| Home | Wiring | Plumbing | Kitchen/Bath |
|
The roof for a small one-story addition is generally either a shed roof, with a single sloping surface, or a gable roof, with two sloping sides that meet at a ridge. An architect can help you choose which style is best suited to your house and recommend whether to match the slope of the existing roof, which you can measure as shown below. -- Basic Roof Structures: Shed roofs are the simpler to build, and most have a gentle slope that is easier to work on. You can omit ceiling joists if you build a roof that is nearly flat or if the addition will have a sloping, cathedral- style ceiling. A gable design offers more versatility; allowing you to build the new roof at a right angle to an existing wall or roof or to extend the main roof. A gable must always have ceiling joists, which keep the rafters from spreading apart at the bottom. -- Getting the Details Right: To blend an addition seamlessly with the house, duplicate the existing cornice. In the case of a gable roof that intersects the house roof, also make sure the house and addition eaves meet at the same level. If the house roof has an over hang of at least 12 inches, this can be achieved simply by adjusting the addition overhang. To match a shorter overhang, you must build the intersecting gable with rafters that are the same size as those in the house. Except in that case, there is no need to match the house-roof framing in the addition. Refer to local codes when selecting lumber for joists, rafters, and plates for fastening rafters to the house. The shown table provides information needed to cut rafters and plates. CAUTION----Check siding and roofing materials for lead and asbestos; follow the procedures listed there if either substance is present. SAFETY TIPS --- Protect your eyes with safety goggles when you are hammering, drilling, or cutting. Put on a hard hat when working in the attic or with overhead roof framing, and wear shoes with nonslip rubber soles whenever you climb on a roof. Measuring a roof slope. • Mark the top of a level 12 inches from one end with a piece of tape. • Hold that end against the underside of a - rafter inside the attic or under a rake board outside the house. With the level horizontal, set a ruler or carpenter’s square vertically at the tape. • Note the distance in inches from the level to the underside of the rafter or rake board. This figure, the unit rise, is the vertical rise of the roof in a 12-inch run; the rafter illustrated at left has a unit rise of 6; that is, the reading indicates a 6-inch rise for 12 inches of run. TOOLS:
MATERIALS:
51 52 Table ---- Unit rise --- Common-rafter end-cut heights for various lumber sizes --- Jack rafters Foot Setback angle: 2 x 4 2 x 6 2 x 8 2 x 10 2 x 12 Fitting rafters to the roof. This table provides specifications essential to the discussion that follow, organized according to rafter slopes given as unit rise. It lists the vertical end cuts for five sizes of rafters, figures needed not only for cutting the rafters but also for helping determine the size and placement of a rafter plate or a ridge beam. The last two columns apply to jack rafters, short rafters that support an addition roof wherever it overlaps the main roof. One column indicates set back, the placement relative to the valley formed where the two roofs meet. The other indicates the angle at which to tilt the blade of a circular saw from its normal position of 90 degrees to fit the foot of a jack rafter to the roof beneath it. TWO CORNICE STYLES 52ab--- A closed cornice. For a roof without an overhang, rafters and ceiling joists are trimmed flush with the wall top plate. A plywood filler strip and a frieze board, installed after the roof sheathing, attach to the heels of the rafters. A drip edge is fastened over the frieze, and vents, positioned below any gutter to be added, allow air to enter the space under the roof. Gable ends are finished with a spacer board and a rake board, both the same width as the frieze, nailed through the sheathing to the end rafter. A box cornice. In this style, the rafters project to frame an overhang at the eaves. A notch called a bird’s mouth near the end of each rafter fits over the addition wall. The over hang is enclosed by a fascia and a ventilated plywood soffit that meets a frieze board nailed to the sheathing at the top of the wall. A drip edge covers the joint between the fascia and the roof sheathing. Gable ends are finished as in the closed cornice, with the addition of a piece called a pork-chop that covers the cornice end. In a wide box cornice , horizontal supports called lookouts frame and support the soffit. ROOF SHEATHING; DRIP EDGE; GABLE SHEATHING; SPACER BOARD; HEEL; FILLER STRIP 52 ==== PROVIDING FOR VENTILATION 53a--- Ventilation of the enclosed space under an addition roof prevents hot air from accumulating there in summer and heating the house. In winter, adequate circulation clears out any warm, moist air that might escape from the house, preventing condensation on cold rafters and roof sheathing that can ruin insulation and rot wood. The most efficient arrangement of vents allows hot air to rise through weatherproof openings along the ridge, pulling in cooler outside air through openings at the eaves. Comparable vents are available for a shed roof where it meets the house wall. If ridge venting is impractical, you can substitute a vent in the top of the gable. At the base of the roof, install either a full-length ventilating strip along the entire eave or individual vents (photo graph)—rectangular models for installation in a soffit, or circular ones for the frieze board where there is no overhang. Before deciding on any ventilation scheme, however, check your local building code for ventilation requirements and accept able venting methods. ==== BUILDING A SHED ROOF 1. Mounting