Does Wet & Forget Actually Work on a Roof?
Yes — slowly, and only if you can get it onto the roof evenly. Wet & Forget and the products like it are genuinely effective at killing the organism behind black roof streaks. What they don't do is remove it. The product kills the growth and then leaves the dead material on the shingles for wind and rain to wear off over the following three to twelve months. That's not a flaw in the product — it's the design, and it's printed on the bottle. So the honest answer depends on what you actually want: if you're willing to wait most of a year and you can safely reach the whole roof, it works. If you want the streaks gone before your daughter's graduation party in three weeks, it will not do that, and no amount of re-spraying makes it faster.
What it does well
Give it credit where it's earned. These products use a quaternary ammonium compound rather than bleach, which means no chlorine smell, no immediate risk to most plants below, and no pressure anywhere near your shingles — and pressure is the thing that actually destroys roofs. On a roof with light, early streaking, sprayed on a cool overcast day with no rain coming for a few hours, it will kill the growth and the roof will look meaningfully better by the following season. If your streaking is mild, you're comfortable on a ladder, and you're not in a hurry, this is a reasonable thing to do and we'd rather tell you that than pretend otherwise.
The three reasons it disappoints people
1. Coverage. The bottle assumes you can reach every square foot. From a ladder at the eave, you can reach the bottom four to six feet of a roof plane and not the rest — so the bottom clears up, the top stays black, and now the roof looks worse than it did. A hose-end sprayer doesn't fix this; it dilutes the product and still can't carry to a ridge two stories up.
2. Time. Three to twelve months of weathering is the stated mechanism. Most people re-apply at week six because "it didn't work," which wastes the product and doesn't speed anything up.
3. It doesn't stop it coming back. The black on your roof is a bacteria, Gloeocapsa magma, and it's arriving on the wind from every other roof in the neighborhood. Killing this generation doesn't change the conditions — a shaded north plane under a tree will re-streak, whichever method cleared it.
What a professional soft wash does differently
The method isn't the interesting difference — the outcome is. A roof soft wash uses low pressure and a cleaning solution to kill the growth and rinse it off the same day, so the roof is visibly clean when we leave rather than in nine months. It's applied from the roof itself or from a system that reaches the ridge, so the whole plane is treated evenly instead of the bottom six feet. And nobody in your household goes on a ladder. That's the trade: the DIY route costs about $30 and a Saturday and pays off next spring; the professional route costs more and pays off that afternoon. Both are legitimate — they're answers to different questions.
What's not legitimate on a roof is pressure. See whether soft washing is safe for shingles — the short version is that a pressure washer strips the granules that are the shingle's entire UV protection, and it will void most manufacturer warranties.
So which should you do?
Spray it yourself if the streaking is light, you can safely reach the whole roof, and next spring is soon enough. Get it done if the streaks run to the ridge, the roof is two stories or steep, you want it gone this month, or you've already tried a bottle and only the bottom cleared. If you're not sure which you've got, text a photo of the roof to the number on this site — we'll tell you honestly which one you're looking at, and we'll say so if the bottle is the better buy.
Common questions
Does Wet & Forget really work on roof algae?
Yes, it kills the growth — but it doesn't remove it. The product is designed to let wind and rain weather the dead material off over three to twelve months, so the roof clears up gradually rather than the day you spray it.
How long does Wet & Forget take to work on a roof?
Three to twelve months for the dead growth to weather off, depending on how much rain and wind the roof gets. Re-applying sooner doesn't speed it up; it just uses more product.
Why did Wet & Forget only clean the bottom of my roof?
Because that's as far as the spray reached. From a ladder at the eave you can cover the bottom four to six feet of a roof plane, so that section clears and the rest stays streaked. It's a coverage problem, not a product problem.
Is Wet & Forget cheaper than having the roof cleaned?
Per bottle, yes. It's roughly $30 plus your time versus a professional soft wash. The difference is speed and coverage: the DIY route clears gradually over the next several months and only where you could reach, and a professional wash clears the whole roof the same day.
Can I pressure wash the algae off my roof instead?
No. High pressure strips the granules off asphalt shingles, and those granules are the shingle's UV protection — it shortens the life of the roof and voids most manufacturer warranties. Roofs get low pressure and a cleaning solution, never a pressure washer.