Why the Shaded Side of Your House Turns Green

Walk around almost any house in Northern Virginia and you will see the same pattern. The side that gets sun looks fine. The north side, or the wall tucked behind a row of trees, has gone grey and green, usually in streaks that run down from the roofline or the corners of the windows. It looks like dirt, so plenty of people grab a garden hose and a brush on a Saturday and then wonder why it keeps coming back. It is not really dirt. It is a living film, and that changes how it should be cleaned.

Why the shaded side goes green

Mildew, algae and the occasional bit of moss need three things: moisture, something to feed on, and not too much direct sun. Our humid summers and damp springs supply the first. Pollen, dust and bits of tree debris that settle on siding supply the second. The north face of a house, and any wall shaded by oaks or maples, supplies the third, because it stays damp long after the sunny side has dried out from the morning dew.

The streaks usually start where water lingers: under window sills, along the bottom course of siding near the ground, behind downspouts that splash back, and below a gutter that overflows. Once a thin film takes hold, it holds moisture against the surface, which feeds the next layer of growth. That is how a wall goes from a faint haze to solid green in a couple of seasons.

Vinyl is not damaged by this the way raw wood would be, but the film dulls the colour, traps dirt, and makes the whole house look older than it is. On painted siding, a film left alone for years can stain the finish more deeply and leave a ghost of the streaks even after cleaning.

What a pressure blaster does to siding

The tempting fix is to rent a pressure washer and blast the green off. It does remove the visible colour, often quickly. The trouble is what else it does. Siding is a set of overlapping panels, and the joints at the bottom of each panel are not sealed. Aim a hard stream upward and water gets behind the panels and into the wall side of the siding. Point it at a window frame and it can push water past the seals. Even when nothing leaks, a narrow high-pressure stream leaves pale tracks where the nozzle passed, and on older painted surfaces it can lift the paint right along with the growth.

There is also the root problem. Blasting knocks the top off the growth but does not always kill what is underneath, so the green tends to creep back faster than people expect. The wall looks clean for a short stretch, and then the streaks return along the same lines, because the conditions that made them have not changed.

What a soft house wash does differently

A soft wash flips the approach. Instead of using force to scrub the growth off, it uses a cleaning solution to break the growth down, then rinses with low pressure, closer to a strong garden hose than a blaster. The solution goes on from the ground, sits for a few minutes while it works, and is rinsed away. The mildew and algae are killed rather than just scraped, which is why the surface comes up evenly and tends to stay clean longer.

Low pressure is also kinder to everything attached to the wall: window seals, light fixtures, dryer vents, painted trim and the soft lower edge of each siding panel. Hall Son's offers this kind of house wash, and the way they describe it is plain: increase your home's curb appeal, remove unsightly stains and extend the life of your siding. Their before and after photo shows grey lap siding streaked with green on one side of a curved line and even blue-grey siding on the other.

Small things that help before and after

A little preparation makes the result better and protects what is around the house.

• Close windows and doors, and mention any window that does not seal well so it can be rinsed gently.
• Move cushions, grills, toys and planters away from the walls so every foot of siding can be reached.
• Point out delicate plants near the foundation, since they sit in the path of the rinse.
• Show the crew where outdoor outlets and light fixtures are, so no one sprays straight into them.
• Trim shrubs that touch the wall, which hold moisture against the siding and block access.

After the wash, a few habits slow the return of the green. Trim branches so sun and air reach the shaded wall. Keep gutters clear so they do not overflow down the siding. Redirect any downspout that splashes onto the wall. None of this stops growth completely on a north face, but it stretches the time between washes.

Getting your siding looked at

Every wall is different, and the right approach depends on the siding type, its age and how heavy the growth is. Hall Son's Services starts every job with a free estimate, and would rather look at the house than guess from a description. Call or text 571-336-2395 to set up a FREE Estimate.