An Ounce of Prevention

Thanks to Grant Kirker for writing this article spotlighting how homeowner’s can better maintain their wood decks. Kirker is a Research Forest Products Technologist at FPL in the Durability and Wood Protection Research unit.

The experimental test block setup used to examine the role of accumulated leaf litter on material performance and wood durability in aboveground exposure. The test block is surrounded by untreated pine and the channel between the block and frame is filled with leaf litter. The two black fittings on the top are able to take repeated moisture measurement using a pin-type moisture probe. USDA Forest Service Forest Products Laboratory

Benjamin Franklin is credited with the saying “an ounce of prevention is worth a pound of cure”, which suggests that taking steps to avert a problem before it starts is far better than taking corrective steps after the problem arises. Forest Products Laboratory (FPL) Wood Durability and Protection researchers, in collaboration with research partners at Oregon State University, attempted to apply this concept to a situation close to home for many homeowners—the wooden deck.

The global wooden decking market in 2020 was valued at $15 billion USD, of which the North American markets made up about 35%, or $5.25 billion USD1. In a 2019 National Association of Homebuilders (NAHB) survey2, 20.3% of all new houses included decks. Although this estimate is lower than historical averages, the global pandemic has led to an increasing interest in outdoor living spaces, which will likely cause this market to increase. Wood is an excellent building material for outdoor decking because of its  reasonable cost and low maintenance requirements; if properly installed and maintained, a wood deck can provide a long-lasting benefit to the homeowner.

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Research in Progress – Building Safer Balconies

The scene is iconic, Juliet on her balcony calling out into the night, “O Romeo, Romeo, wherefore art thou Romeo?”, and Romeo calling up from the garden below to his star-crossed love, desperation in his heart. It is a scene that is known nearly all around the world. To many, it is what gives balconies their romantic appeal.

Construction in Charlotte, North Carolina. (Photo credit: Home Innovation Research Labs)

What a different scene it would have been if Shakespeare was not only a writer but an engineer who understood the difficulties of balcony architecture and construction. Balconies would be viewed with less rosy lenses if Shakespeare, instead of giving Romeo “love’s light wings,” gave him a balcony with moisture-driven rot and the moment he began to climb towards Juliet, the structure unmoored and flattened him under piles of destabilized building materials.

Although it may be lighthearted to imagine Romeo in a different balcony scenario, between 2001 and 2016 there have been approximately 239 balcony and deck collapses in the United States alone. In just two high-profile balcony collapses in Berkeley, CA and Chicago, IL, a total of nineteen fatalities resulted. As buildings age, construction defects become fatal defects.

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Derailing the Ice Dam: Moisture Management is Key in Colder Climates

Ice dams have plagued roofs in cold-climates for years. When accumulated snow melts and flows down a slanted roof, it remains liquid, insulated under a blanket of snow. When it reaches the eves, it encounters freezing air, and ice accumulates forming a dam. This ice dam impedes the proper drainage of melt water and precipitation from the roof, and may result in leaks, and damage to ceilings, walls, and insulation.


Weather conditions conducive to ice formation, limited ceiling insulation, and inadequate air leakage control have resulted in formation of ice dams on this home. Control of heat and moisture flows is an ongoing area of research at FPL.

But ice formations aren’t only a threat to the building — the dangers ice can pose to occupants and bystanders are real. In 2010, for example, falling ice killed five and injured 150 people in St. Petersburg, Russia, following a particularly cold winter.

Roof ventilation is the most commonly used strategy for the prevention of ice dams, but researchers aren’t convinced that ventilation is a silver-bullet to halt ice dam formation. A comprehensive review of studies concerning roof ventilation by various research organizations throughout North America was completed by former Forest Products Laboratory (FPL) researcher Anton TenWolde and Bill Rose of the University of Illinois. That review, which was published 13 years ago, still serves as a good guide for how to limit the likelihood or severity of ice damming.

The full article, published in ASHRAE journal, is available here. After a careful evaluation of the selected studies, TenWolde and Rose offer the following conclusions on how homeowners, designers, and builders can help derail the dams:

1. Indoor humidity control should be the primary means to limit moisture accumulation in attics in cold and mixed cli­mates; we recommend attic ventilation as an additional safe-guard. 

