Sometimes, a pressure washer is too much of a good thing. Here's how it might help or harm your deck, roof, car, and more, according to CR's experts.

A pressure washer makes quick, satisfying work of blasting away gunk. Whether you’re cleaning your walkways or stripping old paint from a deck, nothing compares to the unbridled power of these machines.

Pressure washers (aka power washers) can be gratifyingly money-saving, too. With the rising cost of construction materials these days, power-washing an old, stained deck can alleviate the need to buy a new one—and potentially save you thousands of dollars.

But a pressure washer is not the right tool for every job, and it’s easy to get carried away. “You might be inclined to pressure-wash just about everything around the house, but that’s not always a great idea,” says Dave Trezza, the test engineer who oversees pressure washer testing for Consumer Reports.

The powerful stream of water emitted by these machines can damage paint. It can also nick or etch wood, he says, and even certain types of stone.

What’s more, these machines can be quite dangerous when handled improperly (see “Pressure Washer Safety ,” below).

So when should you break out the pressure washer—and when would a scrub brush and garden hose do? We asked CR experts, who’ve evaluated and rated over three dozen pressure washer models, to weigh in on a range of common surfaces. Here’s what they said.

It depends. Composite decks from brands such as TimberTech and Trex often resist deep staining, and clean nicely with a light scrub, followed by a rinse with a garden hose. But if that isn’t enough, check the terms of the warranty to make sure pressure-washing won’t void it.

Decks made from South American hardwoods, such as ipe, cumaru, and tigerwood, will hold up to a power wash just fine. Those made of pressure-treated wood—typically Southern yellow pine, a soft wood— are generally okay, too, assuming you don’t hold the nozzle too close.

Check your owner’s manual to confirm which nozzle and setting the manufacturer recommends for decking, and how far away from the surface it should be held. Generally speaking, it’s best to start with a low-pressure nozzle on an inconspicuous spot to make sure the spray is not etching or marking the wood. Once you find the right nozzle, setting, and distance, work along the length of the board, following the grain of the wood.

Gas or electric pressure washer? This is a relatively light job, so a corded electric washer, like the one below, is all you need. Recommended pressure: 800 to 1,000 psi

No. Tempting as it might be to blast away unsightly moss and algae, using a pressure washer to clean your roof is dangerous, not to mention potentially damaging. We never recommend using a pressure washer while you’re perched on a ladder because blowback could throw you off balance. The powerful stream of water can also loosen roof shingles, and if your shingles are asphalt, it can strip them of the embedded granules that help extend the life of your roof.

Instead, spray down the roof with a cleaner that kills mold and moss, or apply a 50-50 mixture of bleach and water in a pump sprayer and let the moss die on its own. Make sure to build up pressure in your pump sprayer from the safety of solid ground before climbing a ladder to spray your roof.

If there’s an excessive amount of shade, trim overhanging branches or cut down trees to allow sunlight to hit the roof. That will keep moss from growing in the first place.

No. Plenty of people use a pressure washer to clean their car, but it can do more harm than good. Using a pressure washer can damage or nick the paint, which could lead to rust. A car wash will usually get the job done—so will a garden hose and soapy sponge. Use a little elbow grease and a specialized wheel cleaner on the wheels and other problem spots.

Yes, if it’s concrete. Concrete driveways (and walkways) can readily withstand a powerful cleaning without etching. Generally, a finer nozzle will be more effective at spot-cleaning grease stains. For mold or mildew, use lower pressure and coat the surface in suds first.

Gas or electric pressure washer? Any machine capable of producing at least 1,500 pounds per square inch (psi) of pressure can clean concrete, but the work will go much faster if you choose a model producing 2,500 to 3,000 psi, like the gas-powered option below.

Sometimes. Vinyl siding is pliable and can typically withstand pressure washing without much concern. The same goes for fiber cement siding. Aluminum siding, however, can dent, so it’s best to start on the lowest pressure setting with a broad nozzle, and save more concentrated blasts for problem spots.

Wood clapboard siding can be effectively washed, too, but if your house was built before 1978, have the exterior paint tested first by a lead-remediation specialist licensed by the Environmental Protection Agency. If you knock old lead paint loose, it will settle permanently in your soil, where it can get kicked up by children playing outside or can be tracked into the home.

Whatever the material being pressure-washed, make sure to prevent water from becoming trapped between the siding and your home’s sheathing, because the moisture promotes mold. Repair or replace loose, damaged, or missing siding, and take special care not to spray water directly into any gaps around doors, windows, or under the lap joints on siding runs. Don’t pressure-wash shingle siding—the pressure can knock the shingles loose.

Gas or electric pressure washer? If you’re looking to do this job just once every few years, you can probably rent a pressure washer. Or the budget-friendly electric model below will do the trick without breaking the bank. Recommended pressure: 1,200 to 1,500 psi

Pressure washers come with nozzles ranging from 0 degrees to about 40 degrees. The higher the number, the wider the spray pattern, and the less concentrated (and potentially dangerous) the stream of water. Consumer Reports recommends against using a 0-degree nozzle because it poses an unnecessary safety risk. Water concentrated to such a fine point can pierce a range of surfaces, including protective boots.

If your pressure washer came with a red 0-degree nozzle, toss this nozzle out. The next size, a 15-degree nozzle, will do just fine for detail work, such as removing moss from the grooves between pavers. And always wear hearing protection, safety goggles, and protective footwear and gloves while you work.

To test pressure washers, including the models here, we evaluate how much pressure each model can produce. Those that generate a higher level of pressure (measured in pounds per square inch) receive a higher score than those with a lower pressure output.

We measure how long each pressure washer takes to strip paint from painted plastic panels. The shorter the time, the higher the pressure output. And because almost all pressure washers are loud—you’ll need earplugs or an earmuff-style headset to use them—we assess each machine’s noise levels, too.

Finally, we evaluate how easy a machine is to use. Gas-powered machines with features that make them easy to fuel up earn higher ratings than those that don’t have those features. Pressure washers that automatically shut off when the oil is low—which prevents overheating—rack up points as well.

Regardless of a model’s performance, CR recommends only those that do not include a 0-degree nozzle, which we believe poses an unnecessary safety risk to users and bystanders.

Is your deck looking tired and dingy? On “Consumer 101,” Consumer Reports test engineer Dave Trezza demonstrates the correct best way to revive your outdoor space.

More from Consumer Reports: Top pick tires for 2016 Best used cars for $25,000 and less 7 best mattresses for couples

Consumer Reports is an independent, nonprofit organization that works side by side with consumers to create a fairer, safer, and healthier world. CR does not endorse products or services, and does not accept advertising. Copyright © 2022, Consumer Reports, Inc.

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