Roof sheathing covers the rafters or roof joists. The roof sheathing is a structural element and, therefore, part of the framing. Sheathing provides a nailing base for the finish roof covering and gives rigidity and strength to the roof framing. Lumber and plywood roof sheathing are the most commonly used materials for pitched roofs. Plank or laminated roof decking is sometimes used in structures with exposed ceilings. Manufactured wood fiber roof decking is also adaptable to exposed ceiling applications.
Roof sheathing boards are generally No. 3 common or better. These are typically softwoods, such as Doughs fir, redwood, hemlock, western larch, fir, and spruce. If youre covering the roof with asphalt shingles, you should use only thoroughly seasoned wood for the sheating. Unseasoned wood will dry and shrink which may cause the shingles to buckle or lift along the full length of the sheathing board.
Nominal 1-inch boards are used for both flat and pitched roofs. Where flat roofs are to be used for a deck or a balcony, thicker sheathing boards are required. Board roof sheathing, like board wall sheathing and subflooring, can be laid either horizontally or diagonally. Horizontal board sheathing may be closed (laid with no space between the courses) or open (laid with space between the courses). In areas subject to wind-driven snow, a solid roof deck is recommended.
Roof boards used for sheathing under materials requiring solid, continuous support must be laid closed. This includes such applications as asphalt shingles, composition roofing, and sheet-metal roofing. Closed roof sheathing can also be used for wood shingles. The boards are nominal 1 inch by 8 inches and may be square-edged, dressed and matched, shiplapped, or tongue and groove. Figure 3-1 shows the installation of both closed and open lumber roof sheathing.
Figure 3-1.-Closed and open roof sheathing.
Open sheathing can be used under wood shingles or shakes in blizzard-free areas or damp climates. Open sheathing usually consists of 1- by 4-inch strips with the on-center (OC) spacing equal to the shingle weather exposure, but not over 10 inches. (A 10-inch shingle lapped 4 inches by the shingle above it is said to be laid 6 inches to the weather.) When applying open sheathing, you should lay the boards without spacing to a point on the roof above the overhang.
Nail lumber roof sheathing to each rafter with two 8-penny (8d) nails. Joints must be made on the rafters just as wall sheathing joints must be made over the studs. When tongue-and-groove boards are used, joints may be made between rafters. In no case, however, should the joints of adjoining boards be made over the same rafter space. Also, each board should bear on at least two rafters.
Plywood offers design flexibility, construction ease, economy, and durability. It can be installed quickly over large areas and provides a smooth, solid base with a minimum number of joints. A plywood deck is equally effective under any type of shingle or built-up roof. Waste is minimal, contributing to the low in-place cost.
Plywood is one of the most common roof sheathing materials in use today. It comes in 4- by 8-foot sheets in a variety of thicknesses, grades, and qualities. For sheathing work a lower grade called CDX is usually used. A large area (32 square feet) can be applied atone time. This, plus its great strength relative to other sheathing materials, makes plywood a highly desirable choice.
The thickness of plywood used for roof sheathing is determined by several factors. The distance between rafters (spacing) is one of the most important. The larger the spacing, the greater the thickness of sheathing that should be used. When 16-inch OC rafter spacing is used, the minimum recommended thickness is 3/8 inch. The type of roofing material to be applied over the sheathing also plays a role. The heavier the roof covering, the thicker the sheathing required. Another factor determining sheathing thickness is the prevailing weather. In areas where there are heavy ice and snow loads, thicker sheathing is required. Finally, you have to consider allowable dead and live roof loads established by calculations and tests.
These are the controlling factors in the choice of roof sheathing materials. Recommended spans and plywood grades are shown in table 3-1.
Table 3-1.-Plywood Roof Sheathing Application Specifications
Plywood sheathing is applied after rafters, collar ties, gable studs, and extra bracing (if necessary) are in place. Make sure there are no problems with the roof frame. Check rafters for plumb, make sure there are no badly deformed rafters, and check the tail cuts of all the rafters for alignment. The crowns on all the rafters should be in one directionup.
