A much covered and discussed topic, and a lot can be said about it hence I will stick to my experience and write from that.
Moss vs. Algae & Lichen
There are well over 100 different types of moss, and to be honest I used to mix it up with algae or lichen. Now I know these are different so I will separate them out here. Algae and lichen are much less destructive to roofs then moss, so much so that is it usually unnecessary to remove them for other than cosmetic reasons, though if only algae or lichen is present on a roof they come off easier too.
Thus moss is the real “enemy” of your roof let that be asphalt, tile or cedar roof albeit the effects can vary on the different types of roofs.
I break down the damaging effects to two main categories. First one is water damage: moss holds moisture and roofs with moss on them dry out slower which speed up the wear and tear effect on the roof. In freezing temperature this can result in freeze damage as well to the roof.
Second main type of damage is the root damage. The roots of the moss force the material of the shingles apart speeding up its demise.
The end results in both cases are shortened roof life span (need re-roofing sooner) and potential early damage to the roof.
There is a secondary effect of moss on roofs, usually less observed or noticed by others than professional gutter cleaners. Just as your roof slowly ends up in your gutter system, so does the moss but it rolls off the roof much faster and with much greater clogging potential. It can and does clog your gutters, downpipes, drain tiles, and can also spatter all over your yard.
Moss is the number one “dirt” we find in gutters under tile roofs. But the moss doesn’t take it easy on asphalt or cedar roofs either.
Why does the moss grow on your roofs? There are many reasons for that: shade from surrounding trees, dampness of climate, grime and other minerals depositing on your roof from the environment.
Especially in Greater Vancouver we value our green and treed environment, but it raises the need for roof maintenance at the same time. The question you should be asking is do you value your roof, your home, your investment as much as the surrounding trees and the beautiful views?
Next you may be asking, how do you get rid of the moss? Chemicals, pressure washing, brushing, moss dance in the rain?
Sort of a combination of all the above… Sort of, save maybe the dance, though it may do some good for your health (make sure it’s done in warm rain!).
Truthfully I had been looking for a GOOD moss killing product for what is seems like ages. I tried several different ones without a real peace of mind that I did a good job. But at last a few years ago I have come across the solution, a powder, dissolving in water and sprayed on mossy surface that actually kills the roots of the moss immediately. It is peroxide based, no toxic effects, safe for humans, pets, plant AND building material!
This product is called Roof Wash, which is phosphate free, non toxic and environmentally friendly. And since I discovered it we figured out different other uses for it too. It’s awesome for washing down the face (outside) of gutters for example.
Brushing off moss is generally accepted so no point wasting much “typing breath” on it, but pressure washing on the other hand seems to be frowned upon by some, especially by roofers.
Sure, one can debate over it endlessly I think. But when it all comes down to it, you have two things in the balance: negative effects of moss on your roof vs. negative effects of method of removal.
One side may say, pressure washing will remove the protective layer of the shingles (the toxic substances the shingles were treated with to withstand the elements, mainly rain water and sunlight). But then again the moss is cracking the shingles, not to mention that the initial roof treatment doesn’t last forever. In actual fact after a year your cedar roof could (and ideally should) be treated for maximum life span. Such treatments last 5-7 years.
Asphalt and cement tile roofs do not have such life lengthening treatments but they loose their resistance to the elements also much sooner than at the end of their life span. So moss control and treatment may be even more crucial to lengthen their life span.
And certainly in the wrong hands the pressure washer can only cause damage, but that’s true for every tool. Most people do not know that the pressure is adjustable, the fan tip exchangeable and with the proper wood restorer your cedar roof can look like new. So does your cement tile or asphalt roof (without wood restorer, doh!).
As a matter of fact professionals dealing with cedar roof installation and preservation claim that pressure washing is the only way to completely clean the roof.
The age and condition of the roof are major factor in determining what method to use. I always inspect the roof and based on my findings make my recommendation. If a roof is so worn out that it’s warped, cracked and brittle I never recommend anything more than praying it only, and let the elements do the removal of dead moss.
Another fact to consider when removing moss is that however it is coming off it will inevitably bring with it some of the roof. But that “loss” of roof is out weighted by the damage it does to the roof if left untended.
But in reality we are happy to do brushing and spraying. It doesn’t get you a prefect result but if that’s not priority then it’s not a problem. The point is to stop the moss from keep eating at your roof!