Wood-Frame House Construction, Chapter 4, Completing the Shell
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Chapter 4 COMPLETING THE SHELL Page Flashing and other sheet-metal work . 100 Exterior covering materials . 115 Materials ( 100 ), Flashing ( 100 ), Wood siding ( 116 ), Horizontal Gutters and downspouts ( 103 ). siding ( 117 ), Siding for horizontal, vertical, and diagonal applications ( 117 ), Attic ventilation . 105 Siding for vertical application ( 117 ), Siding with sheet materials ( 117 ), Types and location of roof ventilators ( 106 ), Wood shingles and shakes ( 117 ), Area of ventilators ( 107 ), Hip roofs ( 108 ), Other exterior finishes ( 119 ). Flat roofs ( 108 ). Exterior covering installation . 119 Windows and exterior doors . 109 Bevel siding ( 120 ), Drop and Window materials and styles ( 109 ), similar sidings ( 121 ), Vertical and Single- and double-hung windows ( 112 ), diagonal siding ( 121 ), Plywood and Casement windows ( 113 ), other sheet sidings ( 122 ), Stationary (fixed) windows ( 113 ), Comer treatment ( 122 ). Material Awning windows ( 115 ), Horizontal sliding window transition ( 124 ). Wood shingles and units ( 115 ), Specialty windows ( 115 ), shakes ( 124 ), Stucco finish ( 125 ), Sliding glass doors ( 115 ), Masonry veneer ( 126 ), Aluminum and Exterior doors and frames ( 115 ). vinyl ( 126 ). Exterior trim . 127 Material used for trim ( 127 ), Cornice construction and types ( 128 ), Rake or gable-end finish ( 133 ), Cornice return ( 134 ). 99 Completing the Shell The topics discussed in this chapter are specific tasks Flashing related to completing the construction of the shell of the house. Their order of presentation does not necessarily Flashing should be used at the junction of a roof and a reflect the sequence of performance. wood or masonry wall, at chimneys, over exposed doors and windows, at changes of siding material, in roof val- Flashing and Other Sheet-Metal Work leys, and in other areas where rain or melted snow might penetrate into the house. Sheet metal work normally consists of installing flash- ing, gutters, downspouts, and sometimes attic ventilators. Material changes. Horizontal gaps formed at the Flashing is often provided to prevent wicking action by intersection of two types of siding material often require joints between moisture-absorbent materials. It can also be Z-flashing (fig. 87). For example, a stucco-finish gable used to provide protection from wind-driven rain or melt- end and a wood-siding lower wall should be flashed at ing snow. As previously indicated, damage from ice dams their juncture (fig. 87A). is often the result of inadequate flashing. An upper wall sided with vertical boards, with horizon- Gutters are installed at the cornice line of a house with tal siding below, usually requires some type of flashing pitched roof, to carry the rain or melted snow to the (fig. 87B). When the upper wall, such as a gable end, downspouts and away from the foundation area. They are projects slightly beyond the lower wall (fig. 88), flashing especially needed on houses with narrow roof overhangs is usually not required. The bottom edge of the siding is where poor drainage away from the foundation wall is cut back at an angle to provide a drip edge, as shown in often the cause of wet basements. the figure. Materials Doors and windows. Head flashing, which is similar to Z-flashing, should be used over door and window Aluminum is the material most commonly used for openings exposed to rain. Windows and doorheads pro- sheet-metal work; other materials commonly used include tected by wide overhangs in a single-story house do not zinc-coated (galvanized) steel, copper, and vinyl plastic. ordinarily require such flashing. Head flashing should be Aluminum flashing for roof valleys should have a mini- started behind the siding and should be bent out and down mal thickness of 0.019 inch. Thickness for gutters should over the top molding over the window or door. When be 0.027 inch and for downspouts 0.020 inch. Copper for building paper is used on the side walls, it should lap the flashing and similar uses should have a minimal thickness top edge of the flashing. of 0.020 inch. Aluminum is not normally used where it will come in contact with concrete or stucco, unless it is Flat roofs. Flashing is required at the junction of an insulated against reaction with the alkali in the cement by exterior wall and a flat or low-pitched built-up roof a coat of asphaltum or other protection. (fig. 89). When a metal roof is used, the metal is turned up on the wall and covered by the siding. A clearance of Galvanized (zinc-coated) sheet metal is also frequently 2 inches should be allowed at the bottom of the siding for used. Zinc coatings come in two weights, 1.25 and 