Investigating CLT’s Ability to Fight Fungus

The growing reputation of cross-laminated timber (CLT) as a sustainable, cost-effective, and innovative building material has prompted researchers at the Forest Products Laboratory (FPL) to build upon past research and investigate the material’s ability to fight against fungus.

Intact cross-laminated timber panel section (left); 4-in. cube cut from panel section for scaled-up decay testing.

(A) Intact cross-laminated timber panel section; (B) 4-in. cube cut from panel section for scaled-up decay testing.

Praised for its many benefits, including speed of construction, cost, sustainability, excellent thermal and sound insulation, and fire restriction qualities, the pre-fabricated building material has made a name for itself in the construction and worldwide mass timber market. CLT has already made an appearance in a variety of high-rise apartment buildings in the Pacific Northwest and Southeast United States, urging scientists in the Lab’s Durability and Wood Protection Unit to further examine how the timber fairs against a rainy, humid climate.

The study builds upon past conclusions that untreated CLT is susceptible to mold and a variety of fungi. While decay can be reduced with preservatives such as boron, researchers are using more methods to investigate resistance treatments.

Scientists have implemented soil block assay tests on numerous random samples of CLT, and also plan to conduct mass loss and x-ray density profiling to assess decay in CLT.It is hoped that this exploration will help researchers develop more targeted fungal reduction methods for CLT.

The project will conclude in early 2017.  For more information on CLT and fungal resistance, read the full Research in Progress report.

Blog post by Francesca Yracheta

Decayed Wood Does Not Hold Paint: Contractor Tips From FPL

The following was adapted from the Forest Products Laboratory’s Wood Handbook, Wood as an Engineering Material.

Decayed wood does not hold paint. Although this may be self-evident to some, it is one of the main tips researchers at the Forest Products Laboratory (FPL) have for contractors across the nation. Even in new construction, where new wood is expected to be free of decay, contractors can do several things to keep it that way.

If possible, paint all end grain surfaces with an oil-alkyd primer, such as the ends of siding and trim, brick molding, railings, balustrade, posts, beams, and edges of panel products.

When repainting, inspect wood for decay. Problematic areas include end grain of balustrade, brick molding, siding that butts against a roof, and the bottoms of posts on porches. Decay often occurs in the center of the wood, so the surface can appear sound; probe several areas with an ice pick to ensure the wood is sound. Replace boards having decay.

Decay and paint failure in a wood railing. Decayed wood does not hold paint.

Siding intersecting a sloping roof should have a gap (50mm) between the end grain of the siding and the roof shingles. Check for a finish on the end grain; if there is no finish, treat the end grain with a water-repellent preservative (WRP), prime, and top-coat. If there is already a coating on the end grain, keep it painted.

End grain of siding that butts directly against roof shingles (not a recommended construction practice) is not accessible for painting, however you can try to wick WRP into the end grain from a wet brush.

Insects seldom cause problems with finishes. However, when repainting a structure, it’s a good idea to inspect it for termite tunnels and carpenter ants. A termite tunnel is a sure sign of infestation.

Presence of carpenter ants may indicate decay in the structure. Carpenter ants do not eat wood, but they often tunnel out decayed areas to build their nests. Note that woodpecker holes often indicate insect infestation, as woodpeckers will destroy the wood to get to the insects beneath.

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

Wood vs. Water: Who Will Win - the Cabin or the Creek?

One thing we know for sure at the Forest Products Laboratory (FPL) is that wood and water don’t mix, and keeping wood dry is essential to long service life. So really, 80 years seems like a good, long life for a log cabin built with untreated logs, situated just 10 meters from a river in an area with high snowfall, and subjected to the subsequent spring melting that follows each winter. For this cabin, maintained by Michigan Technological University’s School of Forest Resources and Environmental Science, staying dry has been nearly impossible.

Otter River cabin, 2013.

Otter River cabin, 2013.

Situated on the north branch of the Otter River in the Portage Township of Upper Michigan, the cabin was built in 1934-1935 by the Civilian Conservation Corps and housed workers who used the site as a trout hatchery. In 1955, it was transferred to Michigan Tech with promise that public rights of hunting, fishing, and trapping would be maintained. Over the years, continuous maintenance was needed due to periodic flooding and the resulting log deterioration.

