Patience is a Virtue: Impressive Decades-Long Research Results

It takes patience to be a scientist. Research can be time consuming, especially when you’re working in the Forest Products Laboratory’s (FPL) Durability and Wood Protection research work unit. Part of the unit’s mission is to treat wood with preservatives and track how long the treated wood can fend off decay. Success can mean decades-long studies.

FPL's Valley View field test site.

FPL’s Valley View field test site.

Recently, several members of the unit made the trek to FPL’s Valley View test site, an unassuming field west of Madison, Wisconsin, where wood in various forms is left to endure the elements, often for years. Their mission: to inspect treated wood stakes that have been buried in the ground for 40 years.

Ground-level view of intact 2 x 4 field stake showing no visible deterioration after more than 40 years of soil contact.

Ground-level view of intact 2 x 4 field stake showing no visible deterioration after more than 40 years of soil contact.

The stakes, both solid wood and plywood, are made of southern pine or western wood species, such as Engelmann spruce and Douglas fir. They were treated with either chromated copper arsenate (CCA) or ammoniacal copper arsenate (ACA).

The results are astounding. There were stakes, both solid wood and plywood, that achieved a rating of 10 (sound) after 40 years of soil contact.  For comparison, untreated southern pine stakes in contact with soil typically last one-and-a-half to two years before failure (rating of zero) due to decay fungi.

Field tests are critical for establishing the durability of treated wood products that are used for construction of decks, bridges, and utility poles. The test site at Valley View has been maintained by FPL since the early 1950’s. Matched sets of many of the studies at Valley View site are also installed in the Harrison Experimental Forest in southern Mississippi, which represents a more severe decay environment due to increased average temperature and rainfall.

Examples of sound (rating of 10) 2 x 4 (top) and plywood field stakes after 40 years of soil contact in southern Wisconsin.

Examples of sound (rating of 10) 2 x 4 (top) and plywood field stakes after 40 years of soil contact in southern Wisconsin.

Blog contributed by Grant Kirker, Amy Bishell, and Stan Lebow.

Cottage Conundrum: FPL Fields Cleaning Question

All of the research and expertise of the Forest Products Laboratory (FPL) is in vain if we can not use it to better the American people. Our lab receives many questions for our researchers, and recently, we received a question regarding T1-11 cedar siding.

Dear FPL,

I have a problem with the T1-11 Cedar Siding (plywood) that was installed on my cottage back in 1986. There is a black fungus on all sides of the cottage, even where the front is exposed to the direct sunlight most of the day. I would like to have a vinyl siding installed over the T1-11, but am not sure if I should have that done without washing the siding first. I have washed the siding in the past and the fungus has returned! I would appreciate any advice you can provide to me.

Cottage Conundrum

Luckily, Mark Knaebe, a Natural Resources Specialist with the Forest Products Marketing Unit, had the answer!

Dear Cottage Conundrum,

The most important question here is what do you really have?  If you used what is commonly called a “wood bleach” which is oxalic acid, and it brightened up, then you have iron stain. Iron stain is not a fungus.  


Although it leaves wood black, iron stain is not a fungus. The above example was caused by corroded fasteners.

If you used a chlorine or oxygen bleach and it brightened it up, then you have mildew, which is a form of fungus, and it may not be a huge problem except it that it can keep wood wetter for longer periods which can promote decay fungus (rot) in very wet areas.

I would not give up on the T1-11 just yet.

If you are set on putting vinyl over it, and it is iron stain, you can ignore the black. If you have mildew, you should find out why first, and if bleach cleans it up, you could just bleach and put up the new siding paying special attention to proper flashing. Traditional penetrating coatings contained oils which are food for mildew so as soon as sunlight destroys the mildewcide, mildew can quickly return, so that might be what you’re experiencing.

The bottom line is find out what it is first, and take the next steps from there. Thanks for your question!



Temperature Down, Danger Up : Wood Heating Caveats From FPL

Most residential buildings in the United States employ wood as a primary construction material, and increasingly, commercial buildings are following suit. Although researchers at the Forest Products Laboratory (FPL) have spent the better part of a century formulating new treatments and methods for improving the fire durability of wood, fire safety remains a serious consideration, particularly during the winter.

