chapter 4. PLYWOOD PANEL SIDING

4.1 PLYWOOD PANELS

4.1.1 Construction and in manufacturing processes has seemed, in the author's opinion, to make plywood Plywood panels are made up of an uneven more vulnerable to exposure conditions. number of layers or plys. Depending upon The authors also believe that the overall the type of panel, these plys can vary in quality of construction plywood has thickness fiom a few hundredths of an inch decreased in recent years. It appears that for fine hardwood face veneers to 1/4 inch core laps, core gaps, voids, glue skips, and or so for the inner plys of some sheets of use of decayed material have increased. construction grade plywood. This range These deficiencies are prone to cause excludes lumber core plywood. Remember failure. that these veneers are made by a sharp blade cutting a rotating log set in a lathe. For example, American Plywood Associa- After the log is cut, the veneers are tion standards for inner plys, unfortunately, flattened to make the plys. During the are based solely on the structural cutting and flattening process, the plys characteristics of different wood species. develop a continuous series of small cracks, This basis allows a wide range of inner called lathe checks, every 118 inch or so plys that may or may not have a relation- across the entire face of the sheet that was ship to the face veneers. Some currently toward the inner side of the log. This used inner plys are made of woods that "loose" side on face veneers is placed to decay rapidly when exposed to limited the interior of the panel during layup. amounts of moisture. Obviously, if these panels are to be used in moist or wet con- ditions, the potential for decay is very high. 4.1.2 Quality The reputation of plywood siding has The designer and builder should make sure grown over time, through experience with their specifications clearly control plywood the performance of panels manufactured quality, or else they should make sure the from larger and more mature trees. The final installation appropriately uses products increasing use of different species and of conforming to the commercial standards. smaller logs from generally younger forests, Some mills produce a higher quality of with changes in the manufacturing process, product than required by the industry have changed the character of the product. standards. When this higher quality is The effect of these changes in raw material desired, it must be specifically called for. chapter 4 - 2 WOOD: DETAILING FOR PERFORMANCE

4.1.3 Panel Size and Grade 318-inch smooth-surface plywood panel might provide sufficient stiffness and The normal size of construction plywood resistance to bowing when applied to wood panels is 4'-0" wide by various lengths, studs 16-inch on center; a 318-inch textured starting at 8'-0". Nine- and ten-foot-long panel (not permitted in the APA 303 siding siding panels are readily available, while series - see Table 4-1) will probably not be panels longer than that are available only sufficient because its effective thickness through special-order scarf joining. The may be reduced to less than 114-inch by plywood industry states that panels should the grooves. be installed with 118 inch vertical gaps between sheets. The details in this manual do not show that recommended gap, Table 4-1 because we believe it is unrealistic to frame 303@Siding Face Grades a building with standard spacings that will Patches yield a 4'-0-118" module. The author's Grade' concur with the industry that a 118" gap is wood Svnthetic desirable. While some manufacturers pro- 303-OC (Clear) Not Permitted Not Permitted duce siding panels at minus 118" to provide 303-OL (Overlaid. Not Applicable for Overlays for this gap, panels are also made at minus e.g. MDO Siding) 1/16" and at 4'0". We hope the industry 303-NR Not Permitted Not Permitted will standardize at the 3'11-718" width so (Natural Rustic) that this important expansion gap can be 303-SR Not Permitted Permitted as consistently provided. We believe that the (Synthetic Rustic) Natural-Defect best way to achieve this is through user Shape Only demand. 303-*-W Limit * Not Permitted Not Permitted Limit * 4.1.4 Texture Limit * -Any Combination Face veneers for plywood panels are 'W-wood patch. S-synthetic patch, S/W-both wood and available in a wide range of textures, synthetic patches. The maximum number of patches species, and overlays. It is important to allowed in 303 grades is 6, 18, or 30. One of these numbers will appear where the * is shown above. consider these surface properties in relation to the proposed use and exposure. Textures, such as the grooves that cut deeply into the panels, effectively reduce 4.2 WEATHERING PROPERTIES the thickness of the plywood and decrease its resistance to bowing and cupping. These Weathering is less obvious on saw-surfaced types of textures also expose the inner plys panels, and less surface deterio-ration will of the panel and the gluelines to the show than on smooth-surfaced plywood, but weather, which often results in less-than- rough-sawn panels have a thinner surface satisfactory performance. For example, a veneer due to the manufacturing process. WOOD: DETAILING FOR PERFORMANCE chapter 4 - 3

