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A process for forming algae-resistant roofing shingles comprises producing algae-resistant granules, and adhering the granules to a shingle stock material. The algae-resistant roofing granules are produced by a process that includes coating mineral particles with a clay-silicate binder including a m

A process for forming algae-resistant roofing shingles comprises producing algae-resistant granules, and adhering the granules to a shingle stock material. The algae-resistant roofing granules are produced by a process that includes coating mineral particles with a clay-silicate binder including a metal oxide algaecide and small organic particles. The particles are heated to cure the binder, and the organic particles pyrolyse to form pores in the coating. The release of the algaecide is controlled by the structure of the granules.

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We claim: 1. A process for producing algae-resistant roofing shingles, the process comprising producing algae-resistant roofing granules, and adhering the granules to a shingle stock material, the algae-resistant roofing granules being produced by a process comprising:(a) providing inert base partic

We claim: 1. A process for producing algae-resistant roofing shingles, the process comprising producing algae-resistant roofing granules, and adhering the granules to a shingle stock material, the algae-resistant roofing granules being produced by a process comprising:(a) providing inert base particles;(b) forming first intermediate particles by coating the inert base particles with a first mixture including;at least one algaecidal material, anda void-forming material, the void-forming material releasing gaseous material at temperatures above 90 degrees C., and having an average particle size no larger than 2 mm,to form a first layer on the inert base particles;c) forming second intermediate particles by coating the first intermediate particles with a second mixture including a coloring material and not including a void-forming material; and(d) heating the second intermediate particles to decompose the void-forming material and form pores in the first layer to produce the roofing granules.