EXTERIOR WALL COVERINGS
Because siding and other types of exterior wall covering affect the appearance and the maintenance of a structure, the material and pattern should be selected carefully. Wood siding can be obtained in many different patterns and can be finished naturally, stained, or painted. Wood shingles, plywood, wood siding (paneling), fiberboard, and hardboard are some of the types of material used as exterior coverings. Masonry, veneers, metal or plastic siding, and other nonwood materials are additional choices. Many prefinished sidings are available, and the coatings and films applied to several types of base materials may eliminate the need of refinishing for many years.
One of the materials most used for structure exteriors is wood siding. The essential properties required for siding are good painting characteristics, easy working qualities, and freedom from warp. Such properties are present to a high degree in cedar, eastern white pine, sugar pine, western white pine, cypress, and redwood; to a good degree in western hemlock, spruce, and yellow popular; and to a fair degree in Douglas fir and yellow pine.
The material used for exterior siding that is to be painted should be of a high grade and free from knots, pitch. pockets, and uneven edges. Vertical grain and mixed grain (both vertical and flat) are available in some species, such as redwood and western red cedar. The moisture content at the time of application should be the same as what it will attain in service. To minimize seasonal movement
due to changes in moisture content, choose vertical-grain (edge-grain) siding. While this is not as important for a stained finish, the use of edge-grain siding for a paint finish will result in longer paint life. A 3-minute dip in a water-repellent preservative before siding is installed will result in longer paint life and resist moisture entry and decay. Some manufacturers supply siding with this treatment. Freshly cut ends should be brush-treated on the job.
Some wood siding patterns are used only horizontally and others only vertically. Some may be used in either manner if adequate nailing areas are provided. A description of each of the general types of horizontal siding follows.
Figure 4-1.-Types of wood siding.
Fiberboard and Hardboard
Fiberboard and hardboard sidings are also available in various forms. Some have a backing to provide rigidity and strength, whereas others are used directly over sheathing. Plywood horizontal lap siding, with a medium-density overlaid surface, is also available as an exterior covering material. It is usually 3/8 inch thick and 12 or 16 inches wide. It is applied in much the same manner as wood siding, except that a shingle wedge is used behind each vertical joint.
A number of siding or paneling patterns can be used horizontally or vertically (fig. 4-1). These are manufactured in nominal 1-inch thicknesses and in widths from 4 to 12 inches. Both dressed and matched and shiplapped edges are available. The narrow and medium-width patterns are usually more satisfactory under moderate moisture content changes. Wide patterns are more successful if they are vertical-grained (to keep shrinkage to a minimum). The correct moisture content is necessary in tongue-and-groove material to prevent shrinkage and tongue exposure.
Treating the edges of drop, matched, and shiplapped sidings with water-repellent preservative helps prevent wind-driven rain from penetrating the joints exposed to the weather. In areas under wide overhangs or in porches or other protected sections, the treatment is not as important. Some manufacturers provide siding with this treatment already applied.
A method of siding application, popular for some architectural styles, uses rough-sawn boards and battens applied vertically. These can be arranged in three ways: board and batten, batten and board, and board and board (fig. 4-2).
Figure 4-2.-Vertica1 board siding.
A number of sheet materials are now available for use as siding. These include plywood in a variety of face treatments and species, and hardboard. Plywood or paper-overlaid plywood, also known as panel siding, is sometimes used without sheathing. Paper-overlaid plywood has many of the advantages of plywood besides providing a satisfactory base for paint. A medium-density overlaid plywood is not common. Stud spacing of 16 inches requires a minimum thickness of panel siding of three-eighths inch. However, 1/2- or 5/8-inch-thick sheets perform better because of their greater thickness and strength.
Standard siding sheets are 4 by 8 feet; larger sizes are available. They must be applied vertically with intermediate and perimeter nailing to provide the desired rigidity. Most other methods of applying sheet materials require some type of sheathing beneath. Where horizontal joints are necessary, they should be protected by simple flashing.
