San Francisco Apartment Association

Maintenance Corner

Maintaining Exterior Wood Structures

by Steve Saarman

Wood is a common element in the exterior of many San Francisco buildings. It is used for constructing stairs, decks, walkways (connecting building levels), carports, trellises, fences, retaining walls and more. In all of these uses, wood has an intrinsic aesthetic value, but may also contribute to a life-safety issue if not proactively maintained for decay.

Questions that immediately come to mind are: What is wood decay? What causes wood to decay? How is it detected? What are some “best practices” to prevent it?
Fungi, the source of wood decay, are a very primitive form of plant life. There are four conditions that must be present for wood-decay-producing fungi to grow on wood: oxygen; temperatures ranging between 32 degrees and 90 degrees for optimal growth; moisture above the fiber saturation point of wood, generally greater than 25%; and an available nutrient source—glucose in the cell walls of wood itself.

If any of these conditions are eliminated, decay fungi can’t survive. Therefore, the easiest condition that can be controlled though conscientious construction practices and sound waterproofing details is the moisture content of the wood.

Wood-decay-producing fungi are an essential part of our ecosystem. They help recycle organic products. The way this type of fungi causes wood decay is through its enzymatic growth process. As a fungus grows, it produces spreading hyphae, which are like roots. The tips of these hyphae produce and secrete enzymes, which attack the linkages bonding glucose or sugars in the cell walls of the wood. The movement of the enzymes and released sugars need a film of liquid water to make the whole process flow. So, without liquid water as the transport medium or a wood-moisture content greater than 25%, the whole process shuts down and wood won’t decay.

Early Detection
Advanced signs of wood decay are obvious. As the cellulose in the wood is attacked, the surface of the wood eventually shrinks, collapses and cracks. At this advanced stage of wood decay, the wood’s structural integrity is lost. Stepping back in time, at what point does this presence of decay-producing fungi compromise a wood member’s structural integrity? We know that once decay has set in and environmental conditions don’t change, it continually grows and expands, thereby degrading the wood and causing significant loss of strength. The Forest Products Laboratory in Richmond, California has shown that a 1% loss in weight (or water loss due to cell-wall destruction) results in between a 6% and 50% reduction in strength, as measured in terms of toughness—the ability to withstand impact. A 10% weight loss equates to over a 50% loss in toughness. So, how does one detect such seemingly insignificant and almost invisible loss in the weight of an existing wood member?

The Forest Products Laboratory has devised two definitive methods. The simplest method to detect surface wood decay is called the “pick test.” This can reliably be used to detect the presence of nonvisible decay when the wood weight loss is in the range of 5% to 10%. Unfortunately, this percentage already equates to roughly a 35% to greater than 50% toughness loss in the wood strength. And, remember, even with that much loss, there is still no visible decay.

The pick test involves using an ice pick, awl or a small-headed screwdriver to penetrate approximately one-quarter inch into the wood surface where fungal decay is suspected. Next, rotate the tool to pry up a splinter parallel to the grain. The appearance of this splinter and the sound it makes when it is pried up is indicative of the level of fungal decay. A clean, long, sharp splinter that “snaps” indicates nondecayed wood. With fungi-infested wood, the splinter will be small, brittle and will often break across the grain with little or no sound. Always set up a “control,” by testing a few different areas well away from any areas of suspect. This way, you can see how that individual piece of wood reacts. In general, if a structural wood member fails the pick test at a critical load point, that member should be replaced. If in doubt, contact a structural engineer to run load calculations and make an evaluation.

rotting posts/></p> <p>The next test is less subjective. It is called the “immunodiagnostic wood-decay test.” It positively detects decay fungi in wood when there is, again, no visible damage. It utilizes wood shavings from a drilled hole. This test has the advantage of determining what is happening inside a wood timber and not just on the surface, as with the pick test. The wood shavings are soaked in a special extraction fluid for two hours with a test strip of litmus-type paper. The extraction fluid reacts with any fungal antigens present in the shavings and leaves a telltale blue line indicating positive fungal infestation. Again, when a positive result is obtained on a structural timber, a structural engineer should be consulted.</p> <p>The early detection of wood decay fungi is a wakeup call. It should lead to proactive ways to extend a wood member’s useful life by developing a program to address individual, as well as project-specific, needs.</p> <p><span class=Controlling Moisture Content
Best practices to be employed to control wood-decay fungi for exterior wood structures always hinge around controlling the moisture content in the wood. There are two basic options available. The first option utilizes sloped surfaces to allow gravity to shed water, in conjunction with sealing the exterior wood surface. Under wetter environmental conditions, the other option is to use pressure-treated wood or apply topical preservative chemicals, which basically poison the wood, so it becomes unsuitable to wood-decay fungi as a source of nutrients and energy.

