Why this usually happens
Shade and dampness — north sides, under trees, and low spots that hold water.
Organic film from pollen, leaves, and dirt feeds the growth.
Concrete, interlock, and natural stone are porous and hold moisture.
Why you should not ignore it
It's a real slip-and-fall hazard, especially when wet — and a liability if guests or customers use the walkway. Moss also works into joints and cracks, widening them over time.
This problem is common on north-facing driveways, shaded side walkways, backyard patios, and interlock areas that stay damp after rain. In Durham Region, spring moisture, leaf debris, snow melt, and summer shade can all keep porous surfaces wet long enough for algae and moss to take hold. Because these surfaces are used by family, guests, delivery drivers, tenants, and customers, traction matters as much as appearance.
What to do next
- A garden-hose rinse will not last — the growth lives in the pores and comes right back.
- Proper cleaning uses pressure washing for concrete, or a controlled method for pavers and stone, with a treatment that kills the algae and moss so it stays clear longer.
A safe exterior cleaning approach
Our concrete and driveway cleaning removes algae, moss, and stains and restores safe traction — for driveways, walkways, and patios across Durham Region.
Related DSS services and local pages
Frequently asked questions
Will pressure washing damage my interlock or pavers?
We adjust the method for the surface to avoid damage and joint-sand loss.
How long until the green comes back?
With proper treatment, far longer than a rinse; shaded areas may need periodic cleaning.
Is the green stuff dangerous?
Mainly as a slip hazard — it makes wet surfaces very slippery.