An exterior-grade plywood should always be used for siding and can be obtained in grooved, brushed, and saw-textured surfaces. These surfaces are usually finished with stain. If shiplap or matched edges are not provided, the joints should be waterproofed. Water-proofing often consists of caulking and a batten at each joint and a batten at each stud if closer spacing is desired for appearance. An edge treatment of water-repellent preservative will also aid in reducing moisture penetration. When plywood is being installed in sheet form, allow a 1/16-inch edge and end spacing.
Exterior-grade particle board might also be considered for panel siding. Normally, a 5/8-inch thickness is required for 16-inch stud spacing and 3/4-inch thickness for 24-inch stud spacing. The finish must be an approved paint, and the stud wall behind must have corner bracing.
Medium-density fiberboards might also be used in some areas as exterior coverings over certain types of sheathing. Many of these sheet materials resist the passage of water vapor. Hence, when they are used, it is important that a good vapor barrier, well insulated, be used on the warm side of the insulated walls.
Nonwood materials are used in some types of architectural design. Stucco or a cement-plaster finish, preferably over a wire mesh base, is common in the Southwest and the West Coast areas. Masonry veneers can be used effectively with wood siding in various finishes to enhance the beauty of both materials.
Some structures require an exterior covering with minimum maintenance. Although nonwood materials are often chosen for this reason, the paint industry is providing comparable long-life coatings for wood-base materials. Plastic films on wood siding and plywood are also promising because little or no refinishing is necessary for the life of the building.
Siding can be installed only after the window and doorframes are installed. In order to present a uniform appearance, the siding must line up properly with the drip caps and the bottom of the window and door sills. At the same time, it must lineup at the corners. Siding must be properly lapped to increase wind resistance and watertightness. In addition, it must be installed with the proper nails and in the correct nailing sequence.
One of the most important factors in the successful performance of various siding materials is the type of fasteners used. Nails are the most common, and it is poor economy to use them sparingly. Galvanized, aluminum, and stainless steel corrosive-resistant nails may cost more, but their use will ensure spot-free siding under adverse conditions. Ordinary steel-wire nails should not be used to attach siding since they tend to rust in a short time and stain the face of the siding. In some cases, the small-head rails will show rust spots through the putty and paint. Noncorrosive nails that will not cause rust are readily available.
Two types of nails are commonly used with siding: the small-head finishing nail and the moderate-size flathead siding roil.
The small-head finishing nail is set (driven with a nail set) about 1/16 inch below the face of the siding, The hole is filled with putty after the prime coat of paint has been applied. The more commonly used flathead siding nail is nailed flush with the face of the siding and the head later covered with paint.
If the siding is to be natural finished with a water-repellent preservative or stain, it should be fastened with stainless steel or aluminum nails. In some types of prefinished sidings, nails with color-matched heads are supplied.
Nails with modified shanks are available. These include the annularly (ring) threaded shank nail and the spirally threaded shank nail. Both have greater withdrawal resistance than the smooth-shank nail, and, for this reason, a shorter nail is often used.
In siding, exposed nails should be driven flush with the surface of the wood Overdriving may not only show the hammer mark, but may also cause objectionable splitting and crushing of the wood. In sidings with prefinished surfaces or overlays, the nails should be driven so as not to damage the finished surface.
The minimum lap for bevel siding is 1 inch. The average exposure distance is usually determined by the distance from the underside of the window sill to the top of the drip cap (fig. 4-3). From the standpoint of weather resistance and appearance, the butt edge of the first course of siding above the window should coincide with the top of the window drip cap. In many one-story structures with an overhang, this course of siding is often replaced with a frieze board It is also desirable that the bottom of a siding course be flush with the underside of the window sill. However, this may not always be possible because of varying window heights and types that might be used in a structure.
Figure 4-3.-Installation of bevel siding.
One system used to determine the siding exposure width so that it is approximately equal above and below the window sill is as follows:
When this system is not satisfactory because of big differences in the two areas, it is preferable to use an equal exposure distance for the entire wall height and notch the siding at the window sill. The fit should be tight to prevent moisture from entering.
