Can roof algae get onto the pavers?

Walk the property from the roofline down: tile, glass, stucco, then the drive. TRI Alliance's list names every one of those surfaces.

ARMA states that “all species are transported through the air and tend to collect and grow on roofing structures and other outdoor surfaces,” a description that does not stop at the roofline. TRI Alliance's tile guide names the wider list directly: “many climates are conducive to mildew, algae and moss growth on everything from glass, stucco, siding and driveways to roofing products including concrete and clay roofing tiles.”

A driveway and a concrete or clay roof tile sit on that one list. Neither document names a route from a roof to a paver: ARMA describes the organisms traveling through the air, and TRI names the surfaces a climate favors growth on. TRI's guide is a roof maintenance document, so its driveway mention is a tile body's observation about other surfaces, not a study of pavers.

The CDC's journal defines the kind of film involved, and its definition is about attachment to a surface.

It describes a biofilm as “an assemblage of surface-associated microbial cells that is enclosed in an extracellular polymeric substance matrix,” and says that “Microorganisms attach to surfaces and develop biofilms.” Attachment, the paper adds, occurs most readily on surfaces that are rougher, more hydrophobic and coated by conditioning films. The paper is about biofilms in general, not roofs or pavers, so reading it beside ARMA is a bridge: a biofilm on a paver is one that attached to that paver.

ARMA's line about algae spores spreading from rooftop to rooftop on wind or animals describes an airborne route to a neighboring roof, not a roof draining growth onto the pavers underneath it. No source read describes rinse water or runoff as the way algae reaches a paver surface.

Whatever settles on a paver surface is treated according to what that paver is made of, and that is where roof rules and paver rules stop being the same conversation.

The Natural Stone Institute's care page carries its own outdoor line on algae and moss for pool, patio and hot tub areas. It is written for natural stone, not for concrete pavers and not for a roof, so a natural stone pool deck follows the institute's page. The paver cleaning page quotes that line alongside the three paver materials and their rulebooks, and sets out what a paver cleaning removes from concrete pavers.