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Are your home’s walks, siding, or deck getting a bit grimy? One solution may be a good power washing.

Power washers put the water pressure from garden hoses on steroids. When inserted into a power washer, the 50 pounds per square inch (psi) of water pressure provided by most hoses leaps to 1,000, or maybe even 4,000 psi on large units used by contractors. This concentrated spray can scour away dirt, mold, and paint from any surface—but too often scours away some of the surface itself. The water jet is also powerful enough to tear flesh. So it makes sense to use care when choosing a contractor or when doing the work yourself. Here are some tips.

Do It Yourself or Hire Someone?

Power-washing equipment used to be for contractors only—big, noisy, powerful machines that were too expensive and unwieldy for home use. But manufacturers now produce affordable, low-end models that are compact, lightweight, and easy to use. These models will supply enough power for DIYers and almost all power-washing projects, and are available at tool rental shops.

But just because you can get a model you can handle doesn’t mean you’ll want to take on the task. Like painting a house, power washing doesn’t require a great deal of know-how, but it does require a great deal of time: if you’ve never done it before, you’ll be surprised how long it can take to spray clean a moderately dirty patch of siding or deck.

Seeking Good Help

While there are specialty power-washing services, many house painters, landscapers, siding contractors, and others also routinely do this work. So if you have a positive track record with one of those contractors, that’s a good place to start. You can check for recommendations from subscribers by looking under any of those service categories here at Checkbook.org. Ask any company you are considering if it owns its own power washing equipment; that might help keep the price down and ensure that the company has some experience with the work.

If you are starting from scratch with a service, ask it to provide you references in your neighborhood. And ask each contractor you’re considering to provide proof of both worker’s compensation and liability insurance.

Shop the Job Around

power washing a patioTo make sure you get a good price, ask at least three contractors to submit fixed-price bids for the work. Provide each company with the same job specs. “Clean the siding” may seem to cover it. But you may as well lay out all the details, even if they seem obvious. For siding, specify which walls, and include trim, soffits, and facias. For gutters, specify that water should flow free afterward. For decks, include railings, balusters, and steps.

Finally, insist that all contractors agree to protect exterior lights and outlets from damage and be responsible for other damage they might cause. In the case of broken windows, it will be clear who is responsible. But there are gray areas—for example, decking that has been power washed clean but has had so much soft wood scoured away that its surface has become corrugated and uncomfortable to walk on in bare feet. Try to address these more subjective issues with an agreement that states all materials shall be left without structural or surface damage that alters their appearance.

Once you’ve chosen the contractor, get all the terms in writing. Most power-washing projects are one-day events, so you probably won’t want to bother with an elaborate contract—a signed copy of the specs you gave contractors plus the company’s fixed price should do. No matter how big the job, it’s reasonable to expect that all payment is reserved until the job is done.

Tips for Doing It Yourself

Be careful. Although most power washers are point-and-shoot machines that don’t require a lot of finesse, you can still do significant damage to your home, yourself, and others if you don’t observe some cautions. Be sure to take the time to read equipment instructions. Below are some basic guidelines—

  • Guard against electric shock. The Consumer Product Safety Commission (CPSC) reports that almost 2,500 injuries related to power washers require emergency room treatment every year. Do not use adaptor plugs that might defeat grounding systems, wear rubber-soled shoes, and keep high-pressure spray away from electric wiring, receptacles, and lights.
  • Never test water pressure against your hand or foot or point the wand at a person. Even at low settings, the water pressure from equipment can tear flesh.
  • Wear eye protection.
  • Maintain a distance of at least eight or 10 inches between the spray-wand nozzle and the surface being cleaned. Avoid spraying upward under siding and flashing; try to angle the spray down and away from doors, windows, and vents.
  • Match the pressure to the material and the job. Start with a diffused spray and then dial it up to strike a balance: enough force to clean the surface, but not enough to cause damage. Unfortunately, there are no set rules—so much pressure for concrete, so much for brick—because even similar materials vary in hardness, level of deterioration, and how they were installed. A wall of old, brittle, wood shakes may become kindling at the same pressure that handles mold on newer siding with no problem. It will take a little time to figure out the right match of pressure and materials.
  • Seal vents. If you accidentally shoot high-pressure water into your attic, you might cause mold problems.
  • Make sure the unloader valve is operating to divert pressure when you’re not spraying. This key part of a power washer is like the pressure release valve on a water heater. It keeps the system from overloading, breaking, or worse. It kicks in when the trigger is released, the wand is no longer spraying, and the machine is still pumping high-pressure water with nowhere to send it. The valve creates a diversion loop that cycles the flow back to the inlet side of the pump. If the valve malfunctions—you’ll hear a chattering sound if it happens—you should have the valve replaced with one that has equal or better ratings than the original in both gallons per minute (gpm) and pressure (psi).