b'YOUR TOOLKIT FOR BUILDING EXCELENCEpolystyrene,polyurethane,andpolyvinylchloridebetween moisture contents of 30% and 40% db. Most other (PVC), and natural organic materials such as wood andwood decay fungi grow optimally between 50% and 60% other plant-based fibers, wool/hair and natural rubber. db. Decay fungi cannot continue to grow at wood moisture Photodegradation results in chalking, embrittlement,contents below 20% db, and water-soaked wood will not cracking, discoloration, and loss of strength of sun- decay because it lacks the air required for fungal growth.exposed materials (i.e., roofing, siding, exterior trim,Preservativewoodtreatmentstakeawaythe window and door frames, exterior caulks and sealants).foodsourcefordecayfungi,typicallybyheavy The slow and constant surface erosion and discoloration ofmetaltoxicitywithcopperastheheavymetal.unfinished exterior wood is a result of photodegradation.Wood decay in existing structures is generally the result of Weathering is a term commonly used to refer to the effects(1) poor preservative treatment in a wet environment (Figure of photodegradation involving exposed building materials. 2), or (2) water in contact with non-preservative-treated Figure 1 shows the roof of a building constructed in thewood at a location where evaporation of the water is limited. mid 1960s. Several original fiber reinforced plastic (FRP)The latter is generally the case when water gets drawn by skylights were replaced with new FRP panels aroundcapillary action into tight interfaces between components 2007.Since that time the remainder of the originalwhere there is little to no airflow. Concrete and masonry FRPpanelssuccumbedtophotodegradationleavingproducts can transmit and hold moisture at fairly high large holes in the roof.In 2022, the owners insurancelevels so wood that is not properly treated with preservative company refused to extend insurance on the buildingmust be isolated from any concrete and/or masonry that until the holes were properly covered.The owner met thisis (1) on the building exterior, (2) not isolated from soil, demand using the white steel panels shown in figure 1.(3) is routinely washed, or (4) otherwise exposed to water. For the ultimate defense against photodegradation, use stone,Isolation is normally accomplished by placing a rigid plastic clay brick, cast-in-place concrete, and precast concrete unitsbarrier between the wood and concrete/masonry (Figure on exterior walls.Similarly, on your roof use slate, clay tile,3).Where a wood support post is bearing on concrete in precast concrete shingles, and materials overlaid with stones. an exterior application, place polyurethane construction adhesive (e.g., Loctite PL Premium) between the wood post and concrete so that it squeezes out as the post is set in place.The adhesive will bond to the wood and concrete, and prevent precipitation from getting drawn (by capillary action) into the tight interface between the components. Ongoing research (Figure 4) has shown this to be very effective in preventing the decay commonly found at the base of wood posts bearing directly on exterior concrete.FIGURE 1. REPLACEMENT OF DETERIORATED FRP PANELS WITH WHITE CORRUGATED STEEL PANELS.Wood Decay FungiWood decay fungi are microorganisms that attack cellulose and, in some cases, the hemicellulose and lignin in wood. They require air, a suitable temperature, and water for survival. Optimal temperature for the growth of decay fungi varies by fungal species but is typically around 77F (25C), and most decay species do not grow below 41F (5C) nor above 104F (40C). Wood decay fungi require a wood moisture content at or above the woods fiber saturation point, typically 28% to 30% on a dry basis (db) before they will start growing. Fungi responsible for brown rot (called dryFIGURE 2. SEVERE WOOD DECAY.rot by some in non-scientific communities) grow optimally continued on page: 14 FRAME BUILDER - JAN2023 / 13'