Since 1998, Michigan Tech and FPL have cooperated to conduct periodic inspections of the cabin. The latest inspection, completed in 2013, utilized various techniques to analyze the condition of the log cabin.

Some decay was obvious with visual inspection alone.

Some decay was obvious with visual inspection alone.

Visual inspection is the simplest method for locating deterioration. Here, researchers look specifically for evidence of water intrusion and damage, especially near the foundation, and evidence of structural failure of the timbers. Many of the logs showed signs of severe deterioration, with the ends of several having been entirely destroyed.

Sound transmission measures the time it takes for stress waves to travel between sensors placed on opposite sides of a timber. Significantly longer transmission times, relative to the base times known for several species, indicate the presence of deteriorated wood. Nearly half of the tested logs produced results in the ‘deteriorated’ or ‘severely deteriorated’ range.

Micro-drilling resistance test of wall timber.

Micro-drilling resistance test of wall timber.

Micro-drilling resistance is a commercially developed technique based on the underlying premise that degraded wood is relatively soft and will have low resistance to drill penetration. Researchers conducted micro-drilling resistance tests in areas of the timbers they believed contained decayed wood based on results from the visual assessments and stress-wave testing. Test results showed that many logs were deteriorated, with most having an outer shell of solid material but a severely degraded core.

In the end, these test results showed that water had finally won the war over wood. Researchers recommended the cabin be dismantled, salvaging any possible historic materials (hardware and any sound timber), and suggested a smaller structure be built on the site using the salvaged materials where appropriate.

For more information on the project, see this FPL Research Note.


Improving Log Defect Detection

The location, type, and size of defects in hardwood logs affect the value of the resulting lumber, so knowing what’s going on inside the tree before it is sawn is valuable. Turns out, you don’t have to be a superhero with x-ray vision to see inside a tree. Several technologies have been developed to do just that, but they each have their limitations.

High-resolution laser scan image of a log with detected defect areas highlighted and acoustic waves passing through.

High-resolution laser scan image of a log with detected defect areas highlighted and acoustic waves passing through.

High-resolution laser surface scanning of hardwood logs can gather data relating to defects on the surface of the log, which can be used to generate maps of defects inside. However, surface inspection can miss unsound or rotten areas inside the log.

Acoustic evaluation, which involves measuring the speed of sound waves traveling through logs, is very accurate at determining soundness, but provides no data about the location of the defect.

Can combining these methods determine the soundness of a log as well as the location of the defects? FPL researchers are working to find out.

FPL Research Forest Products Technologist Xiping Wang, along with partners at the U.S. Forest Service’s Northern Research Station and the University of Minnesota Duluth Natural Resources Research Institute, are examining the technical feasibility of combining acoustic wave data with high-resolution laser scanning data.

Researchers are hoping to develop a combined scanning approach that uses these data to identify potentially unsound defects and facilitate sawing of each log to optimize value.

See this Research in Progress report for more background information and details on the specific approach of the study.

Home Wreckers in Search of Moisture: Tips for the Homeowner

The work of FPL’s Durability and Wood Protection Research Unit is broad in scope and includes studies into damage and contamination by decay fungi, mold, and termites. All these household pests are attracted to excess moisture, which can result from inadequate surface drying of condensation, leaks in pipes and foundations, poor ventilation, or flooding.

Homeowners are increasingly concerned about moisture management and indoor air quality. However, chronic moisture problems in a home can lead to more than poor indoor air quality—persistent high moisture can lead to a cascading biological succession from mold to decay to termite damage.


Blue-black color on walls shows evidence of mold growth. (Photo used with permission from A&J Specialty Services, Inc.)


Contamination with mold can render a home unlivable, and cleanup may require gutting the entire structure. In some cases, cleanup costs for toxic molds can equal the value of the home!

  • Mold occurs on the surface of wood exposed to excessive humidity or wet/dry cycling.
  • Visible mold growth is a good indicator of damp conditions or excess moisture.
  • Water vapor in humid air will not wet wood sufficiently to support decay fungi, but it will permit mold growth.
  • Mold, though unsightly, causes insignificant strength loss to structural wood components.
  • Common mold fungi can cause allergic symptoms; however, some molds (Stachybotrys sp.) produce mycotoxins, which cause illness and make homes uninhabitable.
  • New York City Department of Health and the U.S. Environmental Protection Agency have established guidelines for the assessment and remediation of mold fungi in indoor environments.