Deck Doctor


What can harm a deck?  Answer 11

Though pressure-treated lumber, older growth Cedar and Redwood, and many domestic and exotic hardwood species are naturally resistant to decay from insect attack, NO WOOD TYPE OR SPECIES is immune to the damaging effects of water absorption and sun exposure.

FACT: All exterior wood needs protection from outdoor weather! Why? Because the damaging effects of outdoor weather on exposed, unprotected wood begins IMMEDIATELY and lasts FOREVER.

Weather’s Damaging Effects on Wood (Dimensional Change from Water/Sun -Exposure Cycles)

The day you finish installing a deck or other woods structure is the day that trouble can start! Morning dew, rainwater, melting ice and snow are quickly absorbed by unprotected wood, causing it to soften and swell. Direct exposure to the punishing heat from the sun causes wood to dry out and shrink. Continuous, repetitious cycles of wetting and drying, swelling and shrinking can cause wood to eventually crack and check, or worse, split, cup or warp – all of which lead to premature wood degradation and expensive repairs.

Surface Graying from Sun (UV) Exposure

Unprotected wood is also subject to a variety of other “damage”. Surface degradation caused by constant exposure to the sun’s harmful, ultra-violet rays is a common occurrence on decks, docks, fences and other outdoor wood structures. After consistent sun exposure over 1-3 years, the result is usually surface “graying”. The gray color is indicative of wood fiber degradation on the surface, and this graying will grow darker as years of sun exposure take their toll. Like sun-burnt skin, the grayed surface layer is permanently damaged, and must be removed to get to the sound, healthy layers underneath. A “grayed” surface not only signals the beginning of structural damage, it is often times considered unattractive by homeowners. In the case of Cedar and Redwood, this graying process can occur within 30 days.

Mildew Staining from Moisture Collection

Persistent moisture exposure from excessive rain, humidity or simple shade cover can create ripe conditions for the growth of mildew, mold or other fungi organisms that feed on wood. The result: black, brown or green “staining” that first appears as small “specs” which can eventually spread across the wood surface in mass formations, discoloring it and making it look unattractive at best, unsightly at worst. Wood Rot and Decay from Insect or Fungi Attack Termites and other wood boring insects also can wreak havoc on wood that is not pressure- treated, including all forms of pine, hemlock and fir, plus “new growth” Cedar and Redwood. These insects use wood for a food source, and eventually cause the wood to rot and decay.

Weather Exposure is Especially Tough on Decks

As horizontal surfaces, decks retain water, ice and snow for prolonged periods of time, accelerating damage to dimensional stability. The dampness attracts pollen and fungi spores which collect and cause ugly discoloration. And, decks are subject to 40 - 50 % more direct sunlight than comparable vertical surfaces. This results in unique thermal stresses to the wood. For example, on sunny summer days, the temperature on a deck floor board can vary as much as 50º F between the sun-exposed top surface and the damp, shaded underside. In addition, decks are subject to foot traffic, physical wear and tear, ground-in dirt and food stains.

“Pressure-Treated” Does Not Mean Weatherproofed

In most areas, over 80% of decks are constructed with pressure-treated lumber, (usually pine) which has been injected with a chemical compound that protects it against rot and decay from termites and other wood destroying insects. But pressure-treating does not provide weather protection for the surface. In fact, it actually makes the wood surface more porous – even more susceptible to moisture damage. To combat moisture damage, a water repellent coating should be applied to the wood surface to prevent moisture absorption and, ultimately, dimensional change, or worse, structural failure.


How should “pressure-treated” lumber be protected from moisture damage?

There are many competitive, all-purpose water sealers on the market which claim to “seal” water out. In reality, many of these products only last a short time before allowing the damaging water back into the wood. With some products, this happens in as quickly as 3 months. To properly protect outdoor wood from water and moisture penetration, choose a water repellent sealer or water repellent pigmented finish that will prevent water damage (like splitting and warping) for a MINIMUM 3 YEARS.

FACT: All water repellent sealers ARE NOT the same. Why? Because not all are formulated with active ingredients that can prevent water penetration for a 2-year period and beyond. Cedar and Redwood Also Need Weather Protection In some areas, especially the western United States, many decks are built with wood species native to the region, such as Cedar or Redwood. But regardless of species, wood is wood– a porous surface that will absorb water and eventually show signs of water damage. Cedar and Redwood lumber also need water repellent protection to prevent splitting, warping or other dimensional degradation. In decades past, Cedar and Redwood lumber was harvested from “old growth” trees that were primarily “heartwood” – the center core section of the tree containing high levels of resins and oils which are naturally resistant to decay and insects. However, much of the Cedar and Redwood lumber used in deck and fence construction today is cut from younger, “second growth” trees that have a higher sapwood content and are less resistant to biological enemies like mildew, mold and other fungi attack. Therefore, to safeguard against rot and decay from fungi attack, Cedar and Redwood should also be treated with a n EPA-registered wood preservative.

What type of coating will deliver preservative protection to Cedar and Redwood?

The only coatings that contain enough mildewcide to preserve wood from rot and decay are “EPA-registered wood preservatives”. They generally contain more than double themildewcide of standard water sealers and pigmented finishes/stains for exterior wood. EPA-registered fungicidal wood preservatives are few and far between in the marketplace, and require a major investment by their manufacturer in gaining federal and state approvals to allow them to be sold in retail stores. But, they are worth the effort in locating and using, because they provide the optimal wood protection possible against unsightly mildew, mold and algae growth on decks, siding, roofing and fencing. Look for the EPA registration number on the front of the can.

FACT: All pigmented finishes and stains ARE NOT the same. Why?

Because a few actually include an EPA-registered fungicidal preservative to protect against rot and decay of the wood surface.  Remember, if you choose a coating without a preservative, you risk the potential for decay from fungi attack, which can destroy your customer’s investment and your reputation for quality exterior wood care.

Hardwood and Composite Lumber also Benefit from Weather Protection and Regular Maintenance With the recent emergence of plastic/wood composites and exotic hardwoods as substitute decking lumber. In the case of exotic hardwoods, which are generally harvested and imported from Central and South America, their most visible weakness is that they succumb to UV sun damage very quickly, many times in as few as 2-3 months. Mahogany and Ipe’ that are very dark brown in color will tend to lose their deep surface color, and “silver out” rather rapidly under average sun exposure. In addition, these very dense decking lumbers still can exhibit cracking and checking after exposure to outdoor rain, ice and snow.










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