going to destroy the wood [by drying] it out so much faster than you were prepared for." Cantor has seen novices clean decks with straight bleach, resulting in surfaces "dried out like a piece of fruit."

Cantor said a professional can clean the average 15-by-20-foot deck with stairs in about three hours. The undersides are not typically cleaned. He said his firm would charge about $350 to $375.

Materials

Avoid pointing the unit's tip underneath vinyl or aluminum siding. Doing so might blow the siding off or trap moisture underneath, creating a breeding ground for mildew and mold, Toledo said.

Cantor agreed. "We're totally against cleaning gutters with pressurized water because water can go underneath your shingles," he said.

Aluminum siding requires special precautions "because it's painted and you can strip off the paint," Beckman said. Harsh cleaning agents can discolor painted aluminum surfaces.

Toledo said many consumers are unsure about the proper cleaning of composite and exotic decking materials. Comparing them to "an exotic car," she said, "Our units are safe to [clean them] but, again, you have to be very careful." Use a low-pressure nozzle, stay at a safe distance and test an inconspicuous area first. When in doubt, contact the manufacturer or distributor, or hire a professional.

Prior to a production ban in January 2004, manufacturers of deck and playground equipment injected chromated copper arsenate -- an arsenic-based pesticide -- into pressure-treated lumber. Controversy continues over the proper care of this aging wood.

The District-based Environmental Working Group advises people not to pressure-wash CCA-treated wood, as the spray can spread residual arsenic. Instead, it recommends either replacing high-traffic surfaces with newer lumber treated with alkaline copper quaternary, or ACQ, or cleaning the deck with a soft brush. Seal it every six months. If a power washer is used, the Environmental Working Group suggests directing the spray away from gardens and play areas.

The Consumer Product Safety Commission suggests that people concerned about their CCA-treated decks manually clean them with soapy water and brush and seal them annually.

Safety

The top of a slippery two-story ladder is no place for an introductory lesson in pressure washing, said Mary Pat McKay of the American College of Emergency Physicians. The water's backward thrust could push a person off a ladder.

Accidentally raking the jet stream across a bare leg can cause a laceration or puncture wound. Other injuries are caused by forcible injection of cleaning fluids "deep into soft tissue," McKay said. "The tissue can just die because it's sort of blown apart by the pressure and then by the presence of the [cleaning] fluid in the tissue."

McKay suggested taking sensible precautions. Wear eye protection, keep water away from electrical sources, and don't operate a gas unit inside a home or garage. Damaged high-pressure hoses should be replaced, not repaired. Do not attempt to clean pets using a pressure washer.

Powermate's Beckman said only approved cleaners should be used in pressure washers. He recalled one incident in which a man tried to pressure-wash his car's engine using gasoline.

"Guess what?" Beckman said. "Now you've made a flamethrower. That's one of the warnings in our manual."