How to Pressure Wash a House: PSI, Technique, and What Pros Do Differently

The most common mistake homeowners make when washing their house is starting with the pressure washer set too high. The second most common is rinsing before the soap has had time to work. Both shortcuts produce the same result: a house that looks clean for a month and is noticeably dirty again by fall.

Here's how the process actually works — whether you're doing it yourself or evaluating a contractor.

First: Should Your House Be Pressure Washed or Soft Washed?

These are two different methods, and the right one depends on your siding.

Pressure washing uses mechanical force (high PSI) to blast contaminants off the surface. It works well on hard, durable surfaces: concrete, brick, and some types of masonry. On house siding, it's often the wrong tool.

Soft washing delivers cleaning solution at low pressure (under 300–500 PSI) and lets chemistry do the work. The surfactant and sodium hypochlorite mix kills algae, mold, and mildew at the root rather than just blasting off the surface layer. Results last significantly longer — typically 1–2 years on siding vs. 3–6 months for a pure pressure wash.

For most homes in Michiana, soft washing is the right approach for the siding. Pressure washing (at appropriate PSI) still handles concrete driveways, patios, and some masonry.

PSI Guide by Siding Type

| Siding Material | Safe PSI Range | Notes | |---|---|---| | Vinyl | 1,300–1,600 PSI | Never spray upward into seams — water enters the wall cavity | | Aluminum | 1,200–1,500 PSI | Dents easily; keep nozzle moving | | Wood / cedar | 1,000–1,400 PSI | Higher PSI "furs" the grain; go with 40° nozzle only | | Brick / mortar | 1,500–2,000 PSI | Avoid mortar joints; high pressure erodes them | | Stucco | 1,200–1,500 PSI | Stay 24+ inches from surface; cracking is irreversible | | HardiePlank (fiber cement) | Garden hose + brush | James Hardie recommends against pressure washing; use soft wash only | | Painted surfaces | 1,200–1,500 PSI | Anything higher strips paint, especially on older homes |

A note on pre-1978 homes: If your home was built before 1978, there's a real chance the exterior paint contains lead. EPA regulations require certified contractors to handle wash water and debris from lead paint surfaces. If you're unsure, test before washing.

The Top-to-Bottom Rule (and the Exception)

Rinsing always goes top to bottom — gravity takes the dirty water down and off the surface rather than over clean areas you've already rinsed.

Soap application is the exception: apply detergent bottom to top. Here's why this matters — if you apply soap from the top down, the soap running down the lower sections creates "clean streaks" against the dry siding. Those streaks can permanently etch vinyl and aluminum. Applying from the bottom up ensures the soap runs over already-soapy surfaces.

Step-by-Step Process for House Siding

1. Preparation

• Cover all exterior electrical outlets, light fixtures, and HVAC units with plastic sheeting or contractor bags
• Close all windows and doors — water finds every gap at pressure
• Wet down all landscaping beds, shrubs, and grass within 10 feet of the house before you begin
• Move vehicles away from the wash zone

2. Pre-Rinse

Use a 40° (white) nozzle at low-to-medium pressure. Rinse the entire section you're about to treat with clean water. This cools the surface (warm vinyl is more receptive to cleaning chemistry) and wets the surface so soap adheres rather than beading off.

3. Detergent Application

Switch to a 65° (black) soap nozzle — this drops pressure dramatically and widens the spray fan for coverage. Working in manageable sections (one wall face at a time on a smaller home):

• Apply soap bottom to top, keeping the nozzle 2–3 feet from the siding
• For sodium hypochlorite-based house wash mix: let it dwell 5–10 minutes
• Do not let it dry — if it starts drying, mist with clean water to keep the chemistry active

You'll often see the algae visibly change color from green to tan or white as it dies. That's the chemistry working.

4. Rinse

Switch back to the 40° white nozzle. Rinse top to bottom, overlapping passes slightly. Keep the nozzle angled slightly downward — never shoot upward under vinyl or aluminum panels, which creates a direct path for water into the wall cavity and insulation.

Maintain 12–18 inches from the surface for the rinse pass. Closer than that, even a gentler nozzle can leave pressure marks on vinyl.

5. Post-Rinse of Landscaping

Before moving to the next section, rinse all shrubs, grass, and beds again with clean water. Diluting the sodium hypochlorite overspray protects plants from bleaching.

Why Plants and Paint Are the Two Big Risks

Plants: Sodium hypochlorite at full concentration will bleach and kill vegetation on contact. Professional crews are meticulous about wetting everything down before, during, and after the wash. If a contractor arrives and doesn't spray down your landscaping first, stop them.

Paint: Pressure at the wrong angle, wrong distance, or wrong PSI will strip paint in a single pass. Old latex paint on wood trim is especially vulnerable — it can lift and peel from water intrusion at the lap joints even before direct pressure contact. If your trim paint is already cracking or peeling, pressure washing will accelerate it dramatically. Discuss this with your contractor before booking.

What to Expect When a Pro Shows Up

A professional house wash on a typical single-story Michiana home (1,500–2,000 sq ft of siding) takes 2–4 hours. A two-story home adds another hour or two depending on how the rig is set up.

Here's what the visit should look like:

1. Contractor walks the perimeter with you and notes any damaged siding, open gaps, or areas to avoid
2. All landscaping is pre-wetted before any chemical is applied
3. Chemical is applied low-pressure using a dedicated soft wash pump — not a pressure washer with a soap tip
4. Dwell time is observed (this is the step budget operators skip)
5. Rinse passes from top to bottom
6. Post-rinse of landscaping

You should not see them cranking pressure on your vinyl siding. If the cleaning relies on water force rather than chemistry, the results won't hold.

DIY vs. Professional: The Honest Breakdown

| Factor | DIY | Professional | |---|---|---| | Equipment cost | $200–$600 (purchase or $80–$120/day rental) | Included | | Chemical mixing | You source and mix SH + surfactant | Included | | Results duration | 3–6 months (typically) | 12–24 months (soft wash) | | Risk of damage | Higher (especially on second story, delicate siding) | Lower with experienced operator | | Time | 4–8 hours for most homes | 2–4 hours | | Two-story access | Ladder required | Ground-level long-range equipment |

For single-story homes with vinyl siding in good condition, a capable DIYer with the right equipment can get good results. For two-story homes, homes with wood siding, older painted surfaces, or heavy algae growth, professional soft washing is a better investment.

---

Wondering what your house would cost to wash professionally? Get a free written estimate — we'll walk the property, identify the right approach for your siding type, and give you a written number with no surprises.