Mold and Green Algae on Vinyl Siding

Green film and black spots are not the same problem

The broad green or grey film across an elevation is algae and mildew. It is surface growth, it responds well to a cleaning solution, and it comes off completely.

The small black spots that look like someone flicked ink at the wall are something else: artillery fungus. It grows in damp hardwood mulch, and it fires spores several feet toward light-coloured surfaces. Those spores land with an adhesive that bonds to vinyl, and washing does not remove them. Scraping each one off by hand is genuinely the usual method, and even then a stain often remains. Knowing which of the two you have changes what is realistic.

Why siding grows it here

Organic growth needs moisture, shade, and something to feed on. Humidity supplies the first, and the north elevation or anything under tree cover supplies the second. That is why a wall can be spotless on one side of a house and green on the other. Sprinkler heads that hit the wall, dense shrubs planted tight against it, and downspouts discharging beside it all keep the surface damp enough to sustain growth.

What removes it

A sodium hypochlorite solution mixed for exterior cleaning, applied at low pressure and given dwell time, kills the growth at the root. That last part is what matters: pressure alone removes the visible layer and leaves a root structure that regrows within weeks. Killing it gives a considerably longer gap before it returns.

Applying low to high avoids streaking on dry siding, and rinsing high to low with the spray angled downward keeps water out from behind the panels.

The mistake that causes real damage

Vinyl panels overlap so water sheds outward. Spraying upward into those laps at pressure forces water behind the siding into the wall cavity, where it sits against sheathing and insulation. Nothing looks wrong from the outside, and the consequence appears much later as mould inside the wall. This is the single most common way siding cleaning causes damage.

Keeping it off longer

Cut shrubs back so air moves along the wall. Adjust irrigation so it is not spraying the siding. Make sure downspouts discharge away from the wall rather than beside it. If artillery fungus is the problem, replacing hardwood mulch near light-coloured walls with a non-wood alternative removes the growing medium, which is the only reliable fix.

Having it washed

We soft wash siding and treat the growth rather than blasting it off, and we will tell you honestly if what you have is artillery fungus and how much of it will realistically come off. Call (504) 688-3995 for a free written estimate.

Slowing the regrowth

Cut shrubs back so air can move along the wall. Adjust or redirect irrigation heads that hit the siding. Extend downspouts so they discharge away from the foundation instead of beside it. Clear gutters so they stop overflowing onto one section of wall. Where it is practical, thinning a branch that shades a wall all day does more than any product, because the growth follows the damp rather than the dirt.

Does it damage the siding?

On vinyl it is mostly cosmetic in the short term, though it holds moisture against the surface and traps dirt. On painted wood it matters more, because sustained damp works on the paint film and then on the timber underneath. On stucco it works into the pores and becomes progressively harder to remove fully. The case for dealing with it promptly is strongest on painted timber.