How often should you wash your house?
Every 12 to 18 months for most homes in the Midlands. Heavily shaded properties and anything near open water need it closer to annually; an exposed house on an open lot can stretch past two years. The interval is set by how long your walls stay damp, not by the calendar.
That range is a starting point, not a rule, and the reason it is a range matters. Two identical houses on the same street can genuinely need different schedules if one sits under an oak canopy and the other does not. Anyone who gives you a single number without looking at your property is quoting you an average.
The good news is that a house tells you when it is ready, reliably, and well before it becomes a problem. Once you know which signals to read you can stop guessing — and stop paying for washes a year earlier than you needed them.
Why does the north side always go first?
Because it gets the least direct sun, so it stays wet the longest. Algae and mildew need moisture to establish, and drying time is the only variable that consistently limits them. A north wall can hold overnight dew until mid-morning while the south wall dried an hour after sunrise.
This is the most useful thing to understand about exterior cleaning, because it explains almost every pattern you will notice on your own house. The north elevation goes green first. The shaded corner behind the garage goes next. Under the eaves, where rain never quite reaches to rinse and the surface stays damp, growth holds on longest of all.
It also explains something that confuses a lot of homeowners: why one wall can look years older than the rest of the same house. It is not uneven paint or a bad batch of siding. It is drying time, compounding season after season.
In practice this means the north side is your gauge. If you want one place to look to decide whether it is time, look there — the rest of the house is running behind it.
What actually changes your interval?
Four things, in rough order of impact: tree cover and shade, proximity to open water, what your walls are made of, and which direction they face. Everything else — how new the house is, how much it rains, what colour the siding is — matters far less than these four.
What is not on that list is worth noting too. Rainfall barely moves the interval, because rain both wets the wall and rinses it, and those roughly cancel out. Siding colour affects whether you notice growth, not whether you have it — green algae is simply more visible on cream siding than on grey. And a newer house is not more resistant; it has just not had time yet.
- Tree cover. Overhanging canopy blocks the sun that would dry your walls and drops organic debris that feeds growth. A mature oak over the house can halve your interval on its own.
- Proximity to water. On a lake or river lot, humidity and overnight condensation never fully release. Waterfront properties around Lake Murray routinely need attention about twice as often as a house a mile inland.
- Wall material. Porous surfaces hold moisture in the surface itself, so stucco, unsealed brick and older painted wood stay hospitable longer than smooth vinyl does.
- Orientation. North and north-east elevations dry slowest. A house whose long side faces north effectively has more vulnerable wall area than one whose long side faces south.
How do you read your own siding?
Look at the north wall in good light. A faint uniform dulling means you have months of margin. Visible green or grey patching means schedule it. Dark streaking under the eaves, or growth you can feel with a fingertip, means it is overdue — the organism is established rather than just present.
The fingertip test is the most useful and least known. Run a finger down a clean-looking section of shaded siding and look at it. If it comes away with a grey or greenish film, you have a colony that has not yet become visible from the street. That is the ideal moment to wash: the growth has not had time to work into the surface, and it comes off completely.
Three other signals are worth checking. Look at the gutter faces — those black vertical tiger stripes are the earliest visible sign on most houses, because gutters stay damp longest. Check the lowest course of siding, where soil splash keeps a permanent damp line. And look at the north-facing window frames and sills, which collect and hold moisture in a way flat wall does not.
If you want a single habit that keeps you ahead of it: look at the north wall each spring, after the pollen has washed off but before summer humidity settles in. That is the point in the Midlands year when what you see is what you actually have.
What happens if you leave it too long?
Growth moves from sitting on the surface to working into it. On painted wood it lifts and fails the paint film. On stucco and brick it holds moisture in the surface, accelerating deterioration. And once moss or lichen establish, cleaning stops being enough — the damage underneath them is already done.
There is a genuine threshold here, and it is worth naming plainly because it is where a maintenance cost turns into a repair cost. Algae and mildew are surface organisms; cleaned at any point, they leave nothing behind. Moss and lichen are not. Lichen anchors into the surface with root-like structures, and removing it takes some of the surface with it. On painted wood that means paint. On stucco it means the finish coat.
The other cost is the one people discover when they finally call. A house left long enough often cannot be brought back to clean in a single visit, because what is left after the growth comes off is a surface that has weathered unevenly underneath it for years. We are straight about that when we see it — you will hear what we expect to achieve before we start rather than after.
None of which is an argument for washing more often than you need to. Over-washing is a real thing, particularly on painted surfaces, and paying for an annual wash on a house that needs one every two years is money spent for no result. Read the north wall. It will tell you.
By Ray · Updated