If left unprotected, the surface veneer can different species exhibit a wide range of completely erode away in a few years, performance. The most important perform- exposing gluelines and inner plys. ance variables are dimensional stability, Deterioration of the lathe checks on leaching exposure, and resistance to unprotected textured plywood can become ultraviolet light degradation. Mechanical severe and unsightly, which will ultimately strength, biological activity, thermal lead to the total destruction of the surface properties, and anatomical structure are also ply, leaving gluelines or inner ply exposed often significant. Lathe checks will appear to the weather. When textured, unpigmented sooner and will be more severe on sanded surfaces are desired, large roof overhangs plywood, but they will develop to some and frequent applications of a water degree on all softwood plywood exposed to repellent will probably be necessary to the weather, regardless of species and provide even minimal performance. finishing efforts. Medium-density overlay plywood is an outstanding exception to this Plywood must be protected by a pigmented rule. Redwood heartwood is a durable wood finish if it is to perform well. Surfaces left that has excellent resistance to weathering. unprotected against weather and sunlight It is, however, soft and subject to physical will soon show the lathe checks and other damage. Cedar heartwood has good decay deterioration, which will be hard to hide resistance and excellent coloring, but even with a coat of paint. Because many unfortunately is soft and erodes rapidly paints do not provide much moisture because of degradation by light energy. protection, the designer may be misled in When unprotected or finished only with assuming levels of protection. A clear water repellent, a resawn cedar moisture-resistant paint or opaque stain plywood panel may completely lose its face offers the best level of normal protection veneer in five to ten years. Douglas-fir is available. A quality paint applied over tough and moderately decay resistant, but medium-density overlay plywood panels develops a severe checking pattern and probably offers the highest level of protec- weathers very poorly from a visual tion and performance currently available. standpoint. Finishes are discussed in Chapter 15. All woods are eroded by sunlight. In However, the interactions between panel general, this erosion occurs at about 114- grade and finish choices need to be inch to 113-inch per century on vertical discussed here. Semitransparent stains are walls facing south. The greater the sun generally not satisfactory on overlaid panels intensity, the faster the erosion. As an or on panels with synthetic patches. Paints example of a high erosion rate, a redwood and solid-color stains may be used on any beam in Hawaii exposed to direct sun of the APA 303 sidings but sanded panels showed 5116-inch erosion in 35 years. In should be protected by paint. solid stock, where there is usually sufficient thickness to begin with, this is not normally Weathering is the composite effect of a a major problem. In vertical-grain solid number of degrading factors, and thus stock, the soft and more fragile springwood chapter 4 - 4 WOOD: DETAILING FOR PERFORMANCE

is protected by the ridges of the harder of 8d nails on panels thicker than lD", 6d summerwood. In flat-grain solid stock and if 112" or less, and spaced no more than 6" particularly plywood, with the very thin, at the edges and 12" in the field of the resawn textured plys, this protection does panel, provided lateral forces do not require not exist. The more vulnerable woods, such heavier nailing. Only hot dip~edgalvanized as cedar, are particularly risky. In general or stainless steel should be used. Thermal reference, the UV-resistance ranking of withdrawal can be a serious problem with most woods is inversely proportional to "smooth shanked" nails in some exposure their specific gravity. Specific gravity of conditions. This thermal action can be very common woods can be found in Table 6-1. rapid in aluminum nails, and while slower Unfinished western redcedar, for some in steel and stainless steel, it does occur. reason, erodes much more rapidly than Using nails with annular-ringed nail tips would be expected from this general probably will reduce this thermal relationship; an erosion rate of between withdrawal, but until more is known about 518" and 314" per century is a reasonable this phenomenon, the greater the penetration expectation on a vertical wall facing south the better, with 1- 112-inch the established and subjected to leaching. minimum. Nail patterns must be uniform to avoid localized panel stressing, and the edge clearance condition must be adequate to avoid splitting and unsightly appearance. 4.3 INSTALLATION GUIDELINES (See Chapter 14 for more discussion.) Some comments about machine-driven 4.3.1 Nailing fasteners are in order. These comments Improper panel nailing is one of the most apply to all aspects of construction, with pervasive errors currently found. This defi- specific emphasis here on plywood ciency includes overdriving, shiners or air diaphragms, sheathing, and siding panels. nails, inadequate nail length, excessive Labor and other cost savings of machine- spacing, poor nailing patterns, and driven fasteners are unarguable and should inadequate edge protection. When any of be encouraged. However, nailing gun these deficiencies occurs singly or in manufacturers must face the issue of combination, the potential for panel overdriven nails and the consequences for deformation exists. The more deformation, the building's structural integrity and other the greater the potential for water performance problems. It is essential that infiltration and panel degradation. the nail head position must be pre- determined, with a positive stop. For quality performance of plywood panels, Positioning by friction results in extensive it is necessary to fasten them properly to overdriving and loss of diaphragm strength. the building framing. Nails must go into If a positive head position unit is not solid wood, they must penetrate the framing developed, we believe the acceptance of behind the plywood a minimum of 1-112- machine-driven fasteners by the building inch. At a minimum, this requires the use codes should be reconsidered. WOOD: DETAILING FOR PERFORMANCE

4.3.2 Framing Shrinkage method of application is sometimes used to accentuate the horizontal lines in a build- Longitudinal shrinkage in normal lumber, ing. However, the horizontal orientation even green lumber, is so low that it can makes the panels more vulnerable to generally be ignored. However, some degradation. Two factors contribute to this abnormal types of wood (juvenile and vulnerability: reaction wood) do have significant rates of longitudinal shrinkage, which may cause As noted before, the plywood produc- additional problems as the framing dries. tion process produces lathe checks that run Transverse shrinkage (across the grain) at parallel to the direction of the panel. When plates, joists, and rafters can and often does the panel is placed horizontally, these cause major overall vertical changes. The checks will collect water and facilitate severity of this change is directly related to decay, delamination, or both. The checks the moisture content of the wood at the develop further with weather exposure, so time of installation, the cumulative even if the initial painting provides thickness of the horizontal members, and protection, it is unlikely that this protection the eventual equilibrium of the basic will be maintained. building. The use of 19% KD moisture content framing will reduce shrinkage to Most plywood siding has pattern about half that from using green lumber, grooves in the long direction of the panel. but a significant amount will still take When placed horizontally, the grooves place. In deep-joist multiplate combinations, provide a ledge to hold water, which it can a 314-inch to 1-inch reduction of dimension be absorbed into the panel body. More because of shrinkage is not unlikely for importantly, the grooves expose the end green lumber. Table 4-2 gives some guid- grain and lathe checks of the cross veneer, ance on the average amount of shrinkage which absorb water readily. This end grain that should be anticipated for flat-grain is very difficult to seal, and so even a Douglas-fir, although it usually will be less. groove with sloped edges is a problem. This moisture will not dry readily, thus promoting decay and delamination. 4.3.3 Vertical Installation This is the most common application of If a horizontal application is desired, plain plywood panels, which are usually nailed panels without grooves should be used. The directly to the framing. Properly done, this panel should also be sealed to protect the system will provide good service. Most lathe checks. A medium-density overlay details in this chapter are related to vertical (MDO) surface is excellent. Horizontal installation. joints should not be shiplapped, because shiplap joints have horizontal ledges. 4.3.4 Horizontal Siding Installation Horizontal joints should be flashed and Horizontal application of plywood panel supported by blocking behind the joint, as siding usually is not a good idea. This with vertically applied panels. chanter 4 - 6 WOOD: DETAILING FOR PERFORMANCE

Table 4-2 "B" Dimension for Clearance in Horizontal Joints* (inches)

Cumulative Depth of Moisture Content at Time of Installation Horizontal Members Between Floors Green 12%