Fire retardant test at FPL, 1940s.

A fire retardant test at FPL during the 1940s. FPL research has helped improve building codes, wood treatments, and testing standards.

This danger can be compounded depending on your method of heating. As the temperature goes down, if you choose to heat your home with wood, fire danger goes up. Proper precautions should be taken to ensure that the fire stays contained in the stove or fireplace, lest it spread to the surrounding structure.

According to The Wood Handbook: Wood as an Engineering Material, one of the most important problems associated with home fires is the smoke produced. The term smoke is frequently used in an all-inclusive sense to mean the mixture of pyrolysis products and air that is near the fire site. In this context, smoke contains gasses, solid particles, and droplets of liquid — but why is smoke so dangerous?

Smoke presents a potential hazard because it interacts with light to obscure vision, but the toxicity of combustion products is the primary concern. Fire victims are often not touched by flames but die a s a result of exposure to smoke, toxic gasses, or oxygen depletion. These life-threatening conditions can result from burning contents, such as furnishings as well as from structural materials involved.

The toxicity resulting from the thermal decomposition of wood and cellulosic substances is complex because of the wide variety of types of wood smoke. Composition and the concentration of individual constituents depend on such factors as the fire exposure, oxygen and moisture present, species of wood, any treatment of finishes that may have been applied, and other considerations.

The vast majority of fires that attain flashover (a fire’s sudden spread when an area is heated to its flashpoint) do generate dangerous levels of carbon monoxide, independent of what is burning. Carbon monoxide is a particularly insidious toxic gas and is generated in significant amounts in wood fires.

Even small amounts of carbon monoxide can be toxic because the hemoglobin in the blood is much more likely to combine with carbon monoxide than with oxygen, even with plenty of breathable oxygen present. Generally, two approaches are used to help deal with the smoke problem: limit smoke production and control the smoke that has been produced. The control of smoke flow is most often a factor in the design and construction of buildings.


Wall test conducted in the large vertical furnace at FPL showing a wall panel at the point of fire burn through. Information of this nature is used in building designs to help ensure time for people to exit a burning structure and help contain the fire and smoke.

Draftstops are one useful control measure construction engineers implement. Draftstops are barriers intended to restrict the movement of air within concealed areas of a building. The are typically used to restrict horizontal dispersion of hot gases and smoke in larger concealed spaces such as those found within wood joist floor assemblies with suspended dropped ceilings or within an attic space with pitched chord trusses.

Doors can also be critical in preventing the spread of smoke and fire, even if they are made out of wood. Doors left open or doors with little fire resistance can easily defeat the purpose of a properly fire-rated wall or partition. Listings of fire-rated doors, frames and accessories are provided by various fire testing agencies. When a fire-rated door is selected, details about what which type of door, mounting, hardware, and closing mechanism must be considered.

Finally, keep in mind that smoke rises, and that when evacuating a burning building, clean air can usually be found closer to the ground. For more information on home fires, and tips to keep you and your family safe, visit

For more information about the fire resistance of wood, please see Chapter 18 of The Wood Handbook: Wood as an Engineering Material.





Choose the Right Fasteners to Avoid Unsightly Iron Stains

The following information is from Forest Products Laboratory’s Wood Handbook, Wood as an Engineering Material.

Iron stains occur from rusting of fasteners or by the reaction of iron with tannins in wood. The appearance is different for each of these reactions.

In wood species that lack tannins, iron merely rusts, giving a brown stain to the wood surrounding the fastener. The iron also causes slight degradation of the wood near it (often referred to as “wood sickness”). This discoloration develops over many months or years of exposure.


Iron stain on newly installed wood siding. Poor quality galvanized nails corrode easily and, like uncoated steel nails, usually cause unsightly staining of the wood.

In wood species that have tannins, a chemical reaction takes place between the iron and the tannins. Tannins are just one of the many chemicals (extractives) in wood. Species such as the cedars, the oaks, and redwood are rich in tannins. Iron reacts immediately with the tannins to give a blue-black discoloration.

Steel fasteners are the most common source of iron, but traces of iron left from cleaning wood with steel wool or wire brushes cause iron stain. Poor quality galvanized nails corrode easily and, like uncoated steel nails, usually cause unsightly staining of the wood.

Using the wrong fastener can be costly—it may become necessary to replace all the siding. Therefore, your best bet is to use corrosion-resistant fasteners, such as stainless steel, rather than risk iron stain, particularly when using natural finishes on wood containing high amounts of tannin (such as western redcedar, redwood, and oak).

If using galvanized fasteners, they must be hot-dipped galvanized fasteners meeting ASTM A 153/A specification. Other galvanized fasteners fail. Unfortunately, contractors and their employees may have difficulty recognizing the difference among galvanized fasteners.

Can iron stain be fixed?

If iron stain is a serious problem on a painted surface, countersink the fastener, caulk, spot prime, and top-coat. This costly and time-consuming process is only possible with opaque finishes. Little can be done to give a permanent fix to iron stains on wood having a natural finish.

Removing fasteners, cleaning the affected areas with oxalic acid solution, and replacing the fasteners may not give a permanent fix because residual iron left behind continues to cause staining. Removing the fasteners often splits the siding.

Iron stain occurring beneath a finish is extremely difficult to fix. The coating must be removed before the iron stain can be removed. Oxalic acid will remove the blue–black discoloration. Apply a saturated solution (0.5 kilogram of oxalic acid per 4 liters of hot water) to the stained surface. Many commercial brighteners contain oxalic acid, and these are usually effective for removing iron stains.

After removing the stain, wash the surface thoroughly with warm water to remove the oxalic acid. If even minute traces of iron remain, the discoloration will recur.

For more information, please see chapter 16 of FPL’s Wood Handbook, Wood as an Engineering Material.

To find out more about iron stain, and what can be done about it, please see this FinishLine.

Weathering Walls : FPL Helps Buildings and Builders Breathe Easy

At the core of combating the cold in residential structures is effective insulation — but keeping warmer at home is not as simple as plastering as much polystyrene as possible to your walls and floors. Using more plastic insulation on exterior walls is a surefire way to increase the thermal efficiency of a building, but before you remodel, consider that you may be trapping more than just heat inside of your house.

Popular types of extruded and expanded polystyrene insulation have a much lower permeability than typical wooden building materials such as plywood and oriented strandboard. Because of this, exterior walls may become moisture traps, allowing moisture to enter, but not evaporate. This low-drying potential may lead to the mold growth, and in some cases, decay of walls made with these wood structural panels (WSPs). Unfortunately, little data exists on the real-world performance of these wall combinations, but researchers at the Forest Products Laboratory (FPL) are out to change this.


Wood structural panels (WSPs) are mainstays in residential and commercial construction.


FPL, in cooperation with APA – The Engineered Wood Association and Washington State University, has been studying the hydrothermal performances of walls constructed with WSPs since 2014. Performance testing in the Pacific Northwest was completed last last year, but data for cold climate zones, like FPL’s hometown of Madison, Wisconsin, is still being collected.

Researchers have constructed a “test hut” in Madison using wall assemblies with exterior continuous polystyrene insulation installed over WSPs. In this hut, they will investigate the potential for moisture accumulation and the drying capability of the walls during the colder months of the year.


The interior of the Chamber for Analytic Research on Wall Assemblies exposed to Simulated Weather (CARWASh) at FPL.


But FPL researchers don’t have to wait for the leaves to fall for the testing to begin. Using the on-site weathering chamber, the CARWASh, researchers will be able to run tests on individual wall sections in a computer-controlled and monitored environment. A total of 16 wall assemblies, each 4 feet wide and 7 feet tall, will be tested. Various combinations of water-resistive barriers, and exterior insulation will be used in the test walls, and the CARWASh will provide realistic weather perimeters and controlled water injections to simulate rain intrusion.

The test hut hygrothermal monitoring and CARWASh studies will be completed by July 2016, and the final report will be prepared by September.

With this data, WSP manufacturers will finally know how their products preform when the temperature goes down and the humidity goes up, and whether or not the permeability of the wood balances the impermeability of the insulation. Furthermore, as contractors make improvements to existing structures, and engineers design new buildings to comply with increasingly demanding energy-efficiency codes, they will have peace of mind thanks to FPL research — and breathe a little easier, knowing that their buildings will do the same.

For more information, see this Research in Progress report.