Siding may be installed starting with a bottom course. It is normally blocked out with a starting strip the same thickness as the top of the siding board (fig. 4-3). Each succeeding course overlaps the upper edge of the course below it. Siding should be nailed to each stud or on 16-inch centers. When plywood, wood sheathing, or spaced wood nailing strips are used over nonwood sheathing, 7d or 8d nails may be used for 3/4-inch-thick siding. However, if gypsum or fiberboard sheathing is used, 10d nails are recommended to properly penetrate the stud For 1/2-inch-thick siding, nails may be 1/4 inch shorter than those used for 3/4-inch siding.
The nails should be located far enough up from the butt to miss the top of the lower siding course (fig. 4-4). The clearance distance is usually 1/8 inch. This allows for slight movement of the siding because of moisture changes without causing splitting. Such an allowance is especially required for the wider (8 to 12 inch) siding.
It is good construction practice to avoid butt joints whenever possible. Use the longer sections of siding under windows and other long stretches, and use the shorter lengths for areas between windows and doors. When a butt joint is necessary, it should be made over a stud and staggered between courses.
Siding should be square cut to provide good joints. Open joints permit moisture to enter and often lead to paint deterioration. It is a good practice to brush or dip the fresh cut ends of the siding in a water-repellent preservative before boards are roiled in place. After the siding is in place, it is helpful to use a small finger-actuated oil can to apply the water-repellent preservative to the ends and butt joints.
Drop siding is installed in much the same way as lap siding except for spacing and nailing. Drop, Dolly Varden, and similar sidings have a constant exposure distance. The face width is normally 5 1/4 inches for 1- by 6-inch siding and 7 1/4 inches for 1- by 8-inch siding. Normally, one or two nails should be used at each stud, depending on the width (fig. 4-4). The length of the nail depends on the type of sheathing used, but penetration into the stud or through the wood backing should beat least 1 1/2 inches.
Figure 4-4.-Nailing the siding.
There are two ways to apply nonwood siding: horizontally and vertically. Note that these are manufactured items. Make sure you follow the recommended installation procedures.
When the various combinations of boards and battens are used, they should also be nailed to blocking spaced from 16 to 24 inches OC between studs, or closer for wood sheathing. The first boards or battens should be fastened with nails at each blocking to provide at least 1 1/2 inches of penetration. For wide underboards, two nails spaced about 2 inches apart maybe used rather than the single row along the center (fig. 4-2). Nails of the top board or batten should always miss the underboards and should not be nailed through them (fig. 4-2). In such applications, double nails should be spaced closely to prevent splitting if the board shrinks. It is also a good practice to use sheathing paper, such as 15-pound asphalt felt, under vertical siding.
Exterior-grade plywood, paper-overlaid plywood, and similar sheet materials used for siding are usually applied vertically. The nails should be driven over the studs, and the total effective penetration into the wood should be at least 1 1/2 inches. For example, 3/8-inch plywood siding over 3/4-inch wood sheathing would require a 7d nail (which is 2 1/4 inches long). This would result in a 1 1/8-inch penetration into the stud, but a total effective penetration of 1 7/8 inches into the wood sheathing.
The joints of all types of sheet material should be caulked with mastic unless the joints are of the interlapping or matched type of battens. It is a good practice to place a strip of 15-pound asphalt felt under joints.
The outside corners of a wood-framed structure can be finished in several ways. Siding boards can be miter-joined at the corners. Shingles can be edge-lapped alternately. The ends of siding boards can be butted at the corners and then covered with a metal cap.
A type of corner finish that can be used with almost any kind of outside-wall covering is called a corner board. This corner board can be applied to the corner with the siding or shingles end-or-edge-butted against the board.
A corner board usually consists of two pieces of stock: one piece 3 inches wide and the other 4 inches wide if an edge-butt joint between the corner boards is used. The boards are cut to a length that will extend from the top of the water table to the bottom of the frieze. They are edge-butted and nailed together before they are nailed to the corner. This procedure ensures a good tight joint (fig. 4-5). A strip of building paper should be tacked over the corner before the corner board is nailed in position (always allow an overlap of paper to cover the subsequent crack formed where the ends of the siding butts against the corner board).
