7 Questions for Roof Cleaners

Roof cleaning is an unlicensed activity that anyone with a ladder can offer, and the most commonly marketed method is the one that does the most damage. A short list of questions separates companies that will extend your roof's life from companies that will shorten it in an afternoon.

Question 1: What pressure do you use on asphalt shingles?

The single most important question, and the answer should be an immediate and unambiguous none.

High-pressure water strips the mineral granules that are an asphalt shingle's only UV protection. It also drives water up under shingle courses onto the underlayment, and it can break the thermal sealant bonds holding tabs down. The roof looks transformed for one season and then ages at an accelerated rate permanently. It typically voids the manufacturer's warranty as well.

If a company describes pressure washing, power washing, or any variation applied to an asphalt roof, the conversation is over. This is disqualifying, not negotiable, and it does not matter how good the before-and-after photographs look — the damage is invisible in photographs and appears years later.

Note that low-pressure or soft washing, where a cleaning solution is applied at garden-hose pressure and allowed to work, is a different thing and is legitimate.

Question 2: Which direction do you brush?

Downward, with the shingle lay. Always.

Brushing upward against the courses lifts and fractures shingle edges — doing rapidly and by hand the exact damage moss does slowly. A company that has not thought about brush direction has not thought about the roof.

Question 3: What product, and when will you apply it?

You are listening for two things: a named product rather than a vague chemical description, and an understanding of timing.

Moss is metabolically active in the cool wet months and dormant in summer, so treatment applied to dormant August moss accomplishes far less. A company that treats year-round without mentioning this either does not know or is not saying.

Ask also whether they will apply and leave, or apply and immediately scrub. The better answer is to treat, let the moss die and release its grip over subsequent weeks, and let it weather off — which is the gentlest removal available. Immediate aggressive scrubbing after treatment forfeits that benefit.

Question 4: How will you protect plantings and manage runoff?

In Seattle this is both an environmental and a practical question. Roof runoff reaches storm drains that discharge to Puget Sound and salmon-bearing waterways, and zinc and copper compounds are toxic to aquatic life at low concentrations. Bleach-based products are harsh on plantings and on metal flashings and fasteners.

A competent answer covers pre-wetting and covering vegetation, avoiding application before heavy rain, and controlling or capturing initial runoff. A blank look tells you they have not considered it and that your garden is about to find out.

Question 5: Will you walk the roof, and are you insured for it?

Foot traffic damages roofs, particularly older brittle shingles and particularly in cold weather when asphalt is least flexible. Ask how they minimise it and whether they can work from ladders and edges where possible.

Then ask for proof of liability insurance sent directly from the insurance agency rather than handed over as a photocopy, and confirm workers' compensation coverage. Someone falling from your roof uninsured is a problem that becomes yours, and roof work has a genuinely high injury rate.

Question 6: What will you tell me about why the moss is there?

This is the question that separates a service from a solution.

Moss returns because the conditions producing it are unchanged: overhanging limbs, permanent shade, debris accumulation, slow drying. A company that removes moss and says nothing about causes has sold you a recurring appointment. One that points out which limbs matter, which planes are worst and why, and recommends zinc strips or debris management is giving you something you can act on.

Question 7: What did you find while you were up there?

Ask for this before they start, so it is expected. Someone on your roof has the best view anyone will get of it this year. Cracked vent boot collars, lifted or creased shingles, debris packed in valleys, damaged flashing, missing kickouts, granule loss — all of it is visible to a person standing there.

Ask for photographs of anything found. A company willing to do this is worth keeping. It also gives you an annual condition record at no additional cost.

The red flags, compiled

Pressure washing offered for asphalt. Unsolicited door-knocking with urgent findings. Dramatic claims about immediate structural risk without a photographed defect. Pressure to decide during the visit. No proof of insurance, or a photocopied certificate. Vagueness about products. No mention of runoff or plant protection. And a quote that arrives before anyone has looked at the roof.

None of these questions is hostile, and a good operator will answer all seven without friction. The pattern of the answers is the information you are buying — and it costs one phone call.