If you've ever used a pressure washer to clean a deck or concrete walkway, you know that there's no faster way to remove caked on dirt and algae while at the same time removing old, flaking paint. Given the effectiveness of the technique, it seems to make sense to pressure wash your siding, and many homeowners do just that. However, professional contractors caution that, depending on your siding, this is not necessarily a great thing to do. You could end up damaging the siding and forcing water underneath it, and that could cause problems with rot and mold. Not to mention the possibility of creating airborne flecks of lead-based paint, which pose a danger to your neighbors as well as yourself.
Check for Lead
If you're pressure washing a house that was built before 1978, there's a good chance that some of the paint contains lead. The pressure washer is bound to remove some of this paint, and airborne flecks may settle on your lawn as well as those of your neighbors, where curious children and pets could find them. Not only that, you'll end up inhaling some of these flecks yourself. The toxicity of lead is well established. It causes behavioral and developmental complications in children and a host of physical problems in adults. Repeated ingestion can cause death.
To avoid such hazards, test the house paint for lead before you power wash. You can purchase a lead test kit online or at your local big box store. If the test is positive, don't power wash. Scape off old paint manually and collect it for disposal, then scrub the siding with soap and water.
Choose the Right Sprayer and the Right Nozzle
You don't need or want extremely high pressure when washing siding, so you don't need a contractor-grade pressure washer. Several manufacturers offer electric models that develop just enough pressure for general cleaning, and they do a fairly good job on decks and fences as well as on siding. Electric models are easier to use, too. No need to mix gas and pull a starter cord. You can buy an electric pressure washer for just a bit more than it costs to rent a gas-powered one.
Quick-connect pressure washer tips are color coded to denote the width of the spray pattern. The ones you need to clean your siding are white (40 degrees) and green (25 degrees). If you use your pressure washer to apply soap, use the black tip. Its 65-degree pattern is too wide for cleaning but perfect for wetting down the siding. Avoid the yellow tip (15 degrees,) and under no circumstances use the red tip (0 degrees), which is guaranteed to damage your siding. If your pressure washer comes with a turbo tip, it's fine to use this on siding.
Although it isn't recommended, you may want to use the pressure washer to give your windows a quick once-over. If so, use the white or black tip. Even if you're not cleaning the windows, it's best to use a wide-angle tip when you're working close to windows and can't avoid getting some spray in them.
Use Low-Pressure Spray
Anyone who has experience with a pressure washer has seen the damage it can do. It's important to keep the tip 2 to 6 inches away from the surface you're cleaning, depending on the nozzle and pressure, or the high-pressure spray can splinter wood and even form depressions in masonry surfaces. It can also chip mortar from between bricks, especially if the mortar is old. The effects of high pressure spray on vinyl siding can be even more serious. Vinyl gets brittle as it ages, and it can crack or shatter under the force of a pressure washer.
Most electric washers deliver an appropriately low-pressure spray, but if you use a gasoline-powered machine, you'll need to dial the pressure down manually. A spray pressure between 1,500 and 2,400 psi is about right. To compensate for less efficient dirt removal at low pressure, precede spraying by applying a soap solution to emulsify dirt and make it easier to remove. Most pressure washers have a soap reservoir and can automatically create a soap and water solution. Check the instructions on your model before using a bleach-based cleaner. Bleach can deteriorate rubber parts, and many manufacturers recommend against using it.
Avoid Pointing the Sprayer Upward
High-pressure spray has a way of finding gaps in the siding. Water can soak the protective layer underneath siding and may even penetrate that, as well. If moisture gets trapped, it feeds mold and rot fungi, eventually making it necessary to remove the siding altogether to make repairs. Clapboard, shingles and siding tiles are most vulnerable to this type of damage, but it can be a factor with other types of siding, too. Water can find gaps underneath trim boards on plywood and board-and-batten siding, and it can even enter through nail holes.
To best way to prevent water incursion is to hold the tip of the sprayer above the spot you're spraying and point the tip downward. This prevents water from getting underneath horizontal planks. If your house has tile siding, the water may run between the tiles, but the pressure will force it to run off. Never point the sprayer upward when washing clapboard or tiles. Besides forcing water underneath, the pressure could blow off one of the planks or tiles.