Figure 4-5.-Corner board.
Interior corners (fig. 4-6, view A) are butted against a square corner board of nominal 1 1/4- or 1 3/8-inch size, depending on the thickness of the siding.
Figure 4-6.-Siding details:
Mitering the corners (fig. 4-6, view B) of bevel and similar sidings is often not satisfactory, unless it is carefully done to prevent openings. A good joint must fit tightly the full depth of the miter. You should also treat the ends with a water-repellent preservative before nailing.
Metal corners (fig. 4-6, view C) are perhaps more commonly used than the mitered corner and give a mitered effect. They are easily placed over each corner as the siding is installed. The metal corners should fit tightly and should be nailed on each side to the sheathing or corner stud beneath. When made of galvanized iron, they should be cleaned with a mild acid wash and primed with a metal primer before the structure is painted to prevent early peeling of the paint. Weathering of the metal will also prepare it for the prime paint coat.
corner boards (fig. 4-5) of various types and sizes may be used for horizontal sidings ofaall types. They also provide a satisfactory termination for plywood and similar sheet materials. Vertical applications of matched paneling or of boards and battens are terminated by lapping one side and nailing into the edge of this member, as well as to the nailing members beneath. corner boards are usually 1 1/8 or 1 3/8 inches wide. To give a distinctive appearance, they should be quite narrow. Plain outside casing, commonly used for window and doorframes, can be adapted for corner boards.
Shingles and Shakes
Prefinished shingle or shake exteriors are some-times used with color-matched metal corners. They can also be lapped over the adjacent corner shingle, alternating each course. This kind of corner treatment, called lacing, usually requires that flashing be used beneath.
When siding returns against a roof surface, such as at the bottom of a dormer wall, there should be a 2-inch clearance (fig. 4-6, view D). Siding that is cut for a tight fit against the shingles retains moisture after rains and usually results in peeling paint. Shingle flashing extending well up on the dormer wall will provide the necessary resistance to entry of wind-driven rain. Here again, a water-repellent preservative should be used on the ends of the siding at the roof line.
At times, the materials used in the gable ends and in the walls below differ in form and application. The details of construction used at the juncture of the two materials should be such that good drainage is assured. For example, when vertical boards and battens are used at the gable end and horizontal siding below, a drip cap or similar molding should be used (fig. 4-7). Flashing should be used over and above the drip cap so that moisture cannot enter this transition area.
Figure 4-7.-Cable-end finish (material transition).
Wood shingles and shakes are applied in a single-or double-course pattern. They maybe used over wood or plywood sheathing. When sheathing with 3/8-inch plywood, use threaded nails. For nonwood sheathing, 1-by 3-inch or 1- by 4-inch wood nailing strips are used as a base.
In the single-course method, one course is simply laid over the other as lap siding is applied. The shingles can be second grade because only one-half or less of the butt portion is exposed (fig. 4-8). Shingles should not be soaked before application but should usually be laid with about 1/8- to 1/4-inch space between adjacent shingles to allow for expansion during rainy weather. When a siding effect is desired, shingles should be laid so that they are in contact, but only lightly. Pre-stained or treated shingles provide the best results.
Figure 4-8.-Single coursing of sidewalls (wood shingles and shakes).
In a double-course system, the undercourse is applied over the wall, and the top course is nailed directly over a 1/4-to 1/2-inch projection of the butt (fig. 4-9). The first course should be nailed only enough to hold it in place while the outer course is being applied.
Figure 4-9.-Double coursing of side walls (wood shingles and shakes).
The first shingles can be a lower quality. Because much of the shingle length is exposed, the top course should be first-grade shingles.
Shingles and shakes should be applied with rust-resistant nails long enough to penetrate into the wood backing strips or sheathing. In a single course, a 3d or 4d zinc-coated shingle nail is commonly used. In a double course, where nails are exposed, a 5d zinc-coated nail with a small flat head is used for the top course, and a 3d or 4d size for the undercourse. Use building paper over lumber sheathing.
