Should the roof be cleaned before or after the pavers?

Start with what is missing. None of the roofing bulletins and none of the paver documents read for this page publishes an order for a roof wash and a paver sealing on the same property. The sequence on this page is reasoning, and it is laid out here so a reader can check each step against its source.

The first fact is about the roof. Whatever a roof wash puts on the roof comes off it again: rinse water, spent solution and loosened growth drain to the eaves and drop onto whatever is below. That is a mechanism, and no document measures it. ARMA's technical bulletin acknowledges the path when it tells anyone applying its bleach solution to apply it carefully to avoid damage to other parts of the building and the surrounding landscape.

The second fact is about the pavers, and it comes from the Interlocking Concrete Pavement Institute's Tech Spec 5: "The cleaned surface must be completely dry prior to applying most sealers. Allow at least 24 hours without moisture or surface dampness before application." Set next to each other, the two jobs form a dependency. One of them produces water on the pavers, and the other needs the pavers free of water for a full day before it can start.

Run the roof first and the dependency resolves on its own. The roof sheds its water, the pavers below get cleaned, and ICPI's dry condition is counted from after the last water has reached them. By the time a sealer goes on, the roof has nothing left to send down.

Run it the other way and the roof wash lands in the middle of the paver job. If the pavers were cleaned but not yet sealed, runoff on them is moisture, and ICPI's condition of at least 24 hours without moisture or surface dampness has not been met until a full day has passed without it. If the sealer is already down, the water reaches a coat that ICPI says should see no traffic until it is completely dry, typically 24 hours, with water based curing time that varies with the temperature, wind conditions and humidity. What a particular sealer tolerates while it cures is set by that sealer's maker on its label, and no document read for this page publishes a rule for roof runoff onto one.

ICPI's instruction to turn off all automatic sprinkler systems during cleaning, sealing and drying fits the same reasoning, since a sprinkler, like a roof wash, puts water on the surface during that stretch. The full sequence of clean, dry and seal, with its conditions, is laid out under how paver sealing works as a sequence with conditions, and a surface that looks dry is not always dry enough, as why pavers can look dry and still be too wet to seal explains.

The reasoning is general, and a given property can make it matter more or less. These are the questions that decide how much weight the roof-first sealer sequence carries, none of which a document answers for any one house.

• Does the roof drain onto the pavers at all? A patio set well away from the eaves may sit outside the path of the runoff, while a walkway under the drip edge sits right in it. That is a mechanism, and it is specific to the layout.
• Is a sealer going on, or only a cleaning? ICPI's 24 hours is a condition for most sealers. A cleaning with no sealer to follow has no dry time to protect, though a roof washed later still sends water and loosened growth onto a surface just cleaned.
• Were the pavers sealed recently, before anyone thought about the roof? That case has its own page: a roof cleaned right after the pavers were sealed.
• What does the runoff carry? No source measures what diluted roof runoff does to a paver or a sealer. That question is taken up at will roof cleaning stain my pavers.

Runoff passes the windows on its way to the ground, and glass brings a chemistry question of its own: the National Glass Association names bleach and other solutions containing sodium hypochlorite among the materials that may cause harm to laminated glass, and impact windows are laminated, as the window cleaning page sets out. That is why the order of a roof and its windows is argued separately at should the roof be cleaned before or after the windows. A roof that was only just installed raises a timing question of a different kind, answered at how soon a new roof can be cleaned.