Sun & UV Wood Degradation & Wood Rot Treatment and Prevention

Protecting wood in a coastal or other near-water environmental can be more-than-challenging. So many things pose a risk to the integrity of the wood sun as living creatures, the water itself, erosion from sand-blasting, wind, and especially, sunlight.

Sunlight can actually be the main risk factor to many different types of materials. From aesthetic damage (fading and discoloration) to structural (strength degradation to total failure), the harsh light from the sun can be a major pain when it comes to making wood last.

Understanding how the sun causes wood degradation is the first key to solving the issue.

The most common component of sunlight is ultraviolet light—usually just called UV. UV light is also the most harmful aspect of sunlight in relation to wood because it changes and destroys the lignin of wood—the part of wood that hardens and strengthens its cell walls.

This also means that what might appear to be just cosmetic damage is actually much more. Specifically, when wood color starts to change, the wood is being compromised itself. Often, this will manifest first as a change in the wood’s texture. Even more unfortunately, water damage — from bodies of water or even just rain and humidity — will generally make UV damage even worse.

You might first notice a decrease in resilience of the wood, with it becoming more prone to cracking and/or sloughing off of its protective surface. Water can further seep into new cracks in the wood, causing rot, additional fiber deterioration, mold, and Even fungi can get involved. Depending on the specific condition and resulting type of damage, wood rotwet rot or dry rot — could become an issue. Eventually, there could be warping and finally, structural failure from substantive cracking to the point of collapse.

The scientific term for this wood degradation process caused by sunlight is photo-oxidation or wood photo-degradation.

Here is the scientific explanation:

Although the weathering of wood  materials depends on many environmental factors, only a relatively narrow band of the electromagnetic spectrum, the  UV-light portion of sunlight, is responsible for the primary photo-oxidative degradation of wood. The first law of photochemistry, the Grotthus–Draper principle, states that for a photo-chemical reaction to occur, some component of the system must first absorb light. The second law of photochemistry, the Stark–Einstein principle, states that a molecule can only absorb one quantum of radiation at a time (Rabek 1995). The absorbed energy can cause the dissociation of bonds in the molecules of the wood constituents. This homolytic process produces free radicals as the primary photochemical products. This event, with or without the participation of oxygen and water, can lead to depolymerization and to the formation  of  chromophoric groups  such as carbonyls,  carboxyls, quinones,  peroxides, hydroperoxides, and conjugated double bonds (Feist and Hon 1984).

Thus, the photo-oxidation — or wood-degradation — process is usually somewhat complicated and multivariate. For instance, the intensity of the solar radiation impacting the wood can vary by time of day, day of the week, or season. Then, within each of those aspects, there are additional factors such as whether the impact is direct or indirect, and for how long, along with whether there is any intermediary blockage between the wood and sun.

Obviously, all of this ambiguity is also one of the challenges in determining exactly how – or to what extent and how quickly – UV damage occurs with wood. Whether we do this empirically or in a testing environment, all aspects of the equation — from measuring the precise degree of radiation hitting the wood’s surface to the degree and speed of degradation — can be difficult. There is an interesting study on the impact of UV on wood that examines this entire question and process.

The good news is that there are several different methods of protecting wood from UV damage and/or distress.

Paint and Stain Protection as Wood Sealants

Opaque paints and solid stains can be fairly efficient in blocking UB light from wood, making them pretty good wood sealants. However, if not opaque—and more transparent, such is the case when a stain or wash is used that still allows the wood’s natural appearance to be seen through the stain or paint — that protection is greatly diminished. Sure, some UV is still blocked but enough gets through to still cause photo-oxidation to a degree, over time.

One of the greatest considerations in attempting to use paint to block UV as a wood sealant, is pigment present in the stain or paint. Darker colors usually contain more colorant and thus, will typically provide greater protection. Lighter colors usually have less and therefore, less protection. An exception would be lighter colors that use titanium white. This color base or inclusion — titanium white — is actually dense in colorant too, making lighter colors that include it more protective than expected. Furthermore, colors with titanic white also absorb less heat. Meaning, there will be less overall stress on the wood surface over a full day.

Wood Coatings & Injections as Wood Protectants

Wood is not only sensitive to UV radiation, it’s also sensitive to humidity. Then, humidity makes wood even more subject to UV damage (and then fungi and termites in a vicious cycle) as well.

Therefore, it would stand to reason — and it has been proven — that adding protective coatings to wood can prevent some degree of harm from all of the above.

The primary purpose of the wood protectant or coating will be to protect the surface in general. Obviously then, the moisture penetration will become the first path of UV-damage resistance. As mentioned above, adding certain opaque (or close-to-opaque) pigments to other coatings can be helpful.

Another strategy is adding low molecular weight substances that can absorb some of the sun’s incoming UV radiation and dissipate it as heat. Examples are benzotriazole, triazine, and 2-hydroxybenzophenones (Evans and Chowdhury 2010).

One additional, alternative, route of a chemical coating-type process is the injection of formaldehyde resin (PF) into wood cells. These PF’s — PF resins — absorb a good bit of UV light and help to protect wood as well.

Drying Oils and Compounds to Prevent Wood Rot

Liquid oils can be applied to wood that when they react with oxygen, create a solid film that protects wood from moisture and UV impact. Such oils and other coatings that create a semi-physical barrier for wood, not only protect the wood structure beneath them, but also help to preserve the wood’s aesthetics too.

Using Squared Logs or Timbers to Mitigate Sun Damage to Wood

Generally speaking, if all wood exposed to sun / UV could be squared off or installed as “timbers” this could be beneficial in terms of mitigating UV damage. This is because the sun that hit the wood would be evenly distributed. Squared pieces are also not as prone to rain entering the wood since the wood “checks” will face upward due to the cut. Thus, while squared wood pieces are subject to the same stressors as round wood poles, the “harm” is fairly even, instead of focused in one area that could be compromised over time. On the flip side, such squared logs or timbers are not really feasible or cost-efficient in dock or pier building, which can make this “solution” pretty much a non-starter in that area.

Wrapping Wood Piers with Pole Wrap as a Physical Barrier and Protect Wood

Perhaps one of the most effective and efficient ways of protecting wood from UV damage is wrapping the wood in a “sleeve” that is UV resistant. Often going by the term “pile wrap” or “pole wrap” these physical barrier products are generally made of a 100% opaque plastic or other non-porous material that physically shields the wood from all external stressors—including the sun.

Pier Protector is a unique pole wrap — or pile wrap — that is made of HDPE recycled plastic that can last for decades under the water. When it is wrapped around dock poles correctly, it can provide a complete barrier against the elements—and the sun’s UV rays.

Learn more about Pier Protector here.

Evans, P. D., and Chowdhury, M. J. A. (2010). “Photoprotection of wood using polyester-type UV-absorbers derived from the reaction of 2-hydroxy-4(2, 3-epoxypropoxy)-benzophenone with dicarboxylic acid anhydrides,” J. Wood Chem.Technol. 30(2), 186-204.

Feist, W. C., and Hon, D. N.-S. (1984). “Chemistry of weathering and protection,” In: R. Rowell (ed.), The Chemistry of Solid Wood, ACS Advances in Chemistry Series No. 207, p. 401.

Rabek, J. F. (1995). Polymer Photodegradation: Mechanisms and Experimental Methods, Springer, Germany, 664 pp.

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