Figure 3-2 shows two common methods of starting the application of sheathing at the roof eaves. In view A, the sheathing is started flush with the tail cut of the rafters. Notice that when the fascia is placed, the top edge of the fascia is even with the top of the sheathing. In view B, the sheathing overlaps the tail end of the rafter by the thickness of the fascia material. You can see that the edge of the sheathing is flush with the fascia.
Figure 3-2.Two methods of starting the first sheet of roof sheathing at the eaves of a roof: A. Flush with rafter; B. Overlapping rafter.
If you choose to use the first method (view A) to start the sheathing, measure the two end rafters the width of the plywood panel (48 inches). From the rafter tail ends, and using the chalk box, strike a line on the top edge of all the rafters. If you use the second method, measure the width of the panel minus the actual thickness of the fascia material. Use this chalk line to position the upper edge of the sheathing panels. If the roof rafters are at right angles to the ridge and plates, this line will place the sheathing panels parallel to the outer ends of the rafters.
Notice in figure 3-2 that sheathing is placed before the trim is applied. Sheathing is always placed from the lower (eaves) edge of the roof up toward the ridge. It can be started from the left side and worked toward the right, or you can start from the right and work toward the left. Usually, it is started at the same end of the house from which the rafters were laid out.
The first sheet of plywood is a full 4- by 8-foot panel. The top edge is placed on the chalk line. If the sheathing is started from the left side of the roof, make sure the right end falls in the middle of a rafter. This must be done so that the left end of the next sheet has a surface upon which it can bear weight and be nailed.
The plywood is placed so that the grain of the top ply is at right angles (perpendicular) to the rafters. Placing the sheathing in this fashion spans a greater number of rafters, spreads the load, and increases the strength of the roof. Figure 3-3 shows plywood panels laid perpendicular to the rafters with staggered joints. Note that a small space is left between sheets to allow for expansion.
The sheets that follow are butted against spacers until the opposite end is reached. If there is any panel hanging over the edge, it is trimmed after the panel is fastened in place. A chalk line is snapped on the sheathing flush with the end of the house, and the panel is then cut with a circular saw. Read the manufacturers specification stamp and allow proper spacing at the ends and edges of the sheathing. This will compensate for any swelling that might take place with changes in moisture content.
The cutoff piece of sheathing can be used to start the second course (row of sheathing), provided it spans two or more rafters. If it doesnt span two rafters, start the second course with a half sheet (4 by 4) of plywood.
It is important to stagger all vertical joints. All horizontal joints need blocking placed underneath or a metal clip (ply clip). Ply clips (H clips or panel clips) are designed to strengthen the edges of sheathing panels between supports or rafters. The use of clips is deter-mined by the rafter spacing and specifications (see figure 3-3).
Figure 3-3.-Plywood roofing panel installation.
The pattern is carried to the ridge. The final course is fastened in place, a chalk line is snapped at the top edge of the rafters, and the extra material cut off. The opposite side of the roof is then sheeted using the same pattern.
When nailing plywood sheathing, follow the project specifications for nailing procedures. Use 6d common smooth, ring-shank or spiral thread nails for plywood 1/2 inch thick or less. For plywood more than 1/2 inch but not exceeding 1 inch thick, use 8d common smooth, ring-shank or spiral thread nails. When using a nail gun for roof sheathing, follow all applicable safety regulations.
In this section, well discuss the two most common types of roof decking you will encounter as a Builder: plank and wood fiber.
Plank roof decking, consisting of 2-inch (and thicker) tongue-and-groove planking, is commonly used for flat or low-pitched roofs in post-and-beam construction. Single tongue-and-groove decking in nominal 2 by 6 and 2 by 8 sizes is available with the V-joint pattern only.
Decking comes in nominal widths of 4 to 12 inches and in nominal thicknesses of 2 to 4 inches. Three- and 4-inch roof decking is available in random lengths of 6 to 20 feet or longer (odd and even).
Laminated decking is also available in several different species of softwood lumber: Idaho white pine, inland red cedar, Idaho white fir, ponderosa pine, Douglas fir, larch, and yellow pine. Because of the laminating feature, this material may have a facing of one wood species and back and interior laminations of different woods. It is also available with all laminations of the same species. For all types of decking, make sure the material is the correct thickness for the span by checking the manufacturers recommendations. Special load requirements may reduce the allowable spans. Roof decking can serve both as an interior ceiling finish and as a base for roofing. Heat loss is greatly reduced by adding fiberboard or other rigid insulation over the wood decking.