When controlling the moisture content in wood, developing and implementing effective waterproofing design details is very cost effective for both new construction and repairs; it is also the most environmentally friendly option. It is good practice to have all exterior surfaces sloped to manage water runoff. Wood can be milled or installed with a slope, or sheet-metal cap flashings may be used on top to create a similarly sloped surface. How the ends are terminated is extremely important because no water should be let back into the system or into an attached structure. This is often handled with metal saddle flashings integrated with flexible membrane material. As a general rule, all fasteners should be side mounted, if exposed, and not penetrate the top surface. Large fasteners should have predrilled holes that are filled with caulk prior to installing. Nails should be driven straight in or angled slightly upwards to retard water from following the nail into the center of the attached wood member. This is a constant problem with open-space deck boards over wood joists. Top-driven nails that secure the decking will, over time, lead water into the center of the joist causing “center rot.” That is why various companies have developed hardware to specifically address this issue, utilizing side and bottom attachments only (Shadoe Track, Simpson DBT1, Deck Master, etc.). Always remember, any nail, screw or bolt attached to pressure treated material must be, at the minimum, hot-dipped galvanized or stainless steel, not electroplated. The electroplating process does not deposit sufficient zinc on to the steel surface to offer long-term corrosion protection from the chemicals in the wood.

Always maintain proper clearances between wood and soil (six inches) or concrete (two inches). This helps control rain runoff backsplash, water wicking up through the end grain and conditions that conceal termite tubes.

Sealants
When applying a topical finish to a wood structure, there are options available. Finish colors range from clear to stained to solid paint. Each has different aesthetic looks, levels of protection and maintenance-service lives. Most clear wood sealers must be applied yearly. Clear, penetrating epoxy sealers are a costlier option with the benefit of a much longer service life. An epoxy sealer also provides an excellent surface prep for painting because it penetrates deeply and stabilizes the wood surface prior to a primer and paint system. Stains range from transparent through solid. Generally, the more solids present in the stain, the greater the UV protection and the longer the service life. Stains have the advantage over some paints in that they require much less prep and do not peel. A quality paint system with meticulous caulking will provide the best protection for exterior wood structures. Caulking allows for the filling and sealing of cracks, joints and miters (after priming and prior to painting) to really seal the wood surface. Remember, wood is an organic material that will crack, split and move with the passing of the seasons and years. The goal is always to seal or cap penetrations that can lead water into confined areas inside a timber.

When performing decay repairs during maintenance cycles, assuming an engineer deems it unnecessary to replace an entire wood member, cut out at least one foot beyond the decayed area. Chemically treat both the existing wood member and the new filler piece before installation. Some environmentally safer chemical treatment products utilizing sodium borate are Bora-Care, Guardian, Jecta, Shell-Guard and Tim-Bor.

Another product, which has proven to be very effective, but with a different mode of application, is the IMPEL rod. IMPEL rods are molded tubes of concentrated water-diffusible boron. The rods are inserted into strategically placed drilled holes in areas of high risk for wood-decay fungi and sealed tight with caulk. They come in various sizes, are odorless and don’t stain. IMPEL rods are a great proactive tool to provide sustained protection deep in a wood member. They dissolve only when water is present. They are especially good with large, exposed timbers, but must be replaced periodically.

The keys to a successful maintenance program are spotting the symptoms, determining the cause and having an arsenal of cost-effective methods at your disposal to implement a timely repair. Time is always against you when wood-decay fungi have been detected. Ignoring the problem is very near sighted and will be costlier in the long run. So, be proactive!

The opinions expressed in this article are those of the author and do not necessarily reflect the viewpoint of the SFAA or the San Francisco Apartment Magazine. Steve Saarman is with Saarman Construction, Ltd. and can be reached at 415-749-2700. Copyright © 2006 by the San Francisco Apartment Magazine. All rights reserved