Moss Isn’t The Problem. What It Holds Is.
A green roof looks bad long before it is bad. What shortens a shingle's life is the water the moss holds against it, day after day, through a wet winter.
What moss is actually doing up there
Moss does not eat asphalt. It holds water, and that is enough. A shingle is built to shed rain in minutes and then dry; under a mat of moss it stays damp for days, and on this coast that is most of the season.
Damp shingle loses its granules. Freeze-thaw lifts the edges of the courses. The moss then roots into the gap it made, and the next winter starts further ahead than the last one did.
A roof does not fail because something grew on it. It fails because it never got the chance to dry.
Why a pressure washer makes it worse
Pressure takes the green off in an afternoon and takes the granule layer with it. It also drives water up under the courses, which is the one place on a roof water is not supposed to go.
The slower method is a low-pressure treatment that kills the growth and lets the weather carry it off over the following weeks. The roof looks worse for a month and lasts years longer.
What gets looked at before a price is given
A roof is quoted off its condition rather than its footprint, and most of what moves the number is not the moss itself.
- How much granule is already sitting in the gutters — the cheapest indication of how much life the surface has left.
- Whether the growth is sitting on the courses or rooted under their edges.
- The north and shaded slopes, which carry most of it and dry last.
- Overhanging growth, because a roof under a cedar is a roof that will need this again sooner.
- Valleys and flashings, where a mat holds water against a joint rather than against a shingle.
When it is worth booking
Spring, after the growth flush, or early autumn before the rain settles in. Both give the treatment dry days to work in and leave the roof clear for the season that matters.
If there is already moss thick enough to see from the street, the gutters are carrying it too. The two jobs are usually one visit.
The scope and the method are set out on the roof cleaning page, and the low-pressure approach this article describes is soft washing. We work across nine Greater Vancouver communities.
Questions About Roof Moss in Greater Vancouver
How do I know if my roof needs moss treatment?
Visible growth on shingles is the obvious sign, but the more useful test is the north-facing slope in late spring. If it is still carrying a mat after the drier months, the root structure is well established. Gutters filling with granule grit are the other indicator — that is the shingle surface telling you how much life it has left.
How long does a treated Greater Vancouver roof stay clean?
Typically three to five years after a proper low-pressure treatment, against roughly one year when growth is knocked off at pressure. The difference is the root: treatment kills it, so regrowth starts from new spores on a clean surface rather than from the root mass left behind. North-facing slopes and roofs under heavy conifer cover sit at the short end of that range.
Can I pressure wash my own roof to save money?
The machine is available for rent, and the damage to an asphalt shingle is dramatic and invisible on the same afternoon. Granules leave the surface with the moss, water is driven up under the courses, and the roof looks dramatically better for one season before failing several years early. Low-pressure treatment is slower and is the only method that does not shorten the life of the roof it is treating.
Does roof cleaning affect my shingle warranty?
Most shingle manufacturers explicitly permit low-pressure cleaning and warn against pressure washing. If your roof is inside its warranty period, the manufacturer's own maintenance guidance is worth reading before booking either method. We confirm the method and the products used in writing on request, which is what a warranty claim may eventually ask for.
When is the best time of year to treat a roof in BC?
Spring and early autumn are both practical windows — after the growth flush and before the sustained rain sets in, respectively. Both give the treatment dry working days and leave the roof ready for the season that follows. Moss visible from the street usually means the gutters are carrying it too, and clearing both in one visit is the usual approach.