Figure 3-4.-Ends of roof decking cut at a 2° angle.
Figure 3-5.-Plank decking span arrangements.
All-wood fiber roof decking combines strength and insulation advantages that make possible quality construction with economy. This type of decking is weather resistant and protected against termites and rot.
It is ideally suited for built-up roofing, as well as for asphalt and wood shingles on all types of buildings. Wood fiber decking is available in four thicknesses: 2 3/8 inches, 1 7/8 inches, 1 3/8 inches, and 15/16 inch. The standard panels are 2 inches by 8 feet with tongue-and-groove edges and square ends. The surfaces are coated on one or both sides at the factory in a variety of colors.
Roof decking that extends beyond gable-end walls for the overhang should span not less than three rafter spaces. This is to ensure anchorage to the railers and to prevent sagging (see figure 3-6). When the projection is greater than 16 to 20 inches, special ladder framing is used to support the sheathing.
Figure 3-6.-Wood fiber roof decking at gable ends.
Plywood extension beyond the end wall is usually governed by the rafter spacing to minimize waste. Thus, a 16-inch rake (gable) projection is commonly used when rafters are spaced 16 inches OC. Butt joints of the plywood sheets should be alternated so they do not occur on the same rafter.
DETAILS AT CHIMNEY AND VALLEY OPENINGS
Where chimney openings occur in the roof structure, the roof sheathing should have a 3/4-inch clearance on all sides from the finished masonry. Figure 3-7 shows sheathing details at the valley and chimney opening. The detail at the top shows the clearances between masonry and wood-framing members. Framing members should have a 2-inch clearance for fire protection. The sheathing should be securely nailed to the rafters and to the headers around the opening.
Figure 3-7.-Sheathing details at chimney and valley openings.
Wood or plywood sheathing at the valleys and hips should be installed to provide a tight joint and should be securely nailed to hip and valley rafters. This provides a smooth solid base for metal flashing.
ESTIMATING SHEATHING MATERIAL
To figure the roof area without actually getting on the roof and measuring, find the dimensions of the roof on the plans. Multiply the length times the width of the roof, including the overhang. Then multiply by the factor shown opposite the rise of the roof in table 3-2. The result will be the roof area.
Table 3-2.-Determining Roof Area from a Plan
For example, assume a building is 70 feet long and 30 feet wide (including the overhang), and the roof has a rise of 5 1/2 inches: 70 feet x 30 feet = 2,100 square feet. For arise of 5 1/2 inches, the factor on the chart is 1.100:2,100 square feet x 1.100=2,310 square feet. So, the total area to be covered is 2,310 square feet. Use this total area for figuring roofing needs, such as sheathing, felt underpayment, or shingles.
To decide how much lumber will be needed, first calculate the total area to be covered. Determine the size boards to be used, then refer to table 3-3. Multiply the total area to be covered by the factor from the chart. For example, if 1- by 8-inch tongue-and-groove sheathing boards are to be used, multiply the total roof area by 1.16. To determine the total number of board feet needed, add 5 percent for trim and waste.
Table 3-3.-Lumber Sheathing Specifications and Estimating Factor
To determine how much plywood will be needed, find the total roof area to be covered and divide by 32 (the number of square feet in one 4-by 8-foot sheet of plywood). This gives you the number of sheets required to cover the area. Be sure to add 5 percent for a trim and waste allowance.
Decking or Planking
To estimate plank decking, first determine the area to be covered, then refer to the chart in table 3-4. In the left column, find the size planking to be applied. For example, if 2- by 6-inch material is selected, the factor is 2.40. Multiply the area to be covered by this factor and add a 5 percent trim and waste allowance.
Table 3-4.-Plank Decking Estimating Factor
Wood Fiber Roof Decking
To estimate the amount of weed fiber decking required, first find the total roof area to be covered. For every 100 square feet of area, you will need 6.25 panels, 2 by 8 feet in size. So, divide the roof area by 100 and multiply by 6.25. Using our previous example with a roof area of 2,310 square feet, you will need 145 panels.
|David L. Heiserman, Editor||
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Revised: May 18, 2013