Flashing should be installed at the junction of material changes, chimneys, and roof-wall intersections. It should also be used overexposed doors and windows, roof ridges and valleys, along the edge of a pitched roof, and any other place where rain and melted snow may penetrate.
To prevent corrosion or deterioration where unlike metals come together, use fasteners made of the same kind of metal as the flashing. For aluminum flashing, use only aluminum or stainless steel nails, screws, hangers, and clips. For copper flashing, use copper nails and fittings. Galvanized sheet metal or terneplate should be fastened with galvanized or stainless steel fasteners. (Terneplate is a steel plate coated with an alloy of lead and a small amount of tin.)
One wall area that requires flashing is at the inter-section of two types of siding materials. For example, a stucco-finish gable end and a wood-siding lower wall should be flashed (fig. 4-10, view A). A wood molding, such as a drip cap, separates the two materials and is covered by the flashing, which extends at least 4 inches above the intersection. When sheathing paper is used, it should lap the flashing (fig. 4-10, view A).
Figure 4-10.-Flashing of material changes:
When a wood-siding pattern change occurs on the same wall, the intersection should also be flashed. A vertical board-sided upper wall with horizontal siding below usually requires some type of flashing (fig. 4-10, vie w B). A small space above the molding provides a drip for rain. This will prevent paint peeling, which could occur if the boards were in tight contact with the molding. A drip cap (fig. 4-7) is sometimes used as a terminating molding.
DOOR AND WINDOW FLASHING
The same type of flashing as shown in figure 4-10, view A, should be used over door and window openings exposed to driving rain. However, window and door heads protected by wide overhangs in a single-story structure with a hip roof do not ordinarily require the flashing. When building paper is used on the sidewalls, it should lap the top edge of the flashing. To protect the walls behind the window sill in a brick veneer exterior, extend the flashing under the masonry sill up the underside of the wood sill.
Flashing is also required at the junctions of an exterior wall and a flat or low-pitched built-up roof (fig. 4-11). Where a metal roof is used, the metal is turned up on the wall and covered by the siding. A clearance should be allowed at the bottom of the siding to protect against melted snow and rain.
Figure 4-11-Flashing at the intersection of an exterior wall and a flat or low-pitched roof.
GUTTERS AND DOWNSPOUTS
Several types of gutters are available to carry the rainwater to the downspouts and away from the foundation. On flat roofs, water is often drained from one or more locations and carried through an inside wall to an underground drain. All downspouts connected to an underground drain should be fitted with basket strainers (fig. 4-12) at the junctions of the gutter.
Figure 4-12.-Parts of a metal gutter system.
Perhaps the most commonly used gutter is the type hung from the edge of the roof or fastened to the edge of the cornice fascia. Metal gutters may be the half-round (fig. 4-13, view A) or "K" style (view B) and may be made of galvanized metal, copper, or aluminum. Some have a factory-applied enamel finish.
Downspouts are round or rectangular (fig. 4-13, views C and D). The round type is used for the half-round gutters. They are usually corrugated to provide extra stiffness and strength. Corrugated patterns are less likely to burst when plugged with ice.
Figure 4-13.-Gutters and downspouts:
On long runs of gutters, such as required around a hip-roof structure, at least four downspouts are desirable. Gutters should be installed with a pitch of 1 inch per 16 feet toward the downspouts. Formed or half-round gutters are suspended with flat metal hangers (fig. 4-14, views A and B). Spike and ferrule hangers are also used with formed gutters (view C). Gutter hangers should be spaced 3 feet OC.
Figure 4-14.-Gutter hangers:
Gutter splices, corner joints, and downspout connections should be watertight. Downspouts should be fastened to the wall by leaderstraps (fig. 4-12) or hooks. One strap should be installed at the top, one at the bottom, and one at each intermediate joint. An elbow is used at the bottom to guide the water to a splash block (fig. 4-15, view A), which carries the water away from the foundation. The minimum length of a splash block should be 3 feet. In some areas, the downspout drains directly into a tile line, which carries the water to a storm sewer (view B).
Figure 4-15.-Downspout installation: