Why Does Untreated Wood Turn Gray Outdoors?
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Anyone who has left a wooden fence or deck outdoors has seen the color change. The short answer to why does untreated wood turn gray outdoors is that sunlight, mostly its ultraviolet part, breaks down the wood at the surface. The chemistry is described in research reviews on wood weathering and on wood photodegradation. The explanation below follows those reviews step by step. Quoted wording is copied from the fetched pages listed at the end.
Quick answer
- Wood changes color because outdoor exposure degrades its surface. The weathering review calls this “the slow degradation of the wood surface.”
- Ultraviolet light is the main trigger, and lignin is the part of wood that absorbs it most strongly.
- Over a long exposure, the degraded lignin is washed out, and a gray, cellulose-rich layer is left on top.
- Fungi and dust can add to the gray look, according to the same review.
What weathering means
The change has a formal name. The weathering review defines it this way: “Weathering can be defined as the slow degradation of the wood surface that occurs through the combined effect of sunlight, water, oxygen, temperature, and atmospheric pollution.” The review adds that “One of the major disadvantages of wood is the deterioration of its surface when exposed outdoors, known as weathering.”
Color is usually the first visible sign. The review says that “Colour change is the first sign of complex chemical reactions on the surface of wood exposed to weather, initiated by solar radiation, especially the ultraviolet (UV) part of the spectrum.”
Why ultraviolet light does the damage
Ultraviolet light carries enough energy to break chemical bonds. The weathering review explains that “UV radiation with its high amount of energy can cause the breakdown of bonds in the polymeric molecules of wood and photochemical reactions that lead to the depolymerization of lignin and cellulosic polymers in the wood cell wall.” The breakdown affects the structural polymers that make up the wood itself.
Lignin is the most exposed part. The review states that “Of all wood polymers, lignin is the best absorber of UV light.” It also says that “Lignin absorbs more UV radiation than cellulose, resulting in more photochemical degradation reactions that involve free radicals.”
The coatings review describes the same reaction chain. It states that “The conjugated double bonds and phenolic hydroxyl groups within lignin are highly photoactive; upon UV absorption, they readily initiate radical chain reactions, leading to polymer chain breakage and chromophore formation.”
Yellowing first, then gray
Early color change in many woods is yellowish. The weathering review explains that “The phenolic hydroxyl groups in lignin react with UV radiation to form aromatic free radicals (phenoxy radicals), which further react with oxygen to form quinoid structures that are responsible for the yellowing of wood.” So the first shift is often toward yellow or brown tones rather than gray.
The gray look comes later. The review says that “after a long period of natural exposure (depending on climatic conditions) the surfaces of all species turn grey because the photodegraded lignin portions are leached from the wood surface, and a cellulose-rich surface layer remains on top.” In other words, the photodegraded lignin is leached away by weather, and a cellulose-rich layer remains on the surface.
The timing depends on the climate, as the quote notes in its own parenthesis. The review does not give a number of months or years for any particular region, and this explainer does not supply one.
Other contributors to the gray surface
Lignin breakdown is not the only factor. The weathering review states that “The growth of fungi and moulds and the dust particles which penetrate the porous structure of wood may also contribute to the grey colour.” The word “may” is important here: the review presents these as possible contributors rather than the main mechanism.
Water and temperature also appear in the weathering definition quoted earlier, since weathering is a combined effect of “sunlight, water, oxygen, temperature, and atmospheric pollution.” The sources treat these as parts of one process, not as separate causes that can be ranked.
Where the coatings research fits
Many people protect wood with coatings, and the coatings review explains what happens to those layers. It states that “Photodegradation arises from polymer chain scission, chemical structure reorganization, and photo-oxidation of lignin and cellulose, leading to coating chalking, cracking, gloss loss, and color changes.” It adds that “Ultraviolet radiation can penetrate the coating and interact with the wood substrate, inducing lignin photodegradation and cellulose structural damage.”
The review notes that light intensity, temperature, and humidity shape aging, and it lists “lignin and extractives as major contributors to discoloration.” For a general question about untreated wood, the same mechanism explains why lignin is the first component to change.
The weathering review also describes the protection research as a field of its own. It states that “This paper provides a literature survey on the recent research results in protecting wood from weathering.” The topics it covers include “surface treatments of wood with photostabilizers; protection with coatings” along with other methods, so the review treats protection as a separate question from the chemistry of weathering itself.
For related garden questions about wood, see can you use wood chips as mulch and how to use wood ash as fertilizer.
How we chose the sources
Both sources are review articles hosted on PubMed Central, a non-retail archive. The first reviews the weathering resistance of wood. The second reviews the photodegradation of wood coatings and the mechanisms behind it. Both pages returned HTTP 200 when fetched directly, and each quote in this explainer was copied from the fetched text.
What we could not verify
Both sources are reviews. They summarize a body of research rather than measuring one fence, deck, or board. They also cover a range of species and conditions without giving a single timeline for color change.
What the sources do not say
The weathering review says that the surfaces of “all species” turn gray after long exposure, but it does not give a period in years for any species. The reviews also do not compare untreated boards directly with a specific product. This explainer does not make those comparisons, and it does not say how fast a particular backyard board will change.
Frequently asked questions
Is gray wood rotten?
The sources describe gray color as a surface change. The weathering review describes “the slow degradation of the wood surface,” and the gray layer is the cellulose-rich surface that remains after lignin is leached. The sources do not address decay inside the board, so this explainer does not make that claim.
Why does the first color change look yellow?
The weathering review ties yellowing to lignin. Its wording is that quinoid structures are “responsible for the yellowing of wood.”
Does sunlight alone turn wood gray?
Sunlight is the main trigger, but the review lists several factors. It says the process is “the combined effect of sunlight, water, oxygen, temperature, and atmospheric pollution.”
Do fungi cause the gray color?
They may contribute. The review says that fungi “may also contribute to the grey colour,” but it does not describe them as the main cause.
Do coatings stop the color change?
Not by themselves. The coatings review explains that “Ultraviolet radiation can penetrate the coating and interact with the wood substrate, inducing lignin photodegradation and cellulose structural damage.”
Sources
- “Enhancing Weathering Resistance of Wood: A Review.” PubMed Central article. https://pmc.ncbi.nlm.nih.gov/articles/PMC8234773/ (HTTP 200, checked 2026-10-28). Quote: “Weathering can be defined as the slow degradation of the wood surface that occurs through the combined effect of sunlight, water, oxygen, temperature, and atmospheric pollution.”
- “Photodegradation Mechanisms and Anti-Aging Strategies of Wood Coatings: A Comprehensive Review.” PubMed Central article. https://pmc.ncbi.nlm.nih.gov/articles/PMC13166035/ (HTTP 200, checked 2026-10-28). Quote: “Ultraviolet radiation can penetrate the coating and interact with the wood substrate, inducing lignin photodegradation and cellulose structural damage.”