2. To minimize the danger of ice dam formation, heat sources in the attic and warm air leakage into the attic from below should be minimized. The need for venting to avoid icing depends on the climate and the amount of insulation in the ceiling. How­ever, ventilation is necessary in climates with a lot of snow to prevent icing at eaves, regardless of insulation level. 

3. We recommend venting of attics and cathedral ceilings in cold and mixed climates. However, if there are strong rea­sons why attic vents are undesirable, unvented roofs can perform well in cold and mixed climates if measures are taken to control indoor humidity, to minimize heat sources in the attic, and to minimize air leakage into the attic from below. However, ventilation is necessary in climates with a lot of snow to prevent icing. 

4. Ventilation should be treated as a design option in cold, wet coastal climates and hot climates. Current technical infor­mation does not support a universal requirement for ventila­tion of attics or cathedral ceilings in these climates. 

In summary, for each of the most commonly cited claims of benefits offered by attic ventilation (reducing moisture prob­lems, minimizing ice dams, ensuring shingle service life, and reducing cooling load), other strategies have been shown to have a stronger and more direct influence. Consequently, the focus of regulation should be shifted away from attic ventila­tion. The performance consequences of other design and con­struction decisions should be given increased consideration.

Composite Products and Moisture Management.

Composite wood products posses different characteristics than lumber or timbers. Whereas lumber and timbers are solid wood, engineered composites use adhesives to bond various layers, wood chips, or particles together. In terms of moisture and durability, composite products deserve special consideration.

According to Build Green: Wood Can Last for Centuries, composite products are an increasingly common building material used in building construction. Products such as oriented strand-board, commonly called OSB, are composed of large wood chips compressed and held together with adhesive. Composites are an efficient way to use smaller pieces of wood that might otherwise go to waste. Products such as I-joists and rim boards make up a growing proportion of materials used in home construction.


I-joists are set above structural timbers to provide support for floors and roofing.

Knowing about the special needs of composite products can save homeowners from costly repairs. It is very important to minimize wetting and to maximize drying when composites are exposed to water.

Composite products for exterior applications, such as marine-grade plywood, composite siding, or exterior grade OSB, may be treated with preservative chemicals. Wax added to the adhesive provides additional protection from water. Composite products, such as OSB or plywood sheathing and subflooring, can absorb water more readily than solid wood, particularly from cut edges that are wetted repeatedly. Absorbing water causes these products to swell (or in the case of plywood, to delaminate); once these products have absorbed enough water to swell, they dry very slowly and are vulnerable to decay. When they eventually dry, they do not return to their original dimensions and consequently lose strength.


Close-up end view of OSB (left) and wetted OSB (right) that has absorbed water and swelled. (Photo provided by Steve Easley, Steve Easley & Associates, Inc.)


Water and Wood Decay: A complicated relationship

As discussed in Build Green: Wood Can Last for Centuries, water is the main culprit in wood decay. Because of this, buildings should be designed to minimize wetting of wood or to maximize how quickly wood dries when wetted by rain.

As an example, cliff dwelling ruins of the ancient Anasazi people at Kiet Siel (“broken house” in Navajo) in northern Arizona date back to the 13th century. Dwellings in this semi-arid region are sheltered from occasional precipitation by cliffs overhead. Wood beams and superstructures have remained sound for centuries because they have always been too dry to decay.


The cliff dwelling ruins at Kiet Siel in northern Arizona date back to the 13th century.
(Permission from the Wetherill family.)

Dry wood will last indefinitely. It may come as a surprise then that wood can also be too wet to decay. Just like all living organisms, fungi require oxygen to live. When wood is submerged in water, air is driven out of all the cells, and decay fungi cannot grow. As an example, the remains of 34 Byzantine ships dating from between the 7th and 11th centuries were uncovered below sea level in Istanbul, Turkey. The wood remained intact because there wasn’t enough oxygen to permit wood-decay fungi growth (see below). Wood too wet to decay is not likely to be an issue for the homeowner, but these historical examples illustrate the point that wood-decay fungi need the right mix of air, moisture, temperature, and food source material in order to thrive.