the plates. • Mark the wall 2 inches above the height of the top of the rafter plate, calculated as follows: From the length of a side wall in inches, sub tract 5.5 inches (7.5 inches for an addition framed in 2-by-6s). Multiply the result by the unit rise planned for the roof and divide by 12, then add the rafter end cut. • Strip the siding from the house below the mark and between the side walls of the addition. • Cut a ledger and a rafter plate flush to the outer edges of the side-wall top plates. Make the ledger from ceiling-joist stock, the rafter plate from 2-inch lumber at least as wide as the length of a rafter end cut. NOTE: If you plan to remove the house wall for access to the addition, don’t install a ledger; the header you will install serves as a ledger. • Tack both boards in place, then secure them to house studs with 3.5-inch lag screws and washers. For masonry walls, drill holes for lag-screw shields through the plates into horizontal mortar joints. 53 2. Installing the joists. • Nail joist hangers to the ledger at 2-foot intervals, beginning 1.5 inches from one end and setting each hanger flush with the bottom of the ledger. Position the last hanger 1.5 inches from the other end of the ledger. • Cut the joists to fit between the ledger and the outer edge of the top plate of the end wall. • Nail the boards to the joist hangers and fasten them to the top plate with rafter ties. 3. Hanging the rafters. • Look up the vertical end cut corresponding to the roof slope. Draw a slanted line on a rafter board the length of the end cut from one edge of its face to the other. Cut along it. • With a helper, tack the board to the end of the rafter plate and the end-wall top plate, aligning the rafter board’s top with the rafter plate’s front edge and the rafter’s underside with the end-wall top plate as shown in Step 5. • Mark the rafter board where it meets the front of the rafter plate, at the end wall for the bird’s-mouth cut, and at the outer end for the appropriate cornice. • Take down the board, cut it at the marks, and use it as a template for the other rafters. • Fasten rafters to the rafter plate with rafter anchors; attach them to the joists with 3.5-inch nails. • Saw off joist corners flush with the upper edges of the rafters. 4. Installing cripple studs. • To complete the framing, cut 2-by-4 studs, with angled tops, to fit at 16-inch intervals between end-rafter edges and the side wall top plates. Turn the studs so the wide sides face outward. • Toenail the studs to the rafters and the top plates. 54 ERECTING A GABLE ROOF 1. Making ridge-beam supports. • Cut a pair of 2-by-6s to the height of the ridge beam above the planned ceiling, calculated as follows: First subtract 9.5 inches (13.5 inches for 2-by-6 walls) from the addition’s width and divide by 2. Multiply the result by the unit rise and divide by 12. Add the rafters’ end cut, then sub tract the ridge-beam width. • Nail two 1-by-2s, 1.5 inches apart, to the ends of each 2-by-6 . • Mark the house 1.25 inches above a joist set on the addition side walls. Strip away siding below the mark and between the walls. • Install joists at 2-foot intervals, setting end joists 1.5 inches from the ends of the top plates. • Nail and brace a ridge-beam support at the center of each end joist, aligning the support with the bottom of the joist. 2. Mounting the rafters. • Set the ridge beam in the sup ports, then measure and cut rafters as in Step 3, opposite. • On each side of the ridge beam, hold a rafter against the house and draw a line on the siding, parallel to the rafter and 1 inches above it. Strip away siding below the lines. • Trim the ridge beam—to the outer edge of the rake overhang if one is planned, otherwise to the outside edge of the end-wall top plate. • Nail rafters to studs in the house wall. Secure them to the beam and top plates, also as in Step 3, opposite. • Starting at the gable end, mount the remaining rafters, then saw off the joist corners flush with the rafters’ upper edges. • Cut cripple studs to fit between the end-wall top plate and the end rafters and install them (Step 4). ROOFS THAT INTERSECT 56a--- 1. Cutting the ridge beam. • For an intersecting gable roof, in stall joists and set a length of ridge- beam stock in braces. • Where the beam touches the house roof, drive a nail into the at tic. If the nail protrudes between rafters, install a bracing board of rafter stock at that point, face against the sheathing. • Use a board to help draw the roof slope on the beam . • Cut along the line, then trim the other end of the beam as explained, Step 2. • Mark the ridge beam for rafters at 2-foot intervals, then toenail the end of the beam through the roof into the bracing board or the rafter below. ====WORKING SAFELY ON A ROOF==== 56b--- Safety is the paramount concern during roof work. While the slope of your roof is the main factor in determining whether you need to rent or buy special equipment, your personal tolerance for heights is also important: If you feel uncomfortable on a low-slope roof, take the same safety precautions that others might reserve for a steeper one, or consider hiring a professional to do the roofing for you. In general, if your roof slopes less than 4 inches in 12, you can work and move about on it safely by observing common- sense precautions and wearing rubber-soled shoes. For added security, you can create temporary footholds by nailing 2-by-4s horizontally across the surface in an area of the house roof that will be removed as part of the project, or covered by the addition roof. On steeper slopes, hang a ladder from the ridge with ladder hooks, which adjust to fit any ex tension ladder. Models with casters permit you to roll the hooks, with ladder attached, up a finished roof without damaging it. Turn the ladder over for the hooks to grip the ridge. Instead of ladder hooks, you can use metal sup ports called roof brackets. They can be adjusted to fit any roof slope and secured to a roof without damaging the shingles. ==== 56 4. Installing the rafter plates. • Cut rafter plates out of i-inch plywood, making each long enough to accommodate the remaining rafter positions marked on the ridge beam. • Nail the plates between the ridge beam and the cut edge of the main roof, aligning the outer edge of each plate with the inner chalked line. • To mark the plates for jack rafters, drop a plumb line from the rafter positions on the ridge beam to the roof, then extend horizontal lines from these points to the rafter plates. 