Those granules come off your asphalt shingles. As shingles age, the asphalt dries out and loses some of its grip on the granules, and rain washes the loose ones down into the gutters and yard. Heat, sun and monsoon storms in the Phoenix area all speed that up.
What the granules are for
The granules shield the asphalt from the sun. LBNL researchers wrote that “it is essential that the roofing granules absorb the UV component of sunlight.” As a shingle ages, it loses the oils that keep it flexible. InterNACHI, an inspectors’ association, says “Loss of volatiles can degrade the bond between the asphalt and the granules”, which leads to more granule loss.
Why Phoenix roofs shed
The Valley gives that process plenty of fuel. July averages a high of 106.5°F, and the EPA rates the summer UV index here “Very High”. A storm that drenches a hot roof adds stress too, as the shingles swing from hot to cool fast. Monsoon outflows can bring “winds 50 mph or greater” and heavy rain that carries loose grit off the roof. Read more in Arizona heat and asphalt shingles.
When to worry
- A little grit after a big storm: keep an eye on it.
- Grit every time it rains, or dark bare spots on the roof: get it looked at.
- The fiberglass mat showing through: the roof is past treatment.
Rejuvenation won’t put granules back. SoyLei, which makes the product we use, says it “Will not fix severe aggregate loss or exposed fiberglass mat.” SoyLei also says the product is meant to “improve granule adhesion” on aging shingles. That is the maker’s claim, not ours.
After a storm, see our monsoon roof check. If the grit keeps coming, book a free roof evaluation.