Soft wash vs pressure wash: which surfaces need which

More pressure means a cleaner house, right? It's the most natural assumption in the world, and it's how plenty of roofs get stripped, siding gets flooded from behind, and cedar decks end up looking like they grew fur. The right question isn't how much pressure you have. It's what surface you're pointing it at.

Why blasting a roof backfires

Those dark streaks running down a shingle roof are usually not dirt. On most asphalt roofs they're a type of algae that feeds on the limestone filler in the shingles and spreads downhill with every rain. Moss and lichen show up in shadier spots, usually as green or gray clumps.

High pressure knocks the streaks off, but it takes the granules with them. Those little ceramic-coated grains on top of every shingle are the shingle's sunscreen. They shield the asphalt from ultraviolet light and help it shed heat. Strip them and the asphalt underneath dries out, cracks and curls years sooner than it would have. Shingle makers generally warn against pressure washing for exactly this reason, and damage from it may not be covered under the warranty.

There's a second problem: the algae isn't dead. Blasting only removes what's on the surface, and the growth rooted in the texture of the shingle comes back, often within a season or two.

What a soft wash actually is

A soft wash flips the approach. Instead of force, it relies on chemistry and patience:

1. Pre-wet the plants. Beds, shrubs and grass near the house get soaked with plain water first, so their leaves and roots absorb water instead of cleaner.
2. Apply the solution at low pressure. Usually close to what a garden hose puts out. The mix is typically a diluted chlorine-based cleaner with a surfactant that helps it cling.
3. Let it dwell. The solution needs a few minutes to kill the algae, mildew or moss at the root. Good crews keep it from drying on the surface during this time.
4. Rinse gently. Siding gets a low-pressure rinse. Roofs are often left for weather to finish rinsing, depending on the mix and the contractor.
5. Rinse the plants again. A second soaking dilutes anything that dripped or drifted onto them.

Because the growth is killed instead of scraped, the surface usually stays clean longer than it would after a blast.

Surface by surface: which method fits

Here's where each method belongs on a typical house. The pressure figures are general ranges; the right setting depends on the equipment, the nozzle and how far the wand is held from the surface.

General guidance. Surfaces, products and equipment vary, and a pro should check the condition first.

That last row deserves a note. Even on concrete, a bare wand held by hand leaves overlapping "zebra stripes" where some passes cleaned harder than others. A surface cleaner, the round spinning attachment that rides on the slab, cleans at an even distance and gives the uniform finish people are actually paying for.

Protect your plants, pets and drains

The same solution that kills algae on a roof can burn a hydrangea. A careful contractor will soak plantings before and after, cover delicate ones, and keep the mix at the lowest strength that does the job. Ask about it before the work starts, and move potted plants, toys and pet bowls out of the way.

Runoff rules are local, and many cities restrict what can go down the storm drain at the curb, especially water with cleaners in it. A professional should know the rules where they work. If you're doing a small job yourself, direct the rinse water onto the lawn rather than into the street.

Questions to ask before you hire

• Will you soft wash the roof, or pressure wash it? Only one right answer for asphalt shingles.
• What's in your solution, and how do you protect plants? A pro answers plainly and describes pre-wetting and rinsing.
• Do you walk the house first? Loose siding, failing window seals and peeling paint should be pointed out before anyone turns on a pump.
• Are you insured? Ask for proof of liability coverage. Accidents with ladders and chemicals happen.
• What will it look like right after? An honest contractor tells you roof streaks may take days of weather to fully fade.

A lower price from someone planning to hit the whole house with one wand at full blast isn't a bargain if the shingles lose years of life. For a sense of what goes into a fair number, see how pros approach pricing a roof cleaning.

For pros: sharing this with customers

Homeowners who've never heard of soft washing often assume it's a way to charge more for less work. A short explanation of granules, algae and dwell time turns that doubt into trust, and it separates you from the guy with a rental machine and a turbo nozzle. Send this before a roof or house wash so the customer knows why you won't be blasting, why you're soaking the flower beds, and why the streaks may need a week to fade.

It also helps new crew members. The mistakes that turn into damage claims, like skipping the walk around or letting runoff hit the drain, are covered in three mistakes new pressure washing owners make. The rest of the business side, from seasonality to the calls that come in each spring, is on the pressure washing page.