2. Installing rafters. • Cut full-length common rafters for the addition, but leave the eave ends uncut and long enough to extend below the level of the existing roof edge. Make the rafter pair just outside the house roof half again as long as the length from the ridge beam to the top plate. • If a pair of rafters falls inside the house roof , trim the ends as for a closed cornice. • Set aside one of the common rafters for use as a template for the jack rafters. Then install the remaining full-length rafters, starting at the gable end. • Add cripple studs at the gable section. 3. Positioning roof plates. • Extend a chalk line from the point where the top of the ridge beam meets the house roof to a helper on the ground. Keeping the line taut, have your helper hold the line so that it grazes the surface of both the roof and the top edge of the long rafter. Snap the line . • Mark the house roof on the other side of the addition in the same way. • Consult the table above to determine how far inside the chalked lines to position the roof plates. Mark the roof for this set back and snap the corresponding chalked lines. 57 5. Cutting foot angles. • Using the template rafter reserved in Step 2, trace the horizontal side of the bird’s-mouth cut at the midpoint of a piece of rafter stock about twice the length of the rafter (far left). Ex tend the line across the entire board. • Tilt a circular-saw blade to the required foot angle and cut along the mark, creating left- and right-hand pieces. • Cut each jack-rafter pair the same way. Use a shorter length of stock for each pair. 6. Completing the jack rafters. • Measure from the top of the ridge beam to the outer edge of the left-hand roof plate at a jack-rafter location . Mark the distance on the upper edge of the longest left-hand jack rafter cut in Step 5. • Set the ridge end of a template rafter at this mark, aligning its longer edge with that of the jack rafter. Mark the ridge cut on the jack rafter . • Cut the jack rafter at the line. • Measure and cut each left-hand jack rafter in the same way; use the left-hand rafters as tem plates to mark ridge cuts on right-hand ones. • Toenail jack rafters to the roof plates. Fasten them to the ridge beam with rafter anchors. • Install the template rafter. 7. Trimming the rafters. • At each side of the addition, remove a short section of the fascia from the house cornice. • Hold a level against the edge of the roof. Keeping the level horizontal, position it so that the top outer corner of the level touches the top inner edge of the nearest addition rafter . Mark the rafter at this point. • Cut the rafter at the mark to match the ends of the house-roof rafters. • Cut the other addition rafters at the same length and angle as the first. • Match the cornice of the addition to that of the house. JACK RAFTER 58 EXTENDING GABLE ROOFS Two roof treatments. 59a--- In the example at left, an addition is built flush with the front of the main house but set back from the rear. The result is a partial extension of the house roof across the front of the addition. At the rear, the addition roof runs beneath the main roof and meets the house at the gable wall. When the addition is as high and wide as the house, the main roof is extended to cover both sides of the new construction. Putting up the ridge beam. 59b--- • Remove the rake trim from the house roof—on one side for a partial extension of the roof, on both sides for a complete extension. • Install joists and a ridge beam in the usual way. • For the partial extension at left, build a ridge-beam support to hold the beam so that one corner is even with the top edge of the house end rafter. For a complete extension, butt the addition ridge beam to the end of the house ridge beam. • Fasten the addition ridge beam to the house with rafter anchors. 59 Putting up rafters. 60a--- • Cut the rafters and begin installing them at the gable end. • For a partial extension, fasten one of the last rafter pair to the house end rafter. Mark and cut an additional rafter to fit between the soffit board and top plate of the addition side wall. • Have a helper hold a straight board against the undersides of the installed rafters as a guide for the rafter while you nail it to the house wall. • In addition to the usual flashing for a roof, also flash where the sheathing for the addition roof meets the house soffit or the house- roof sheathing along the rakes. Then install new rake trim between the peaks of the house and addition roofs. For a complete extension, attach both inner rafters to the house end rafters. Building an overhung. 60b--- • Match the cornice trim of the addition to that of the house roof, but extend the fascia or frieze board beyond the outermost rafter to the outer edge of any planned overhang at the rake. • Install a rake rafter close to the ends of the fascia and the ridge beam, leaving just enough room outside the rafter for the rake trim. • Nail bracing blocks made from the same size lumber as the rafters between the rake rafter and the outermost main rafter at 16-inch intervals. 60 |
| HOME | Prev: Grafting New Walls to Old | Next: Finishing an Addition Roof |
Tuesday, April 1, 2014 1:07 PST