APA Engineered Wodd Handbooks Episode 14 pdf

12.1 CHAPTER TWELVE DESIGNING AND DETAILING FOR PERMANENCE Ed Keith, P.E. Senior Engineer, TSD The majority of the single- and multifamily homes built in the United States are constructed almost entirely out of wood or use a large percentage of wood products in addition to their primary construction material. Many light commercial buildings also make extensive use of wood construction due to its ease of installation, struc- tural integrity, code compliance, and environmental benefits. Wood has an almost unlimited useful life span if protected from ultraviolet (UV) light, moisture, fire, and insect attack. Whereas most fires occurring in modern wood structures are the result of human error and cannot be totally prevented by good design and construc- tion techniques, ultraviolet degradation, moisture problems, and insect attack can be virtually eliminated by proper design, construction, and routine maintenance. Every element of the structure must be protected from each of the hazards men- tioned above. Traditional framing and construction techniques do a good job of providing protection for most building elements. Many elements of traditional con- struction that we take for granted are there for the express purpose of providing the permanence that we have come to expect in wood structures. Roof overhangs, for example, keep rainwater and direct sunlight off the walls of the structure. The roof overhangs also minimize the amount of water that enters the ground adjacent to basement or crawl spaces. However, designers often are unaware of the real purpose of such traditional building elements and treat them as mere architectural features. Once they are perceived as an architectural feature, they may be eliminated at the whim of the designer. In the case of the example given above, once the roof overhang is removed to save money or alter the look of the structure, the protection it would have provided is also eliminated. Some of the consequences of the seemingly insignificant act of reducing or removing the roof overhang are: • The siding has less protection from UV rays. • The siding has greater exposure to direct rain. • The siding undergoes greater extremes of wetting and drying. 12.2 CHAPTER TWELVE • The useful life of the finish on the wood elements of the exterior is reduced. • The flashing around the windows must be more precisely installed to prevent leaks. • The basement or crawl spaces are more susceptible to water intrusion. Another example of unintended consequences resulting from a simple design change is the decision to replace a trussed roof with a conventionally framed vaulted ceiling. A conventionally framed vaulted ceiling is traditionally formed by using 2 ϫ lumber framing supported at one end by the walls of the structure and supported at the other by a beam at the peak. The roof sheathing is attached to the top of the rafters with the drywall attached to the bottom. Insulation is normally placed be- tween the rafters. The problem occurs with trying to provide adequate ventilation between the top of the insulation and the bottom of the sheathing. While these types of roofs have traditionally performed well, given the amounts of insulation required in most parts of the United States today, it is very difficult to create an air space to provide for the passage of air unless the rafters are specifically oversized for this application. In addition, because each space between the rafters is not connected to any other space, each space must be ventilated independently. The use of skylights interrupts these air passages, if any. The use of ceiling penetrations allows both air and water vapor to be drawn up into the poorly ventilated spaces between the rafters. Any leaks in the roofing due to flashing errors, wind-driven rain, or the formation of ice dams can result in the roof sheathing and lumber framing getting wet. The lack of ventilation will keep the wood from drying out and could result in the decay of the roof framing system. Thus, the designer must take special care in the design of this type of roof to prevent these problems. Of course, any architectural feature can be successfully designed and detailed once the principles for designing for permanence are understood. In the case of the vaulted ceiling, the use of a scissors truss can impart the same effect without any of the unintended consequences. Wood I-joists could also be used, as described in Chapter 5, since they can be purchased deep enough to provide room for insulation and allow sufficient space for ventilation. The knockouts provided in the I-joist can also be used to provide ventilation above the insulation and around roof penetrations such as skylights. See Fig. 5.45. There are numerous factors that impact the long-term performance or perma- nence of wood building components, ranging from ultraviolet degradation due to exposure to the sunlight to predation of the wood structural elements by insects or fungi. The next sections will cover a number of these factors in practical terms, with specific recommendations for the designing and detailing of roof, walls, and foun- dations for permanence. 12.1 FACTORS CAUSING DEGENERATION OF WOOD STRUCTURES There are several factors that can significantly shorten the life of a wood structure, including weathering, decay, insect attack, and excessive moisture intrusion. DESIGNING AND DETAILING FOR PERMANENCE 12.3 12.1.1 Weathering Natural weathering of wood and finishes involves a complex process that is difficult to duplicate in laboratory tests. Without protective treatment, all wood products exposed to the outdoor environment are subject to a number of physical changes. The most obvious, common, and benign is the change of color caused by ultraviolet light. Light woods become darker, and dark woods become lighter, both converging on the color gray. Hardwoods change color more slowly than soft woods. The gray color, made up of partially degraded cellulose fiber and microorganisms, does not extend very far into the surface of the wood and if left unaltered actually protects the wood from further degradation. The wind, rain, and wind-borne sand and debris continually abrade this fragile coating, allowing more degradation to occur. The erosion rate is about 1 ⁄ 4 in. per 100 years. Unprotected outdoor exposure causes other, more damaging alterations to wood products that must be carefully considered when designing the structure, such as splits and checks in exposed wood products. These splits and checks can cause finishes to fail, permit insects and decay organisms to get beneath preservative treatments, and adversely impact the strength of the wood element. Environmental factors causing degradation to sidings and other exposed wood products, glued engineered or solid sawn, include solar radiation, heat and cold, water, normal air contaminants, and wind. The importance of a particular weath- ering factor varies with the geographical location in which the product is located. For example, solar radiation is an important factor in degrading organic polymers in finishes and is usually more severe in the southern part of the United States. Water and temperature variations can also assist in the degradation of finishes and are more likely to occur in the northern states. 12.1.2 Fungi As evidenced by buildings worldwide, wood construction can provide centuries of service life. However, as a natural, organic material, wood is susceptible to deg- radation by organisms under certain conditions. The most common organisms that must be taken into consideration during building design are fungi. Fungi are low forms of plant life that derive their nutrition by using other organic materials as food rather than producing it themselves, as green plants do. For prac- tical purposes, fungi can be separated into decay fungi and nondecay fungi. Decay fungi are probably the most significant organisms responsible for degradation of wood. Degradation by fungi is commonly referred to as rot, decay, brown rot, or dry rot. Nondecay fungi include stains and molds. Decay Fungi. Decay fungi are spread by microscopic spores, which are produced by the fruiting bodies of fungi. Spores are always present in the atmosphere but need proper conditions to begin their growth. Under suitable conditions, the fungi spores grow into thread-like hyphae that spread throughout the host material and may ultimately produce fruiting bodies, which produce spores for further propa- gation. As with all organisms, certain threshold conditions are necessary for survival and propagation of decay fungi. These conditions can be categorized into temper- ature, food, oxygen, and moisture. In most wood structures, decay can be best eliminated by controlling the moisture content of the wood. 12.4 CHAPTER TWELVE Moisture Control. Decay growth in wood requires prolonged conditions where wood moisture content is in excess of 20–25%. The moisture content of wood components is a function of: • Humidity: Eventually, wood will equilibrate to approximately 6–12 percent mois- ture content during service in most geographical locations if un-wetted by rain or condensation. The exact moisture content at equilibrium is primarily a function of relative humidity. The time it takes for wood to equilibrate is a function of member size and can be substantial for large sawn timbers and glued-laminated timbers. • Direct wetting: Exposure to direct wetting leads to elevated surface moisture content over the short term and high moisture content throughout the entire wood member if exposure is prolonged. As an example, due to the large exposed surface area, wood structural panels continuously exposed to several days of rain wetting may lead to a moisture content of 50% or more. • Condensation: Most wood components used in construction are ultimately pro- tected from direct exposure to weather. However, some components may be sub- ject to wetting from condensation (see Section 12.1.4). • Climate: Wood products exposed to weather vary considerably in moisture con- tent as they are always seeking equilibrium with changing humidity or undergoing moisture cycling from wetting due to rain or snow. Climate conditions affect decay potential of wood used above ground and exposed to weather. In areas of high rainfall or high humidity, the moisture content may be elevated. For such cases, good design and construction practices combined with the use of preser- vative-treated or naturally durable woods will minimize risk of decay and help assure good performance. As the exterior exposure conditions and moisture con- tent are a function of climate, a decay probability map can be developed as shown in Fig. 12.1. Decay Control in Wood Construction. The primary method of preventing decay fungi in wood construction involves keeping the wood below the threshold moisture content needed for decay. The following discussion provides an overview of proper design, storage, construction, and maintenance details that minimize the potential of reaching this moisture level. Floors. Since floors are enclosed by the building envelope, they are generally at low risk of decay except in circumstances where they are over a damp soil crawl space or where plumbing leaks lead to localized wet spots. Adherence to the fol- lowing provisions will help ensure good performance. • Crawl space ventilation: Model building codes require a ratio of 1 ft 2 of net free ventilation for every 150 ft 2 of floor area. The ventilation requirements can be reduced to 1 ft 2 for every 1,500 square feet of floor area when a vapor retarder ground cover is placed over exposed soil in a crawl space. See Section 12.1.4 under Ventilation Requirements for more information. • Distance between grade and nearest untreated wood: Codes typically require a distance of at least 6 in. between the grade and nearest untreated wood. All wood in contact with the ground or below grade should be preservative-treated. • Treated sill plates: Wood members in contact with concrete foundations should be preservative-treated or of naturally durable species. DESIGNING AND DETAILING FOR PERMANENCE 12.5 Notes: Lines defining areas are approximate only. Local conditions may be more or less severe than indicated by the region classification. moderate to severe slight to moderate none to slight FIGURE 12.1 Decay probability map. (Source: International Residential Code for One- and Two-Family Dwelling.) 1 Roofs. Wood roof members may be exposed to moisture from leaks, from mois- ture introduced at the time of construction, and from moisture generated from con- densation. Attention to design and construction details can significantly reduce these moisture hazards as noted below. • Attic area ventilation/vapor retarders: Model building codes typically require 1 ft 2 of net free ventilation for every 150 ft 2 of attic area. This provision can be reduced to 1 ft 2 for every 300 ft 2 when a ceiling vapor retarder is used. The same reduction applies for sloped (pitched) roofs when at least 50% of the required vent area is located in the upper portion of the space to be ventilated and is at least 3 ft above eave vents. See Section 12.1.4 under Ventilation Requirements more information. • Low-slope roofs: It is often impractical to ventilate low-slope roofs since they generally do not contain ventable attic space. Experience has shown that low- slope roofs in commercial buildings can perform adequately even with minimal ventilation. These unique cases demand attention to other details to minimize entrapment or accumulation of moisture in the roof cavity. Attention to the fol- lowing provisions minimizes decay hazard in low slope roofs. Penetrations in flat or low-slope roofs such as skylights, roof accesses, and HVAC duct openings pose special problems because they form isolated pockets within the roof framing. Often the installed vents do not draw from these pockets and can result in unventilated areas where moisture can build up. When using I-joists in the roof framing, the factory-installed knockouts can be removed from the joists forming these isolated pockets. The joists can then be installed with the 12.6 CHAPTER TWELVE knockouts on the top and sized such that the knockouts provide a path to an adjacent vented space. See Chapter 5 for more information on I-joists. • Moisture content of wood components: The limited size of the roof cavity (located between the roofing membrane and the lower ceiling or insulation membrane), especially in conventionally constructed roofs, increases sensitivity to entrapped moisture introduced during construction. It is therefore important to specify the use of air- or kiln-dried lumber and to allow a period of drying after roofing and prior to installation of the insulation and ceiling if the wood structural panels were wetted during construction. • Moisture accumulation due to condensation: Condensation in roofs is primarily dependent upon two factors: the roof deck temperature which can cause a con- densation surface on the back side if it drops below the dew point, and the amount of moisture vapor accumulation in the roof cavity. The potential for moisture accumulation in the roof cavity depends upon entrapped moisture discussed above and interior humidity conditions. Interior moisture sources include such things as human occupancy, moisture generated from building use such as manufacturing operations, and moisture infiltration into the building. In some cases, additional venting to the outside of the building and use of vapor retarders are needed to avoid accumulated moisture that can lead to condensation. (See Section 12.1.4 for further information.) • After installation: Once installed, protect wood structural panels as soon as pos- sible with roofing felt or finish roofing material. If panels were wetted prior to roofing installation, allow some time for drying prior to installing the insulation and/or vapor retarder under the roof deck. Handheld moisture content meters are available for making a quick assessment of moisture conditions of the wood components. Target moisture content is 19% or lower. Walls. Wood components used in walls may be subjected to moisture generated from condensation or moisture intrusion. Proper design, construction, and mainte- nance prevent these causes of moisture problems. See Section 12.2.2 for more detailed information. • Vapor retarders: Condensation may occur in the inside surface of exterior wall sheathing in cold winter climates when moist air comes in contact with a cooler surface. Such moisture can come from sources inside the house such as cooking, clothes dryers, and showers. Installing exhaust fans over cooking stoves and in high-humidity areas such as bathrooms or laundry rooms can help vent excess moisture to the outside. Clothes dryers should also exhaust to the outside. Additionally, a vapor retarder should be installed on the warm side of the wall. Failure to protect the wall cavity from water vapor can result in condensation and elevated moisture content of the wall sheathing and studs. See Section 12.1.4 for more information on condensation. • Moisture intrusion: Performance problems with walls can arise when the weather resistance of the exterior finish system degrades and allows moisture intrusion. Virtually any exterior finish system, whether it is wood siding, vinyl siding, stucco or proprietary systems such as exterior insulated finish systems (EIFS), can lose its moisture resistance, resulting in wall moisture problems. Often these moisture problems can be attributed to installation shortcomings such as lack of building paper and inadequate flashing around doors and windows. Wood structural panel sheathing as well as wall framing require protection from exposure to permanent moisture when used in wall systems. DESIGNING AND DETAILING FOR PERMANENCE 12.7 Decay Control in Exposed Applications. Exposed products, such as siding, are often fully exposed to weather and thus have increased susceptibility to elevated moisture conditions. Although siding products will often experience moisture con- tents above the threshold value needed to support decay on an intermittent basis, wood-based siding products have a good history of performance due to the fact that they dry below this threshold value before decay can take hold. Proper archi- tectural detailing, use of flashing and caulking, and adherence to the manufacturer’s installation recommendations are essential for proper performance. For example, if trim is improperly installed around siding, it may trap moisture and/or reduce the drying ability of the siding. This can lead to long-term moisture accumulation that causes decay. Exposed end grain of wood products warrants special consideration such as flashings or other means of protection, since the high capillarity of end grain in- creases water absorption. Use sealants and protective flashing on the end grain of wood members exposed to the elements. Preservative Treatment. Preservative-treated wood products, which are pres- sure-impregnated in accordance with standards of the American Wood-Preservers’ Association, 2 should be specified for applications that involve high decay hazard. See Chapter 9 for further information. Mildew/Mold Fungi. Mildew/mold fungi, also known as nondecay fungi, are low forms of plant life similar to decay fungi. The major difference is that mildew and mold fungi do not cause the structural degradation of wood products. Mold and mildew can be found both indoors and outdoors. Mold and mildew are terms commonly used interchangeably, although mold is often applied to black, blue, green, and red fungal growths and mildew to whitish growths. The presence of high concentrations of mold and mildew in buildings is an indication of high- moisture conditions that may be detrimental to the structure. Mold and mildew may also cause health problems. This section provides basic information on mold and mildew and methods to minimize the high moisture conditions that can lead to high levels of mold growth in structures. In this section, the term mold will be used to cover both mold and mildew unless otherwise stated. Effect of Mold and Mildew Growth on Wood Components. As organic materi- als, mold and mildew can readily grow on wood if suitable moisture is present. Mold can grow on wood if exposed to water or prolonged humidity in excess of 70%. When mold or mildew occurs on wood products during construction, it should be cleaned as specified below and then allowed to dry before being closed in. When mold or mildew occurs on surfaces of wood products in a finished struc- ture, the products should be similarly cleaned. In addition, the source of excessive moisture must be determined and rectified. Control of mold and mildew in wood structures. Since mold and mildew require high-moisture conditions, proper moisture design, construction, and maintenance of structures are necessary to maintain moisture levels below the threshold for mold growth. There are many sources of occupancy moisture that can lead to elevated interior humidity such that mold can grow. Following is a short list of interior moisture loads that can be anticipated in a residential structure: • Shower 0.5 pint per 5 minute shower • Clothes dryer 4.7 to 6.2 pints per load if vented indoors 12.8 CHAPTER TWELVE • Cooking dinner 1.2 pints (plus 1.6 pints if gas cooking) per family of four • Dishwashing by hand 0.7 pints per family of four • House plants 0.9 pints per 6 plants Cleaning Mold and Mildew. Mold or mildew is often mistaken as dirt. A simple test for mold or mildew is to apply a few drops of 5% solution of household bleach. (It is important to use fresh bleach since bleach deteriorates in potency when older than six months.) Mold or mildew will usually bleach within one to two minutes. Areas that don’t bleach are probably dirt. Mold and mildew can be removed with commercial mold/mildew removers, following the manufacturer’s directions, or with a solution of one part household bleach (5% sodium hypochlorite) mixed in three parts by volume warm water. When using bleach, avoid breathing the vapors and contact with skin and eyes. Children and pets should be kept away from these products. Floods. Flooding represents an extremely high mold and mildew hazard level. Mold will start growing within 48 hours after floodwaters recede. Because of the high levels that may be encountered, flood-damaged structures present an extreme risk for mold. The American Red Cross Publication 4477, Repairing your Flooded Home, 3 and Institute of Inspection, Cleaning and Restoration Certification Standard S500-94, Standard and Reference Guide for Professional Water Damage Restora- tion 4 provide guidance on dealing with flood reclamation. Health Issues with Mold and Mildew. Excessive mold and mildew growth can pose a potential health risk. The health aspects of molds are beyond the scope of this publication. The American Lung Association 5 is a source of information on health aspects of mold. 12.1.3 Termite Protection for Wood-Framed Construction Termites occur in every state of the United States except Alaska. The presence or abundance of termites is determined by their environmental requirements such as temperature, humidity, soil moisture, and availability of food. Termite damage can be controlled with proper building practices and preventative measures. Termite Species. Based on their habitat and mode of attack, termites found in the United States can be grouped in three classes: subterranean termites, drywood ter- mites, and dampwood termites. For more information see Chapters 1 and 9. Termite Protection. Techniques for termite protection involve prevention of access to wood or moisture required for termite existence. Job-Site Sanitation. Houses built on land cleared of trees and brush are prob- ably in the midst of subterranean termite colonies in those geographic areas where subterranean termites are known to exist. In these areas job-site sanitation is critical. Proper job-site cleanup includes removal or burning of all debris, lumber, logs, limbs and stumps. The presence of buried wood attracts termites and can lead to infestation of the house. Lumber scraps should be removed from the site prior to enclosing with the wood or concrete floor. DESIGNING AND DETAILING FOR PERMANENCE 12.9 TABLE 12.1 Minimum Clearance for Untreated Wood to Grade (based on 2000 International Building Code, 9 Section 2304.11) Outside grade • To framing and sheathing 8 in. • To wood siding 6 in. Inside grade (crawl space) • To floor joists or sheathing 18 in. • To floor girders 12 in. Construction. Where termites are prevalent, the best protection is to build using techniques that prevent their gaining access to the building. Foundations may be constructed using the Permanent Wood Foundation (PWF), poured concrete or ma- sonry block with a poured concrete cap through which the termites cannot penetrate. Crawl space and attic vents must be screened to prevent access of winged termites during mating season. Required minimum clearances between the ground surface and any untreated wood in the building are presented in Table 12.1. Lesser clearances are also ac- ceptable provided such wood is pressure preservative-treated. With less than 18 in. of clearance under floor framing or less than 12 in. under floor girders, the shallow under-floor space is generally inaccessible for inspection. In such cases any wood that is at or below the level of the floor sheathing (including the floor sheathing itself, the floor framing, girders, posts, rim joists and blocking, and PWF, if used) must be pressure preservative-treated. Proper ventilation and use of vapor barriers on the ground in the crawl space will help prevent the moist conditions that subterranean and dampwood termites favor. Soil Treatment/Wood Treatment. In regions where a termite hazard exists, treat the soil outside of foundation walls, along the inside of crawl space foundation walls, under basement floors or slabs, and at other points of ground contact with termiticides. For under-floor plenum heating/cooling systems use only termiticides that have been approved for plenum applications when treating soil inside the ple- num. If soil treatment is not used in termite hazard regions, preservative-treated wood should be considered for the subfloor sheathing, floor framing, and supports. The foundation walls and underside of the floor structure should be inspected periodi- cally for evidence of termite infestation, especially if untreated materials are used in the floor sheathing, framing, and supports. Plywood should be treated in accordance with American Wood-Preservers’ As- sociation Standards C9 (Plywood Preservative Treatment by Pressure Process) 6 and C15 (Wood for Commercial-Residential Construction, Preservative Treatment by Pressure Processes) 7 or the American Wood Preservers Bureau FDN Standard (for Permanent Wood Foundation) 8 or equivalent code-approved preservative-treating and quality control requirements and should be marked by an approved inspection agency certified to inspect preservative-treated wood, indicating compliance with these requirements. All such treated wood should be dried to moisture content of 19% (18% for plywood) or less after treatment to minimize subsequent shrinkage. 12.10 CHAPTER TWELVE 12.1.4 Reduction of Moisture through Condensation Control Whenever moist air comes into contact with a cooler surface, condensation is likely to occur. The cool surface may be the underside of roof sheathing or the inside of exterior wall sheathing in winter, or the underside of a subfloor in summer when the building is air-conditioned. The only requirements for condensation are moist air and a cool surface. In the winter, the moisture content of the indoor air (usually measured as relative humidity or vapor pressure) is important, as is the temperature of the surface on which this moisture could condense. The amount of moisture in the air outdoors is also some- times a factor. Condensation can be controlled in three ways: (1) reduce the amount of moisture initially in the air; (2) prevent the moisture from reaching a cold surface by intro- ducing a vapor retarder; or (3) carry it away by ventilation. The Language of Condensation Control. Water stays in the air as vapor as long as the temperature of the air and the amount of water vapor are such that the air can hold it. The amount of water in the air, relative to the amount that the air can hold, is called relative humidity. Warm air can hold more water vapor than cold air. Thus, as air with a given amount of water vapor in it is cooled, the relative humidity will rise until a temperature known as the dew point is reached. At this point relative humidity becomes 100%, and some of the moisture will condense as dew. If moist air contacts a surface at or below its dew point temperature, conden- sation will occur on that surface. Water vapor in the air produces vapor pressure, which is a measure of moisture vapor concentration. Air with high vapor pressure tries to escape to or seek equi- librium with air of lower vapor pressure. The vapor can escape either with a flow of air through cracks or openings in the building shell or without it by direct penetration of building materials if a differential vapor pressure exists across the material. Vapor permeance is a measure of the ease with which vapor can penetrate solid building materials. Materials with low permeance are rated as vapor barriers or, more properly, vapor retarders. Changes in construction due to energy-saving features have tended to increase moisture levels within today’s homes. Washers, dryers, cooking, showers, indoor steam rooms, and swimming pools are sources of water vapor within houses. In older houses, air infiltration around doors and windows, and often directly through cracks in the walls, more or less automatically eliminated condensation. With the tighter, energy-efficient houses being built today, control of condensation must be planned. Condensation Control. The first step in the control of condensation involves re- ducing excess moisture inside the home. • Vent clothes dryers to the outside and not into the attic or crawl space. • Install range hoods over cooking stoves and operate them when any appreciable amount of steam is being generated. • Install exhaust fans in bathrooms and vent them to the outside, not to the attic (consider wiring the fan so that it goes on automatically with the bathroom light). [...]... ϭ (30 ϫ 45 ft) ϫ 1 / 1500 ϭ 0.9 ft.2 ϭ 130 in.2 2 Determine Total Net Free Area of Vents Attic vent area ϭ 155 in.2 / vent ϫ 2 vents ϭ 310 in.2 (free area of attic vents is less than that required) 12 .14 CHAPTER TWELVE Crawl space vent area ϭ 46 in.2 / vent ϫ 4 vents ϭ 184 in.2 (free area of crawl space vents is more than that required) 3 Conclusion Crawl space vents meet minimum ventilation requirements,... below This is true for low-slope roofs as well as pitched roofs The most common types of roof sheathing used in residential and wood-framed light commercial structures are wood structural panels such as APA Rated Sheathing and Structural I Rated Sheathing A number of recent hurricanes have emphasized to the design, building, and code-enforcement communities the importance of designing the roof sheathing... felt underlayment 36" wide centered in valley Lap 6" min Asphaltsaturated felt underlayment Ridge Secure with nails (b) 18" No 30 Asphalt-saturated felt (shake felt) Space neighboring shakes 1/ " to 1/ " apart 4 2 Second course Ice dam protection membrane or No 30 asphalt-saturated felt underlayment Joints in neighboring courses should not be in direct alignment – offset joints 11/2" min 11/2" min overhang... upper end Stagger nails at approx 3" o.c Ridge Valley metal formed from approx 24" wide metal, min 4" extension under tile Underlayment Lap valley metal 8" min Recommended: 2" wide metal clips 8" to 24" apart Bend clip over nail heads Lap underlayment 12" min in valley Note: Field underlayment not shown for clarity (a) FIGURE 12.5 (a) Typical metal open valley flashing Figure 12.7 provides the most common... during high winds • As the sheathing forms the foundation for the whole roof system it is imperative that it be sufficiently attached to resist the kind of loads that can be found in high wind situations APA has published nailing schedules for roof sheathing attachment in high-wind areas See Chapter 7 for further information Cathedral Ceilings In the past cathedral ceilings have been the source of some . element. Environmental factors causing degradation to sidings and other exposed wood products, glued engineered or solid sawn, include solar radiation, heat and cold, water, normal air contaminants,. ϭ 155 in. / vent ϫ 2 vents 2 ϭ 310 in. (free area of attic vents is less than that required) 12 .14 CHAPTER TWELVE 2 Crawl space vent area ϭ 46 in. /vent ϫ 4 vents 2 ϭ 184 in. (free area of crawl. in residential and wood-framed light com- mercial structures are wood structural panels such as APA Rated Sheathing and Structural I Rated Sheathing. A number of recent hurricanes have emphasized