As a former licensed Michigan Builder and certified IICRC Water Damage Restoration contractor, I have seen my share of what water can do to a home. The photo above was taken from a trip to the Gulf Coast after Katrina. These days I no longer do water restoration work, but I still occassinally see signs of water damage while window cleaning. Hurricanes are not preventable of course, but hiring a pro to power wash your home can prevent problems later.
Pressurized water sprayed at 3.5 to 10 gallons per minute when forced between your home's siding and sheathing can become trapped in some circumstances This can lead to wet rot and invite critters that use moist wood for their own building projects or consumption. Good process is to use lower pressure and "arch the spray" or else shoot from the top down. Today's technicians have telescoping poles to reach the peaks of your home. I get a lot of calls asking for power washing with my window cleaning business. I know how to do power washing correctly. I don't take it on because doing it correctly is a very physical project and I already have plenty of hard work to do with Window Cleaning.
But if you are going to have your place power washed here are a few tips. Find someone with experience, call a pro. Anyone can go pick up a power washer at Home Depot and call themselves a power washer. The equipment used by the pros today is more advanced. For example they have chemical injection systems for just the right application.
The technician knows how to calibrate the system with flow so that what they put down (dose) to meet the soil (demand) and leave little residue (what is left over). Vinyl siding is designed to keep water off your home that falls from the sky, it can even handle rain that comes at it from a side wind. Even water that gets blown behind siding should drain in normal circumstances. Problems occur when water is sprayed at high pressure up onto siding vertically. For one thing this practice may loosen fasteners. That is why the pro's use extended poles and/or a ladder and/or spray at a downward trajectory.
Another tip is to always check to make sure vinyl or aluminum siding has not been installed over old wood siding. Water pushed up in such cases becomes trapped and causes all manner of problems for the homeowner. Don't powerwash a home in that circumstance.
If you do power wash yourself, dial the pressure on your machine way down and if necessary use a bit of soft agitation (a china brush, or a sponge, something softer than the material being cleaned). Never use anything abrasive on the siding. If you have a tan or gray colored vinyl siding scratches may not show, but if it is one of the new reds, yellows blues, or greens it is a good bet the damage from using the wrong agitation device (life a stiff brush or an abrasive pad) will show up.
There is a large disparity between one siding product and another. Every builder saves a bit on cost someplace. The professional cleaner will carefully test the siding in an inconspicuous place. For most projects, a mild detergent, ladder, a hundred foot or so of garden hose, and a sack of terry towels will suffice. Rust removers and other cleaning agents may be used following label instructions. Sodium hypochorite (bleach diluted with water) is popular for removing mildew. Pre-rinse, lather up, scrub and final rinse. If do decide to DIY, be sure to tightly close the windows and to be careful around the shrubs.
Beware: Watch overhead for where the electrical service comes in, it is either underground or overhead. Cover lamps with a plastic bag. If you use bleach, don't track it inside and rinse it off well. When bleach dries, it leaves a salt that reactivates when it gets wet. Don't track that mess into your home!
Similarly, wood decks require under 500 psi to avoid raising the grain. Cement is a whole other topic. Now you have some practical information. If you hire out the job, ask a few questions now that you understand the process better. I will be Happy to do your windows but no matter how many times you ask me to do power washing the answer will always be no. I don't do power washing because I already specialize in window cleaning and if I tried to do both in a market the size of Greater Grand Rapids, I would just burn myself out eventually,