1.50 ounces per square foot (total weight of coating on both protection from melted snow and water. sides). When 1.25-ounce sheet is used for exposed flash- Ridges and roofs. Ridge flashing or roll roofing ing and for gutters and downspouts, 26-gauge metal is should be used under a Boston ridge on wood shingle or required. With 1.50-ounce coating, a 28-gauge metal is shake roofs to prevent water entry (fig. 90). The flashing satisfactory for most metal work, except that gutters should extend about 3 inches on each side of the ridge should be 26 gauge. and be nailed in place only at the outer edges. Ridge shingles or shakes, which are 6 to 8 inches wide, cover In choosing accessory hardware such as nails, screws, the flashing. hangers, and clips, it is important to avoid the potential for corrosion or deterioration that can occur when unlike Vents. Stack vents and roof ventilators are provided metals are used together. For aluminum, only aluminum with flashing collars, which are lapped by the shingles on or stainless steel fasteners should be used. Galvanized the upper side. The lower edge of the collar laps the shin- sheet metal should be fastened with galvanized or stainless gles. Sides are nailed to the shingles and caulked with steel fasteners. roofing mastic. 100 Figure 87 – Z-flashing at material changes: Valleys. The valley formed by two intersecting roof mineral-surfaced roll roofing in place of metal flashing. planes is usually covered with metal flashing. Some build- As an alternative, one strip of roll roofing 36 inches wide ing regulations allow the use of two thicknesses of can be applied to the valley and covered with asphalt or Figure 88 – Gable end projection material transition without flashing. fiberglass shingles applied continuously from one plane of 1-inch-high crimped standing seam should be used the roof to the other. This type of valley is normally used (fig. 91B). This will keep heavy rains on the steeper only on roofs with a slope of 10 in 12 or steeper. slopes from overrunning the valley and being forced under the shingles on the adjoining slope. Nails for the Widths of sheet-metal flashing for valleys should not be shingles should be kept back as far as possible to elimi- less than: nate holes in the flashing. A ribbon of asphalt-roofing 12 inches for roof slopes of 7 in 12 and steeper. mastic is often used under the edge of the shingles. inches wide for roof slopes 4 in 12 to in 12. 18 7 Roof-wall intersections. When shingles on a roof 24 inches wide for slopes flatter than 4 in 12. intersect a vertical wall, step flashing is used at the junc- tion. Aluminum or galvanized steel is bent at a 90° angle The width of the valley between shingles should and extended up the side of the wall a minimum of increase from the top to the bottom (fig. 91A). The mini- 4 inches over the sheathing (fig. 92A). When roofing felt mal open width at the top is 4 inches and should be is used under the shingles, it is turned up on the wall and 1 increased at the rate of about /8 inch per foot. These covered by the flashing. The siding is then applied over widths can be marked on the flashing with a chalking the flashing, allowing about a 2-inch vertical space string before shingles are applied. between the level edge of the siding and the roof. When adjacent roof slopes vary, for example, where a If the roof intersects a brick wall or chimney, the same low-slope porch roof intersects a steeper main roof, a type of metal flashing is used as that described for the 102 Figure 89 – Flashing at junction of built-up roof and Around small chimneys, flashing often consists of sim- vertical building wall. ple counterflashing applied over step flashing on each side. For single-flue chimneys, the shingle flashing on the high side should be carried up under the shingles. This flashing should extend up the chimney for a distance of about 4 inches above the roof sheathing (fig. 93A). A saddle for better drainage is often constructed on the high side of wide chimneys. It is made of a ridge board and post and sheathed with plywood or boards (fig. 93B). It is then covered with metal that extends up on the brick and under the shingles. Counterflashing at the chimney is then used, as previously described, with lead plugging and caulking. A very wide chimney may have on the high side a partial gable that can be shingled in the same man- ner as the main roof. Roof drip edge. Aluminum drip edge flashing is often used around the entire perimeter of the roof to protect the edge of the sheathing and to reduce the amount of rain- water running down the fascia or blowing under the roof covering (fig. 94). Gutters and downspouts Several types of gutters are available to carry the rain- Figure 90 – Flashing at Boston ridge with wood shingles.