Group 1 Douglas-fir, hem-fir, hemlock, sitka spruce, and southern yellow pine

10" (6" joist) 12" (8" joist) 14" (10" joist) 16" (12" joist)

Group 2 Pines, except southern yellow

10" (6" joist) 12" (8" joist) 14" (10" joist) 16" (12" joist)

Group 3 Incense cedar, redwood, western redcedar

10" (6" joist) 12" (8" joist) 14" (10" joist) 16" (12" joist)

* Note: "B" Dimension is the sum of the potential drying shrinkage value plus 1/4 inch. WOOD: DETAILING FOR PERFORMANCE chapter 4 - 7

4.3.5 Panel Thickness a few years, if the exposure is such that the sealant is actually performing a useful Minimum panel thickness requirements function. Sealants should be restricted to specified by the building codes usually are secondary water-shedding conditions or based solely on structural considerations. remedial work where they might receive the These thickness requirements are not related attention they need. to other performance criteria, such as cupping, bowing, or water infiltration. Panels that experience excessive deflection 4.3.7 Finishes due to inadequate stiffness will probably allow the excess passage of water through See Chapter 15. their joints. Such water penetration exposes the plywood to decay and the membrane (often, unfortunately, not provided) to deterioration. Adequate stiffness is even 4.4 EDGE CONDITIONS more critical in the grooved textured panels Joint detailing for installing panel siding is (see Table 4-3). In addition, the plywood critical in terms of decay-resistant cantilever (the distance between the nail performance of panel siding systems. The line and panel edge) necessary at inter- design of the joints must accommodate mediate floor systems and head and sill proper nailing, drying shrinkage of framing, conditions is critical to avoid panel bowing water-resistant joinery of the exterior from framing shrinkages. Table 4-4 gives panels, and protection of the felt membrane recommended maximum cantilever dimen- from sunlight. Figure 4-1 through Figure sion "C" for different plywood thicknesses 4-17 illustrate some good and bad practices when the cantilever dimension is parallel to of joint detailing. the face grain. The designer should to give balancing plywood cantilever and cross grain shrinkage of plates and joists. 4.4.1 Vertical Joints For vertical joints, both the board and 4.3.6 Sealants batten joint (Figure 4-1) and the shiplap joint (Figure 4-2) are appropriate. Both Good sealants are wonderful materials, but have continuous overlapping of exterior in the authors' opinion they should not be material to protect the building membrane used as a primary membrane material in from exposure to sunlight, excessive water, wood construction. Elastomeric sealants do and mechanical degradation. not stick to wood very well without exten- sive surface preparation. Further, it is Plywood manufacturers recommend a 118- unusual in wood construction to find skilled inch space between panels. This expansion sealant applicators within the normal labor space has been omitted in the drawings for force. More importantly, the bond between the reasons discussed earlier in this chapter most sealants and wood deteriorates within in the section on panel size. Panel edge chapter 4 - 8 WOOD: DETAILING FOR PERFORMANCE

Table 4-3. Recommended Plywood Thickness

Vertically Installed Plywood

- -- - - Panel Thickness 318" 1/2,, 518" & thicker Stud spacing Grooved Solid Grooved Solid Grooved Solid

Marginal OK OK OK OK OK No Marginal Marginal OK OK OK No No No Marginal Marginal OK

Recommended Plywood Thickness

Horizontally Installed Plywood

Panel Thickness 318 " 1/2,, 518" solid + up Stud spacing Textured Solid Textured Solid Textured Solid

The horizontally installed plywood part of this table should be used for overlay plywood. Smooth sanded plywood will develop excessive lathe checks and in moist conditions will . further deteriorate. Textured surface plywood should never be used in the horizontal direction.

