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The Science Behind The "Roof Wash"


That unpleasant-looking growth that’s discoloring and degrading your roof is called  — Gloeocapsa magma. Gloeocapsa magma may resemble algae,  but it’s actually an airborne cyanobacteria! Yes, you read that right.. BACTERIA! YUK!!! This bacteria started getting noticed in the Southern US around 1990. It has since spread to most of the US and Canada and is responsible for creating the “black streak” phenomenon on roofs. The bacteria accumulates over time and starts to show up on roofs as dark or discolored patches/streaks/stains. This discoloration creates a UV barrier that allows the bacteria to grow deeper, causing its host to deteriorate over time.


Obviously, your roof is very important. It protects the home’s interior from the outside elements. Maintenance is necessary to keep the roof performing as it should. All roof types require maintenance. The biggest part of that maintenance is cleaning, but few people know that. Why? In the ’70s, the manufacturers switched from felt-backed shingles to fiberglass-backed shingles, and then added limestone… Gloeocapsa magma LOVES limestone. It also LOVES tile roofs. Roofing tiles, much like the porous structure of limestone, gives it a nice place to live and grow. Metal roofs are also subject to Gloeocapsa magma. Metal is not as porous as limestone or roof tiles, but it still has a layer that is just porous enough to allow the bacteria to grow. Patriot Exterior Cleaning has spent years developing specific soft wash methods and ratios to safely and effectively clean each type of roof with incredible results and no damage.   

How does Gloeocapsa end up on your roof? Commonly, reproductive spores are blown there by the wind. Birds, squirrels, and other small animals also act as spore carriers. Gloeocapsa magma prosper where nutrients, moisture, and heat are present. A sun-warmed, rain-dampened roof is a perfect host environment. Once a colony establishes itself, it will grow exponentially over time until the entire roof is infested.

Many people believe Gloeocapsa magma on a roof is only an optic issue, but prolonged contamination can affect a roof’s service life.  Gloeocapsa magma also absorbs heat (due to its dark nature), causing roof surface temperatures to rise. The result is a hotter home, which means a higher energy bill.


Its biggest effect is obviously on your home’s exterior, but it also poses a human health threat to those who suffer from allergies, asthma, and respiratory problems. Gloeocapsa magma spores can get inside your home because not only are they on your roof, but they end up on your siding, driveways and sidewalks, and are easily tracked indoors. They can even be drawn into your air conditioning systems and populate the ducts.