Prevention beats cleaning — but the free option beats both

Everyone else ranking for this is paid when you say yes. Here's when you shouldn't.

• The roof still has moss or streaking on it. Prevention suppresses new growth and clears nothing. Clean first, then prevent — in that order, or the money is wasted.
• Your roof is open and sunny and has never grown anything. Prevention is insurance against a problem you do not have. Plenty of roofs simply never develop growth, and paying to protect one is a solution shopping for a cause.
• Trimming the trees is available and you have not done it. Do the free intervention before the paid one. If a canopy is keeping the roof permanently damp, treatment is fighting a condition you could have removed.
• You are being sold a recurring treatment subscription on a roof near the end of its life. If replacement is close, algae-resistant shingles at re-roof solve it permanently. Committing to years of treatment on a roof you are about to remove is the wrong purchase.
• Your roof is metal or tile. The whole strip-and-inhibitor approach is built around asphalt shingle. On other substrates the growth behaves differently and the standard prevention pitch does not straightforwardly transfer.
• You are told strips will keep the whole roof clean. They only affect the surface rainwater runs across on its way down from where they are fitted. Slopes and areas outside that path see no benefit, and honest operators say so up front.

Fitting a strip near the ridge means fastening through or under shingles at the highest point of the roof. Done wrong it creates a leak at the worst possible place, and it puts you at the steepest, least forgiving part of the slope to do it. This is the DIY job in this vertical most likely to cost more than it saves.

• Does my roof need cleaning before any prevention goes on?

  The correct answer on a roof with visible growth is yes. An operator who says prevention alone will clear it is either wrong or selling.

• Where exactly will the strips go, and which slopes will they cover?

  Strips only benefit the surface below them, on the slope they sit on. Placement is the whole product.

• How will you fasten them without creating a leak path?

  This is roof penetration work at the ridge. The method should be something they can describe without hesitating.

• If this is a treatment, how often will it need renewing and at what price?

  Get the recurring number, not the first-visit number. A cheap application on a short cycle is not cheap.

• What effect will you honestly promise, and what happens if growth returns anyway?

  Suppression is a fair claim. Guaranteed permanent cleanliness is not, and the gap between them is where disputes start.

• Would trimming my trees reduce the need for this?

  The answer you want is an honest yes where it is true. A contractor who tells you the free option first is a contractor worth using again.

• Is this appropriate for my roof material?

  Strips and inhibitors are an asphalt-shingle playbook. On tile or metal, ask what specifically is being proposed and why.

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