Note: "Texture" here refers to all American Plywood Association 303 siding patterns. WOOD: DETAILING FOR PERFORMANCE chapter 4 - 9

1 -1/2" min. nail penetration

Centerline between

PLYWOOD VERTICAL JOINT BOARD AND BATTEN Figure 4-1 chapter 4 - 10 WOOD: DETAILING FOR PERFORMANCE

1 - 1 /2" min. nail penetration in stud

line between plywood

Siding

k PLYWOOD VERTICAL JOINT SHIPLAP Figure 4-2 WOOD: DETAILING FOR PERFORMANCE chapter 4 - I I

Bonding of sealant to wood is usually very poor and often fails quickly. Subsequently, water can penetrate if no felt is used, promoting decay in stud cavity.

SEALANT OVER MEMBRANE A sealant joint over felt is not acceptable. As sealant fails quickly, water can penetrate and be trapped between the sealant and felt, causing the plywood panel to decay, and reducing the life of the

NO FELT MEMBRANE It is necessary to provide felt behind shiplap and board and batten joints as these joints cannot be made completely water

PLYWOOD VERTICAL JOINT POOR PRACTICES Figure 4-3 chapter 4 - 12 WOOD: DETAILING FOR PERFORMANCE

Table 4-4 of "Z" flashing. This space accommodates "C" Dimension: Siding Free Cantilever Distance the shrinkage of floor joists as well as top (for vertical siding only) and bottom plates of stud walls. Further- more, nailing of plywood panels should not Siding Thickness "C" Dimension restrict the shrinkage movement in the floor 3/8" 3" joists and plates. Therefore, horizontal lf2" 5" joints should be located in between top and 5/8" 12" bottom plates at a floor level. Nailing of the plywood panels should be separated vertically as far as possible within this zone to minimize the nailing restraint of nails should be at least 318-inch from the shrinkage. The recommended maximum free edges, especially for panels in shear walls, cantilever distance ("C" dimension) in where the panels exert large forces against Table 4-4 should be followed in locating the nails. The 318-inch space requirement nails. Figures 4-8 to 4-10 illustrate possible keeps the panel edges from tearing against locations of the horizontal joint in this the nails. The size of nail should be ade- zone. If the panels are restrained in this quate to penetrate the framing at least 1- area, large stresses will develop and cause 112-inch. Therefore, a minimum of 6d nails plywood panels to bow and buckle, as should be used for 112-inch thick and thin- illustrated by Figure 4-7. In one- or two- ner panels. A minimum of 8d nails should story buildings with low membrane expo- be used for panels greater than 112-inch sure to wetting, it may be acceptable to thick, with still larger sizes when gypsum place the membrane continuously behind board or insulating materials are between the flashing. This detail can be an siding and framing. The vertical joints economical alternative. should always be supported by studs behind the joints. The studs must be placed precis- To provide for shrinkage spacing, the ely to accept nails from both panel edges. recommended gap allows the bottom edge Because nail-to-edge distance of 318-inch is of the panel to be at least 114-inch away required, an error of 318", whether due to from the flashing ledge. This prevents stud placement, warped studs, or misnailing, wetting of the upper panel by capillary will result in nails missing the studs. action from the collected water. The flash- ing also keeps water from penetrating the inner plys via the top edges of the lower 4.4.2 Horizontal Joints panels. If the top edge of the lower panel is unprotected, water that collects on this For horizontal joints, the most economical ledge will seep into the center and back of and reliable method is the flashing joint the lower panel, where it is slow to dry out (see Figure 4-4). The space between upper and will thus promote deterioration. and lower panels, the "B" dimension, should be sized to handle shrinkage, as To be effective in wind-driven rain noted in Table 4-1, and covered by strips conditions, the "Z flashing should have WOOD: DETAILING FOR PERFORMANCE

