Pressure washing removes what is on a surface by hitting it hard enough to knock it loose. Soft washing removes it by killing it and letting it rinse away. Both have a place. The difference matters because most of what grows on a South Florida house is alive, and the things it grows on are frequently too fragile to take real pressure.
What pressure actually does
A consumer machine runs somewhere around 1,800 PSI. Professional units go well past 3,000. Concrete does not mind. Almost everything else on a house does, and the damage is rarely obvious on the day.
- On stucco, high pressure erodes the finish coat and opens the surface up. It looks clean and then holds dirt worse than it did before, because the texture is now rougher and more porous.
- On barrel or flat roof tile, pressure drives water up under the laps and into the underlayment, and it strips the surface off the tile. This is the expensive one.
- On wood fencing, anything past roughly 1,200 PSI raises the grain and leaves furring that has to be sanded before stain will sit right.
- On painted surfaces, pressure finds every marginal bit of adhesion and takes it off. Sometimes that is useful before a repaint. On a coat you intend to keep, it is not.
- On screen enclosures and window seals, it tears and forces water past gaskets.
What soft washing actually is
Soft washing runs at low pressure, generally under 500 PSI, and does the work chemically. The standard mix is sodium hypochlorite diluted to the job, a surfactant so it clings long enough to act rather than running straight off, and water. It is applied, given dwell time, and rinsed.
The reason it matters is that the black streaking on South Florida roofs is not dirt. It is a cyanobacteria, commonly gloeocapsa magma, and alongside it sit algae and lichen with root structures that go into the surface. Blasting takes the visible top off and leaves the organism, which is why a pressure washed roof often looks worse again within months while a soft washed one stays clear far longer. Lichen in particular has to be killed. It does not come off mechanically without taking substrate with it.
Surface by surface
| Surface | Method | Why |
|---|---|---|
| Concrete driveway, sidewalk | Pressure, often with a surface cleaner | Takes the pressure and cleans evenly without the striping a wand leaves. |
| Pavers | Pressure, moderate, plus re sanding after | Cleaning removes joint sand. Replacing it is part of doing the job properly. |
| Roof tile or shingle | Soft wash only | Pressure drives water under the laps and strips the tile surface. |
| Stucco and painted walls | Low pressure or soft wash | High pressure erodes the finish coat and finds weak adhesion. |
| Screen enclosure | Soft wash | Screen tears. Frames and seals do not want forced water. |
| Wood fence or deck | Low pressure, under about 1,200 PSI | Higher raises the grain and leaves furring to sand out. |
| Vinyl fence | Soft wash | Cleans the mildew rather than polishing chalked vinyl. |
Cleaning before painting is a different job again
A wash for appearance and a wash for adhesion are not the same specification. Before a repaint the goal is a surface with nothing living on it, no chalk, and no loose material, because paint bonds to what it touches. If it touches a film of algae, that is what it is bonded to.
That usually means a soft wash with proper dwell time rather than a fast mechanical pass, followed by a full rinse and enough drying time before anything is applied. Skipping it is the single most common reason an exterior repaint fails early here, and it is invisible in the finished look on day one.
A quick chalk test settles the question on an older wall: rub a dark cloth across the paint and if it comes away powdery, the surface is chalking and needs both washing and a binding primer before a new coat goes anywhere near it.
How often
Sun, humidity and rain give growth a long season here, so surfaces green up faster than in drier climates. Roofs and north facing walls, which stay damp longest, go first. Annually is a reasonable baseline for most houses, and coastal properties that also collect salt often want it more often. Screen enclosures and pool decks are usually the first thing owners notice.
See pressure cleaning for what we cover, or read why coastal exteriors need different coatings if the wash is the first step toward a repaint.
Common questions
- Can you pressure wash a tile roof?
- It should be soft washed instead. Pressure drives water up under the tile laps toward the underlayment and strips the surface off the tile itself. It also leaves the organism behind, so the streaking returns quickly.
- What causes black streaks on a Florida roof?
- Usually a cyanobacteria, commonly gloeocapsa magma, often alongside algae and lichen. It is alive and rooted, which is why it has to be killed chemically rather than blasted off.
- Will soft washing hurt my plants?
- It can if it is not managed. Pre wetting the beds, covering what is vulnerable and rinsing thoroughly afterwards is part of doing the job properly.
- Do I need to pressure wash before painting?
- The surface has to be clean, but a wash for adhesion is a different specification from a wash for appearance. Paint bonds to whatever it touches, so anything living, chalking or loose has to come off first and the wall has to dry before coating.
