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.Building atop a house is often less expensive than expanding outward, if only be cause such an addition sits above the old foundation. Building up may also be the only way to improve a house on a small or hilly lot. As explained in this section, this style comes with two complications: scaffolding and roof-safety equipment is a must—and a new stairway is usually required.
A New Room over the Garage
A Fire-Resistant Indoor Stair
Outdoor Stairs to Save Space
[A shed dormer for an attic]
An Addition for on Attic
A shed dormer increases living space in a house by adding head room along one side of an attic—as long as the ridge beam is 10 feet or more above the attic floor. The dormer depicted here, which runs from the roof ridge to the house wall, is the easiest dormer to build, as well as the roomiest.
- Important Preliminaries: To maintain support for the house ridge beam, plan for a dormer no more than half the roof length, and no closer to the end rafter than a sixth of the roof length. Dormer side walls must rest directly on rafters.
Allow for access from below; you can replace old attic stairs with some that meet code, or add a new stairway If the addition includes a kitchen or bath, align the pipes with plumbing below. Finally, make sure the attic joists are adequate (a 2-by-6 joist should span no more than 8 feet; a 2-by-8, 11 feet). If not, double them.
Anatomy of a shed dormer.
The frame of a shed dormer consists of three walls and a roof, bearing on four long plates. The front wall has corner posts and conventional window framing set between studs, which run to the top plate of the house wall and may be unevenly spaced to avoid existing joists and rafters. Side-wall studs are 2-by-4s notched to hold the dormer rafters and allow nailing from in side as well as out. They rest on 2-by-4 plates nailed through the roof to doubled rafters. The dormer rafters are the same size lumber as the house rafters. They bear on a rafter plate nailed through the roof to the house rafters. Finally, ceiling joists tie the front wall of the dormer to the house rafters on both sides of the ridge.
Consult our earlier discussion for roof-safety procedures. A project of this scale requires scaffolding and, in addition, ladder hooks and roof jacks.
CAUTION---Don’t our earlier for advice on testing (or, and safely handling, asbestos roofing shingles.)
SAFETY TIPS: Soft-soled shoes are essential when you work on the roof; in the attic, a hard hat helps protect against protruding nails. Put on safety goggles when hammering; add a dust mask for sawing. Ear protectors are a good idea during prolonged noisy work.
ASSEMBLING A SAFE WORK PLATFORM
Raising the sections.
Metal scaffolding and special aluminum-and-plywood planks for the platform are available from tool-rental centers. Scaffolding comes in sections 5 feet high, 5 feet wide, and up to 10 feet long. Assemble it as follows:
• Set the legs of two end frames onto base plates resting on 2-by-12 sills, and bolt on two X-shaped crosspieces. Level and plumb the section with the base-plate adjusting screws.
• Slide a coupling pin into the top of each corner up right. Assemble a second section atop the first, keeping the structure level and plumb with the base-plate screws. To add higher sections, climb the end-frame rungs.
• Hook planks across the top of the scaffolding and in stall guardrails, leaving open the side of the scaffolding toward the house.
Bracing tall scaffolding.
Scaffolding higher than one story must be secured to the house.
• Notch a 2-by-4 to fit around a corner upright, and cut the other end to fit snugly against the house.
• Make a tight loop of baling wire around the upright and also around a 2-by-4 that is set across the jambs inside a window.
• Twist the loop with a strong stick to tighten the wire and secure the frame against the house. Wedge the stick to prevent unwinding.
If no window is convenient, fasten the wire with a 0.5-inch nail driven into a stud; in a masonry wall use a lag screw and an anchor in a mortar joint.
BUILDING THE DORMER’S FRONT WALL
1. Doubling the side rafters.
Strengthen the rafter on each side of the planned dormer by nailing to it a board, or doubler, of the same size.
• Mark and cut two doublers to fit between the ridge beam and outer edge of the house-wall top plate. Copy the rafter’s angled end cuts with a T bevel.
• For each rafter, check which face attaches to a joist; secure a doubler to the opposite face with 3-inch nails, 6 inches apart, in a zigzag pattern. Nail from both sides. Also nail through the ridge beam into the end of each board.
• On the dormer side of each doubled rafter, drill i-inch marker holes through the roof near the ridge and top plate.