sufficient flange width (see Figure 4-4). As located near the top of the floor joist. The an alternative to flashing, staggered lapping top nailing line of the bottom panel should of plywood panels and scarf glue joints are be located in the top plate of the stud wall acceptable for horizontal joints (see Figure or near the lower edge in the joist. 4-4 and 4-12). 4.4.6 Alternative Floor-Level Joints 4.4.3 Panel Attachment (Figure 4-5) (Figures 4-11 and 4-12) When plywood panels must span floor These details illustrate alternative floor-level joists, nails should not be placed at the joint designs. Lapping plywood panels is an panel edges at the horizontal joint as is excellent way to provide a decay-resistant normally recommended, but nailed near the horizontal joint. This detail will require top and bottom of the floor joists or plates, more labor to execute, the joint does not leaving the top and bottom edges unre- rely on flashing. This method can supply strained. The distances from the edge of the as much overlap distance as the exposure panels to the nailing line should not exceed condition requires without using wide strips the "C" dimension of Table 4-3. The panel of flashing, which may be aesthetically should also be sufficiently thick to keep the undesirable. Both details illustrate nailing free ends of plywood panels from bowing patterns that do not restrain the shrinkage and deflecting outward. movement in the floor joist. 4.4.4 Poor Practice - No Expansion Gap 4.4.7 Banded Floor-Level Joints (Figure 4-6) (Figures 4-13 and 4-14) The drying shrinkage of wood is restrained. These details illustrate how a band board As the floor joist and plates shrink, the can be used with the floor-level joint. Band vertical load carried by the studs is boards, also called beltline trim or a belt transferred to the plywood panels. The course, may be aesthetically desirable to induced stress can cause panels to bow emphasize horizontal lines in a building. outward. This detail can be used only if the The addition of the band board complicates lumber is kiln dried to 12% so that no the horizontal joint detail, because band further drying shrinkage will occur after boards must be protected by flashing unless construction. Even then this design is poor they are treated with a preservative. If pre- practice, because moisture will be absorbed servative treatment is used, both the ply- through the lower edge of the upper panel. wood and the band board should be treated because water will be trapped between the 4.4.5 FI-oor-Level Joints two and decay may result, even at the (Figure 4-8 to 4-10) second- or third-story level. In these details, At floor levels, nailing of plywood panels two shrinkage relief joints are needed be- should restrain wood shrinkage as little as cause the band board must be attached to possible. Because most shrinkage occurs at the floor joist. If only one relief joint is the floor joists and plates, the bottom provided, the nails used to attach the band nailing line of the top panel should be board will restrain shrinkage movement. chapter 4 - 14 WOOD: DETAILING FOR PERFORMANCE

Felt

Siding

*

"Z" flashing a "A" min. Wood frame backing

1 - 1/2" min. nail penetration in framing "B" min.

3/8" min.

"A" min.

Note: For "A" and "B" dimensions see Tables 3-1 and 4-1 Felt PLYWOOD HORIZONTAL JOINT BUTT AND FLASHING JOINTS

Figure 4-4 WOOD: DETAILING FOR PERFORMANCE

Bottom nailing line

Note: For "A" and "C" dimensions see Tables 3-1 and 4-3 PLYWOOD HORIZONTAL JOINT DETAIL AT FOUNDATION Figure 4-5 chapter 4 - 16 WOOD: DETAILING FOR PERFORMANCE

Bottom nail line

"C" dim. max.

Band board where "C" dimens- on not practical for sid~ngth~ckness

Note: For "A" and "C" dimensions see Tables 3-1 and 4-3 PLYWOOD HORIZONTAL JOINT DETAIL AT FOUNDATION Figure 4-6 WOOD: DETAILING FOR PERFORMANCE

-Shrinkage restrained by nail

No expansion gap between plywood & "Z" flash~ng

PLYWOOD HORIZONTAL JOINT POOR PRACTICE

Figure 4-7 chapter 4 - 18 WOOD: DETAILING FOR PERFORMANCE

PLYWOOD HORIZONTAL JOINT DETAIL AT FLOOR LEVELS Figure 4-8 WOOD: DETAILING FOR PERFORMANCE chapter 4 - 19

PLYWOOD HORIZONTAL JOINT DETAIL AT FLOOR LEVELS Figure 4-9 chapter 4 - 20 WOOD: DETAILING FOR PERFORMANCE

PLYWOOD HORIZONTAL JOINT DETAIL AT FLOOR LEVELS Figure 4-10 WOOD: DETAILING FOR PERFORMANCE chapter 4 - 21

PLYWOOD HORIZONTAL JOINT DETAIL AT FLOOR LEVELS Figure 4-11 chapter 4 - 22 WOOD: DETAILING FOR PERFORMANCE

0

Note: For "A" and "C" dimensions see Table 3-1 PLYWOOD HORIZONTAL JOINT DETAIL AT FLOOR LEVELS Figure 4-12 WOOD: DETAILING FOR PERFORMANCE chapter 4 - 23

