Will roof cleaning dirty my windows?

Follow the last step of a roof wash and the answer comes into view. The Asphalt Roofing Manufacturers Association's bulletin on algae discoloration of asphalt shingle roofs closes its cleaning procedure with one instruction: "Finally, rinse the solution from the roof by gently spraying the surface with water."

The solution being rinsed is spelled out earlier in the same bulletin: "Discoloration may be lightened by applying a solution of liquid household bleach (sodium hypochlorite) and water." ARMA already has the rest of the building in mind while that solution goes on, telling the applicator: "Be sure to apply it carefully to avoid damage to other parts of the building and the surrounding landscape."

That is the procedure for asphalt shingle. It does not speak for tile or metal, and it does not say what any company's soft washing solution contains. What it does establish is the order of events on the roof: a solution applied carefully, and then water sent down the slope to carry it away. Everything below the roof is downstream of that rinse.

Water that leaves a roof does not stop at the edge. It runs off the eave and down the building, over the wall and over whatever sits below, including glass. What it carries is whatever it picked up on the roof: the rinsed solution and what the wash loosened from the surface.

Two different paths bring roof water to a window. One is the rinse-down itself, the final plain-water pass that clears the solution off the roof. The other is overspray, the solution that drifts beyond the surface it was aimed at. A pane can meet either, and a pane cleaned before the roof is washed can receive both.

Glass is not the only surface on that path. Water leaving a roof also lands on walks and driveways below, and whether roof cleaning stains pavers follows that water to the ground. No source read for this answer measures how much reaches a window or what share of it stays.

Water that runs down a pane and dries leaves behind what it was carrying. That is the whole sense in which roof water dirties glass, and it is why the glass industry's rule is about what is allowed to stay. The National Glass Association's quick-reference guide says: "DO NOT allow water or cleaning residue to remain on the glass or adjacent materials".

The window makers make the same point about timing. The PGT, CGI and WinDoor care instructions tell owners to "Clean and rinse each window or door individually to prevent cleaning agents from permeating for long periods of time." The makers wrote that about window cleaning, not roofs, but a roof solution running down a window sits on the glass and frame just the same.

NGA's own exterior method is built around the same downhill logic. It says "Exterior cleaning should begin at the top of the building and continue to the lower levels to reduce the risk of leaving residue and cleaning solution on glass that has already been cleaned", and it includes checking that the wind is not blowing solution and residue onto glass that is already clean. A roof is simply the highest surface of all.

The residue question is about what dries on the pane. The chemistry question is separate, and it turns on the kind of glass below. The National Glass Association's cleaning paper names "Bleach (or other solutions containing sodium hypochlorite)" in its list of materials that may cause harm to laminated glass, and writes that the plastic interlayer is generally exposed around the periphery of the window glass, where "cleaning fluids and their vapors must be kept away from this area."

Impact windows are built with laminated glass, reasoning from two makers' own lines: PGT says its hurricane windows and doors are made with laminated glass, and Cardinal says its laminated Sea-Storm glass is used in hurricane-resistant windows and doors. ARMA's shingle solution is household bleach, so a roof wash above laminated glass carries a chemistry problem as well as a rinse problem, and keeping that solution off the pane is a requirement of the technique itself. Why it matters at the pane's edge is set out in what is under the roof while a soft wash happens, and the rule for each kind of glass is in impact, Low-E, tinted and tempered glass.

Followed this far, the reasoning points toward an order, and it stays reasoning: neither ARMA nor NGA publishes a sequence for a roof and its windows, and which order any company works in is not stated here. That argument is made at whether the roof is cleaned before or after the windows.

When both surfaces are on the list, the 239 Exteriors estimate form has an option for exactly that: under the service needed, choose More than one service, and describe the roof and the windows in the project details.