Does a dirty roof make a house hotter?

Three documents touch this question, and each was written about a different kind of roof. Before any conclusion, here is what each one covers and what each one says.

Read across, the three do not contradict one another so much as speak about different things. Two speak to roofs built to reflect sunlight. One is a tile maker speaking about its own product. None of them measured an ordinary house under an ordinary roof with growth on it.

EPA defines the stakes for one kind of roof. “A high solar reflectance, or albedo, is the most important characteristic to understand in terms of how well a cool roof reflects heat from the sun away from a building.”

ARMA's algae and moss bulletin makes the one warning of the three, and limits it to those roofs. On highly reflective or “cool” roofs, it says, algae “will undoubtedly have a negative impact on the long-term effectiveness of these roof systems.” Undoubtedly is ARMA's word. So is the scope.

The reasoning behind that is not hard to follow. A cool roof does its job by reflecting sunlight, and the most common roof algae makes a dark pigment: ARMA says the most common type “protects itself from damaging ultraviolet rays by producing a dark pigmented sheath,” and its June 2024 bulletin names Gloeocapsa magma as probably the most prevalent of several algae species involved. A dark layer over a surface meant to reflect is the conflict that reasoning suggests, though ARMA's sentence does not spell out its reason. How large the effect is on a given roof, neither ARMA nor EPA says.

EPA's upkeep line keeps the same focus. Cool roofs may call for “periodic maintenance to keep the roof clean and maximize its reflectance, particularly for low-sloped cool roofs.” Low-sloped is EPA's qualifier, and a pitched tile or shingle roof is not the case that sentence singles out.

A concrete tile maker reaches a different conclusion for its own product. In a September 2020 article, Eagle Roofing Products writes that dirt, mold, mildew, algae, bacteria or other debris will not affect the “energy efficiency of your roof”, and adds that some homeowners may consider their appearance less than desirable.

Eagle's earlier article, from 2019, calls the same buildup of algae, mold and mildew “typical of all natural materials” and says it is not harmful to the integrity of the tile. Both statements are Eagle's, about Eagle's concrete tile. They are not a rating of any other maker's tile, and they do not state what any one roof's reflectance is.

That scope is why this answer does not stretch Eagle's sentence over every roof in town. What each maker publishes about cleaning its own product is set out in the roof cleaning page's section on tile, shingle and metal, and how a white concrete tile goes dark to begin with is covered in why a white tile roof turns black.

The figure people want, how many degrees warmer a room gets or how much harder the air conditioning works, is not in any source read for this answer.

EPA's cool roof page does print temperature and energy comparisons, but they compare a cool roof with a conventional roof. They do not compare a clean roof with a soiled one, so none of those figures is repeated here as the effect of dirt. Lifting a number from one comparison to answer a different question would make this answer easier to quote and less true.

Ultraviolet light is a separate subject that is easy to fold into this one. What the sources describe as UV degradation is aging, not heat: CertainTeed says UV rays reaching unprotected asphalt will age the asphalt prematurely, and U.S. Steel describes chalk on a paint film as a by-product of ultraviolet degradation of the paint system. Neither document connects that aging to the temperature inside a house.

Nor do the sources put a figure on the seasons. GAF says blue-green algae requires moisture and proper temperatures to grow, and why a roof can look darker after a stretch of wet weather is taken up in why a roof looks worse after a wet winter. A darker look is a change the documents describe in appearance, not a measured change in heat.

Because nothing published settles the general question, the specific one goes to people who can measure or rate the roof in front of them.

• An energy professional can look at how one house actually cools, which is a measurement of that house rather than of roofs in general.
• A roof product's maker says whether the product is sold as a cool roof, what its reflectance is, and what upkeep it asks for. EPA names reflectance as the property that matters.
• A roofing contractor speaks to the roof's condition, which is a separate matter from how it looks or how warm the attic runs.

A cleaner can speak to appearance and to the published method, and the line past that is drawn in where cleaning ends and roofing work begins. Why growth shows sooner on a pale surface is its own answer, in how soon white roofs show growth.

Where ARMA and Eagle agree is appearance: both say growth changes how a roof looks, and neither measures how warm a room gets. That is curb appeal rather than comfort, and whether it adds anything to a home is weighed in whether exterior cleaning adds value.