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Ask the Carpenter: How to use a pressure washer properly

Ask the Expert
Start spraying from a distance of 4 feet and work your way closer until you get the desired effect.
Start spraying from a distance of 4 feet and work your way closer until you get the desired effect. jpldesigns

When it comes to heavy duty cleaning outdoors, nothing does the job faster than a pressure washer. Here’s how to use one correctly:

Using a pressure washer to clean siding

I want to be clear that I’m not a fan of pressure washing houses; I prefer hand scrubbing, which is time consuming but thorough. I know many painting contractors who pressure wash because it’s fast and easy, but some people do it the wrong way. Some painters and pressure-washing companies will wash a house from the ground up, directing their spray up the siding. Wood siding is installed in an overlapping fashion. It is designed to keep out falling water, not water sprayed upward. When water is shot upward against siding, it can get behind it. Then it can take a week for the siding to dry out properly.

If you’re going to use a pressure washer, it should be from above, while standing on a ladder or lift, spraying downward. The same thing goes for your roof; spraying up under the shingles can drive water under your roof, creating a leak and promoting rot.

Choosing a nozzle

Selecting the correct nozzle is important to get the best results. Nozzles are marked by spray pattern and are often color-coated to delineate the width of the fan. A wide spray (40 degrees) is good for soft materials like wood. A thin stream (0 degrees) is good for tough stains on concrete or to strip offending buildup from a surface or weeds from cracks in driveways.

Spray pattern

The spray pattern you use is important and can significantly affect your results. Start spraying from a distance of 4 feet and work your way closer until you get the desired effect. Don’t get too close to the work surface, however; you may damage it. Once you find the appropriate distance, maintain it throughout your cleaning.

Work from the top down, using an overlapping pattern to prevent streaking. Be sure to spray at an angle, because a direct hit from a powerful washer can force dirt and grit into the surface rather than wash it away.

Detergents and chemicals

If you are using a detergent, degreaser, or any other chemical to accelerate your cleaning, you should prepare the area around your work surface and ensure you are wearing the appropriate personal protective equipment.

Protect yourself

Always wear eye protection when operating a pressure washer. When you are using harsh chemicals, wear gloves, long sleeves, and pants to prevent direct contact with your skin.

Protect your vegetation

To mitigate the effect of chemicals on your landscaping, you can presoak your plants around the cleaning area with a generous amount of water to dilute any possible contamination. Place a clear tarp over your plants. Be sure to minimize the amount of time they remain under the tarp on hot days.

Take extra care

Turn off the power to exterior outlets. A pressure washer is an extremely powerful tool, so be very careful near windows, electrical components like lights and outlets, and fragile components of your home or deck. And remember: A pressure washer could force water in and behind the exterior shell, accelerating rot and promoting mold.

Finally, treat the nozzle as you would a gun: NEVER point it at anyone or anything you don’t intend to spray.

Rob Robillard is a general contractor, carpenter, editor of AConcordCarpenter.com, and principal of a carpentry and renovation business. Send your questions to homerepair@globe.com or tweet them to @robertrobillard. Subscribe to the Globe’s free real estate newsletter at pages.email.bostonglobe.com/AddressSignUp.