Roofing Granules

Roofing granules are small particles of crushed rock, usually coated with a fired ceramic colour layer, that are pressed into the hot asphalt surface of shingles and mineral-surfaced roll roofing. They form the visible weathering surface of the product.

Purpose

• UV protection: asphalt degrades quickly in sunlight. Granules shade the asphalt and are the main defence against ultraviolet ageing.
• Colour: the ceramic coating gives the roof its colour and blend. Lighter colours reflect more heat, which is useful in hot, sunny climates; darker colours absorb heat.
• Fire resistance: the mineral layer helps the shingle resist burning brands.
• Special functions: some granules are coated with copper or zinc compounds to resist algae, and "cool roof" granules use pigments that reflect near-infrared radiation.

Materials

Granule materials include slate, schist, quartz, vitrified brick, stone and ceramic. The underside of a shingle uses a finer, non-coloured surfacing such as sand, talc or mica to stop shingles sticking together in the bundle.

Granule loss

Some granule loss is normal, especially when a roof is new and loose surplus granules wash into the gutters. Ongoing or heavy loss, however, is a sign of ageing: once the asphalt is exposed, it oxidises and cracks faster. Hail can knock granules off in round patches, which is one of the signs adjusters look for after a storm. Pressure washing a shingle roof can strip granules and is generally discouraged by manufacturers.

Inspection tip

Granules collecting in gutters or at downspout outlets, bare dark patches on shingles, or exposed fibreglass mat all indicate that the roof surface is wearing out.