2. Outlining the dormer.
• Straddle the ridge and, with a helper standing on a ladder or scaffold, use the marker holes you drilled in Step 1 as guides to snap a chalk line above the dormer-side edge of each doubled rafter pair.
• Snap a third line perpendicular to the first two, parallel to the eaves and roughly 2 feet up the roof from the house top plate.
3. Cutting through the roof.
• Set up a 2-by-10 platform on roof jacks slipped under the shingles and nailed to the sheathing.
• Install a remodeling blade in a circular saw and set it to the combined thickness of shingles and sheathing; find this dimension from the marker holes drilled in Step 1.
• Saw along the chalked line that runs parallel to the eaves, extending the cut 1.5 inches beyond the lines that mark the dormer sides. (Surface scars at the cut ends will be covered by side-wall plates.)
• Remove the roof-jack platform.
4. Probing for the top plate.
• Starting at the cut made in Step 3, saw partway toward the eaves along one of the dormer side lines. Stop well before the blade reaches the top plate of the house wall.
• Push a stiff wire through the saw cut while a helper in the attic observes how far the cut has progressed toward the top plate.
Saw a little farther and probe again, stopping when the cut reaches the inside edge of the top plate. Extend the cut to the outer edge of the plate with a saber saw.
• Make a similar cut at the other side of the dormer, then mark the outer edge of the top plate with a chalk line snapped between the ends of the two cuts.
5. Prying up a section of roof.
• Working from scaffolding, cut through the roof along the edge of the top plate.
• Saw the rectangle of shingles and sheathing within the four cuts into manageable pieces, then pry them off and discard them.
6. Setting corner posts.
• Assemble two corner posts from 8-foot 2-by-4s and 0.5-inch plywood spacers. Bevel one end of each post to match the roof slope, then cut the posts to reach 3 inches lower than the ceiling height planned for the dormer.
• With a utility knife equipped with a hooked blade, cut away shingles and ex pose the sheathing next to the bottom corners of the opening.
• Set a post on the sheathing over the doubled rafter, its outer face aligned with the edge of the opening. While a helper plumbs it right to left, toenail it in place front and back.
• Tack a scrap of 2-inch lumber about 6 feet up the roof and nail a brace to it. Plumb the post front to back, then nail the brace to the post. Toenail through the sides of the post into the sheathing and doubled rafter.
• Install the other post in the same way.
7. Nailing the dormer top plate.
• Extend the scaffolding 5 feet upward.
• Standing on the new platform, nail a 2-by-4 to the tops of the corner posts. If you cannot obtain a straight 2-by-4 long enough to span the distance, temporarily splice pieces end to end with scrap nailed across the edges of the boards. Position the splice where it will be supported by a stud or a header in the front wall of the dormer.
8. Framing the front wall.
• In between the house top plate and the dormer top plate, install jack studs and king studs, headers, and rough sills for rough window openings. Nail cripple studs under the sills (above).
• Fasten full-height studs between window openings. Because the dormer studs share the house top plate with joist and rafter ends, the studs may have to be spaced irregularly, but not more than 24 inches apart.
• After framing the front wall, double the dormer top plate, staggering any joints between top-plate pieces. Re move the scrap from the lower joint.
• Plumb the wall again (Step 6), then nail a temporary 1 -by-4 brace from the top of one corner post to the bottom of the other.
ADDING RAFTERS AND SIDE WALLS
1. Aligning a rafter.
• Cut two strips of 4-inch plywood 36 inches long and 8 inches wide. Lay one strip over the doubled rafters as a stand-in spacer for a rafter plate (Step 3).
• Select a rafter board 30 inches longer than the dormer roof. While a helper holds one end of the board so the bottom edge meets the inside corner of the upper plate, set the other end on the spacer, placing the second plywood strip on top of it as a rafter extender.
• Maneuver the rafter board along the spacer to close any angle between the ex tender and the edge of the board, and simultaneously to bring the topmost corner of the extender even with the ridge.
• Tack the extender to the rafter board and have your helper tack the board to the end of the top plate.
2. Marking the rafter ridge cut.
• Slide the spacer up the roof until it touches the underside of the upper plywood strip.
• Set a board with parallel edges on the spacer alongside the rafter board. Draw a ridge-cut line on the rafter guided by the top edge of the board. Set aside the rafter board and extender.
• Mark the roof at the top of the spacer.
• Lay the rafter board on the scaffolding, take off the extender, and make the ridge cut on the line.
3. