Tile vs Metal vs Shingle Roof Wash Considerations

Why this matters

A roof is the one surface where the wrong equipment choice is a claim, not a callback. Run a surface cleaner or a straight wand across composition shingle and you strip the mineral granules that give the shingle its UV protection and fire rating, turning a 20-year roof into a 10-year roof on a job the customer thought was cosmetic. Run the same pressure across standing-seam metal and you can crease a panel or force water up under a lap seam never built to take force from that angle. The trade answer, the only one most shingle manufacturers and the Asphalt Roofing Manufacturers Association (ARMA) will stand behind, is a no-pressure chemical wash: kill the organism with a sodium hypochlorite (SH) based mix and let gravity and rinse water do the rest, never a pump against the deck. That single choice is the difference between a wash that adds years to a roof and one that voids the warranty in writing.

The no-pressure method

The organism you are usually treating is Gloeocapsa magma, a blue-green algae showing up as black or gray streaking, worst where the roof stays shaded and slow to dry. It lives inside the shingle's mineral surface, not on top of it, which is why scrubbing or blasting never removes the root and the streaking returns within a season; killing it chemically does. ARMA's published guidance on cleaning algae-discolored roofs (confirm the current edition, and cross-check against the shingle manufacturer's own care guidance, since some publish a stricter version) calls for a low-pressure or no-pressure chemical application, never a pressure washer against the shingle.

The sequence: pre-wet everything below the roofline the runoff will touch, before mixing anything. Mix the roof-wash solution at a dilution matched to material and soil load (the worked example shows how that number gets set), applied at garden-hose pressure through a pump-up or skid sprayer, never the pressure washer's high-pressure gun. Apply top-down, ridge to eave, in sections sized to what you can rinse before the label's maximum dwell time passes, without scrubbing; the chemical does the work, and agitation is the mechanical action you are avoiding. Let it dwell for the label's window, watching for the streaking to lighten, then rinse ridge-down at garden-hose pressure, no pump, no narrow tip, confirming clear runoff before the next section.

Material by material

The chemistry stays close to constant across all three; what changes is mechanical handling, the dilution ceiling, and the failure mode when you get it wrong.

Composition shingle. The granule layer is the vulnerability: bonded to the asphalt mat but not permanently, and any foot traffic, brush, or pressure stream dislodges it. SH at a roof-rated dilution is generally compatible with the mat, but hold dwell and concentration inside the label's window on an older roof, since less binder holds the granules and an over-strength mix shows up as bleaching and faster granule loss at rinse. Walk a shingle roof only when inspection requires it, at the headlap, never mid-tab.

Concrete and clay tile. Tile is heavier and more chemically inert, so SH exposure is rarely tile's own failure mode; the failure mode is foot traffic and runoff into the unglazed joints. Tile can be walked, but only at the headlap over a batten, never the center or unsupported tail, exactly where an undeliberate boot lands. A cracked tile from a wrong step is often not visible from the ground and shows up as a leak two rains later, so photograph a walked path when one is required. An acid-based treatment misapplied to a stain on glazed tile can etch it; keep treatment local with a rinse boundary. Grout joints absorb runoff and hold a stain if the rinse is rushed.

Metal roofing. Two risks stack. The finish: a Kynar or PVDF factory coat tolerates a properly diluted, properly rinsed mix well, but a lower-grade or aged, chalked finish can spot or dull if the mix dries on it, so metal gets the shortest dwell-to-rinse interval of the three. The metal itself: bare or scratched galvanized steel, and any dissimilar-metal contact point, can corrode faster when chlorine-bearing runoff sits rather than sheets off. Never walk standing seam except on designated walk pads; the pan between seams dents under a boot's point load. Wet metal is the most slip-prone surface of the three.

Runoff management off the roof plane

Pre-wet the drip line before starting, and give the same beds a thorough clean-water rinse after finishing, not just where foam collected; a pooling point at a downspout can sit long enough to burn foliage even after the roof looks clean. Route runoff away from wood decking, unsealed pavers, and anything that stains or etches from chlorine-bearing water. Many jurisdictions regulate wash water with chemical additives entering a storm drain, and some require containment, diversion, or dechlorination first; that threshold varies by municipality and state, so confirm the current local ordinance or your state environmental agency's guidance before the first job in a new area, not after a complaint. Overspray does not respect a property line; confirm wind direction before starting a section.

Controlling the sodium hypochlorite hazard

SH is a real chemical hazard on its own, independent of the roof under it, and needs the same specific, in-clause control as a gas or electrical hazard.

Wear chemical-resistant gloves and splash-rated eye and face protection whenever mixing or applying, since roof application means overhead drift back onto you. And if that does not hold, then - flush contact with clean water for at least fifteen minutes, remove contaminated clothing while flushing, and get medical evaluation rather than waiting to see if irritation passes.

Keep the roof mix segregated from any acid-based product on the truck and mix station, including a rust remover or acid-based gutter brightener staged for the same job, because hypochlorite plus an acid releases chlorine gas. And if that does not hold, then - a sharp, pool-like odor or a yellow-green haze means stop mixing immediately, move upwind, and do not re-enter until the area has ventilated.

