Why the north side of your house turns green

Walk around almost any house in Wayne or Ridgewood in late October and you can pick out the north side without a compass. The back corner by the downspout, the wall behind the shrubs, the stretch of siding under the deck: that's where the green shows up first. From the street it looks like dust. Up close it looks like a stain, and it smears if you rub it with your thumb. That's algae, and vinyl siding is a surprisingly comfortable place for it to live.

Why the north wall goes green first

Algae needs three things: moisture, a little food, and time. North Jersey supplies all three. The north side of a house gets the least direct sun, so after rain, morning dew or a sprinkler pass the wall stays damp for hours longer than the south side does. Pollen, dust and the fine grit that settles on every vertical surface give the algae something to eat. Trees close to the house, and shrubs planted tight against the wall, hold humidity right where the siding needs to dry out.

The siding itself isn't feeding anything. The growth is a film that sits on top and spreads, which is why a wall can look badly stained while the material underneath is perfectly sound. Mildew behaves the same way. One customer on the Modern Powerwash reviews page has fiber cement siding rather than vinyl, and wrote that mildew was growing on some areas of it. Different material, same cause: a wall that stays shaded and damp.

What happens when you blast it

A lot of homeowners rent or buy a pressure washer and go straight at the green. Sometimes it works. More often you end up with stripes, clean lines where the nozzle passed and a dull film in between. Vinyl siding is hung in overlapping panels, and a hard, narrow jet can push water up behind the lap and into the wall. It can also leave swirl marks in the surface or catch an old seam and work a panel loose.

The frustrating part is that you don't need that kind of force. Algae holds on lightly. What it needs is something that loosens the growth and then plenty of water to carry it away. Force is the wrong tool for the job, and on a second-story wall you're also standing on a ladder while you swing it around.

How Soft-Wash handles it

Modern Powerwash describes its Soft-Wash method as "a precise combination of lower pressure and increased water flow, available in both hot and cold options." In plain terms, the crew leans on water volume and a cleaning solution to loosen the growth, then rinses it off, rather than scraping it away with a hard jet. The company says it uses environmentally friendly cleaning products, which matters when the wall you're washing sits right above your hydrangeas.

For the ordinary green film on shaded siding, cold water is usually enough. Heavier buildup is where hot water earns its place, and we get into that in a separate article on hot versus cold. What you should notice afterward is an even color from corner to corner, with no stripes, and no feeling that the siding got sanded. Customers describe the result in simple terms. Greg H wrote that the house "looks brand new" after the job, and Cathy B said everything "looks like new."

Slowing the green down next time

No cleaning makes a shaded wall permanently immune. But a few habits in your own yard stretch the time between washes:

• Trim shrubs and low branches back so the wall gets some air and a few more hours of sun.
• Check that gutters and downspouts aren't overflowing or splashing onto the siding.
• Aim sprinkler heads away from the house, and fix any outdoor spigot that drips against the wall.
• Rake leaves and mulch away from the base of the siding so splash and damp don't climb the wall.

Plenty of Modern Powerwash customers settle into a rhythm instead of waiting for the green to get bad. Linda B wrote that Tom and his crew have washed her house and deck "regularly for the past 15 years," and Bob C said it was his third time using them.

If your north wall has gone green, the house washing page lays out what Modern Powerwash does for siding, and quotes are free. Call (973) 777-6336, or send the form on the site and you'll hear back by the next business day.