Note: For "A" dimension see Table 3-1 PLYWOOD HORIZONTAL JOINT DETAIL AT BAND BOARD Figure 4-13 chapter 4 - 24 WOOD: DETAILING FOR PERFORMANCE

PLYWOOD HORIZONTAL JOINT DETAIL AT FLOOR LEVELS

Figure 4-14 WOOD: DETAILING FOR PERFORMANCE chapter 4 - 25

4.4.8 Floor-Level Joint 4.4.10 Corner Joints (Figures 4-15 and 4-16) (Figures 4-18 and 4-19) Alternatives to providing two relief joints Battens, or comer boards, should be for a band board are to provide proper installed at exterior corners to protect the preservative treatment or to space the board building membrane from sunlight degrada- away from the building surface. Neither of tion at the panel joint. Comers are subject- these methods requires that the band board ed to greater construction abuse, an addi- be flashed. A single relief joint at the tional splitsheet of felt should be provided. center is thus sufficient. The corner detail should include a 118" space at panel edge to accommodate possi- If a preservative treatment is used, both ble movement of plywood sheets as a result plywood and band board should be treated, of framing shrinkage. Corner boards should because water will collect in the small gap also be supported by additional studs at the between the band board and the plywood. corner where needed, and they should be at Because of this, the flashing behind the least 3/4-inch thick and at most 4" wide to plywood should also extend 2-inches above minimize cupping. All nails should pene- the band board in case this space overfills trate studs at least 1-112". with water. The top of the band board (Figure 4-14) should also be sloped to 4.4.11 Joint Sealant minimize water collection at this ledge. An The sealant-to-wood bond is vulnerable. effective alternative to wood treatment is to Although the initial condition may appear provide a separation between the band satisfactory, it can fail rapidly in adhesion board and the building surface (Figure 4- when exposed to weathering. Failure may 15). When this approach is used, the lead to water penetration, which promotes separation space must be at least decay in the wall cavity. Surface applica- one-quarter the depth of the band board. tions of sealant are not effective; butt joints This distance should be adequate to prevent are also vulnerable and cannot be relied clogging from debris. The spacers should upon. A sealant joint over felt membrane is not be continuous, to allow debris to fall also not acceptable. The sealant cannot be through. expected to exclude moisture for the life of the structure, but will interrupt the flow of 4.4.9 Poor Practice - Decay Hazard penetrating water as the water flows down- (Figure 4-17) ward between membrane and siding, caus- ing the plywood panel to decay and Water will collect between the band board reducing the life of the membrane. and the plywood which will promote decay in this area. If the amount of collected water is significant and if no membrane is 4.4.12 No Membrane used, or if decay progresses, water will It is necessary to provide felt membrane penetrate into the stud cavity, causing behind shiplap and board and batten joints. further damage. These joints cannot be made watertight. chapter 4 - 26 WOOD: DETAILING FOR PERFORMANCE

Note: For "A" and "C"

PLYWOOD HORIZONTAL JOINT DETAIL AT FLOOR LEVELS Figure 4-15 WOOD: DETAILING FOR PERFORMANCE chapter 4 - 27

Non-continuous spacers

PLYWOOD HORIZONTAL JOINT DETAIL AT FLOOR LEVELS

Figure 4-16 chapter 4 - 28 WOOD: DETAILING FOR PERFORMANCE

Untreated wood

Untreated band board

No membrane

PLYWOOD HORIZONTAL JOINT POOR PRACTICE Figure 4-17 WOOD: DETAILING FOR PERFORMANCE chapter 4 - 29

PLYWOOD VERTICAL JOINT INTERNAL CORNER DETAIL

Figure 4-18 chapter 4 - 30 WOOD: DETAILING FOR PERFORMANCE

PLYWOOD VERTICAL JOINT EXTERNAL CORNER JOINT Figure 4-19