Hold concentration and dwell time inside the label's rated window for the material, since exceeding either turns a controlled wash into fabric, screen, or metal damage from the mix drying where it was not meant to sit. And if that does not hold, then - a spotted surface or a white residue needs an immediate fresh-water rinse, because SH residue keeps reacting until it is rinsed away or fully dry.

Before starting, cover or shut down anything in the wash path the label warns against wetting directly, most often a powered attic vent or solar array junction. After the final rinse, uncover and restore each item and verify it works before leaving; a covered item left uncovered is now a service call with your name on it.

Ladder and roof-access ratings

No-pressure application from a ladder, working the eave line without stepping onto the roof, is the safer default whenever pitch and eave height make it workable. When a job genuinely requires roof access, confirm the ladder's duty rating against the combined weight of the technician, tools, and mix container, against the current edition of the applicable ANSI standard for the ladder's material, since duty-rating classifications are periodically revised. Read the rating on the specific ladder, not from memory.

Results develop over the following weeks

This is the single most common customer-expectation problem in roof washing, and the one that generates a callback for a job that actually worked. A no-pressure wash does not look finished the moment the water stops: the algae is killed at the time of service, but the dead organic matter releases from the surface gradually, over roughly two to six weeks, as sun and rain carry it away, rather than as an immediate visual result. A customer who watched a neighbor's roof come out streak-free in an afternoon, under a pressure method you would never run here, will call the day after convinced nothing changed.

Set this before starting, not after the complaint call: tell the customer the roof will look largely the same the day you leave, that improvement builds over the following weeks, and give a specific date to expect the bulk of the change by, not a vague "give it some time." Photograph the roof before starting and at completion, dated. A call at the two-week mark resolves with that photo pair; staining still visible past six to eight weeks is a different problem, worth checking whether dwell or coverage was adequate.

Worked example: a mixed shingle-and-metal roof

A job comes in for a 1,800 square foot composition shingle main roof with visible streaking, plus a 600 square foot standing-seam painted-steel porch roof tied in at a shared valley, 2,400 square feet total.

The label's finished-mix coverage rate for a moderately soiled shingle roof runs 150 to 200 square feet per gallon; take the midpoint, 175, as the planning number (start from your own product's label, since coverage is set by the manufacturer and soil load, not by this article). 1,800 divided by 175 is about 10.3 gallons, rounded up to 11 for edge overlap and a second pass on the darkest streaking. The roof-rated dilution for a moderate load runs 1-to-1, concentrate to water, as a starting ratio (confirm your own label, since ratios commonly run 1-to-1 up to 1-to-3); at 1-to-1, 11 gallons finished is 5.5 gallons concentrate and 5.5 gallons water.

The porch roof's finish sheet calls for a weaker mix and a shorter dwell-to-rinse window, so the concentrate ratio is cut roughly in half while coverage stays the same: 600 divided by 175 is about 3.4 gallons, rounded to 4, built at roughly 1-to-3, about 1 gallon concentrate and 3 gallons water. Total concentrate for the visit: 5.5 plus 1 is 6.5 gallons, plus surfactant at the label's stated rate.

It is an 88 degree day with direct sun. Heat makes hypochlorite off-gas faster, so the shortened end of the dwell window applies; the tech works the shingle field in three roughly 600 square foot sections, rinsing each before moving on rather than applying all 1,800 square feet first. The porch roof, with its own shorter window on the same heat, is worked as one section immediately before its own rinse.

Rinsing runs ridge to eave. Shingle runoff clears from milky to clear within the first pass per section, the cue the chemical carried off rather than dried on the surface. The porch roof gets a deliberate second pass at the seams and the valley, where higher-roof runoff can pool briefly against the metal, and pooled contact time is what risks a finish spot.

A heavier soil load would push coverage toward 150 square feet per gallon, raising the shingle field's volume to roughly 12 gallons; a cooler day would let dwell run longer without changing the concentrate ratio. Either way, the metal roof still gets the weaker mix and the shorter interval, regardless of weather.

How to verify you got this right

Confirm rinse water ran clear, not milky or foaming, off every section before moving on. Check landscaping in the drip line for wilting or bleaching immediately after and again at 24 to 48 hours, since a chemical burn does not always show at the moment of rinse. Check the metal-to-shingle transition and any valley for a dried white residue or a dulled spot; either means a section dried before its rinse and needs the fresh-water flush described above. Confirm every covered or shut-down item is restored and functioning, and log the completion date, the follow-up date two to three weeks out, and the mix ratio and coverage rate used on each material.

References

• Asphalt Roofing Manufacturers Association (ARMA), published guidance on cleaning algae-discolored asphalt shingle roofs (confirm current edition and cross-check against the shingle manufacturer's own guidance).
• Shingle and metal-panel manufacturer care and warranty documentation for the specific product installed (dwell time, dilution ceiling, and pressure-washing exclusions vary by manufacturer).
• Local municipal or state environmental agency guidance on exterior-cleaning wastewater discharge to storm drains.
• Current edition of the applicable ANSI ladder standard for duty ratings by ladder material.
• See related: reading rust and corrosion patterns.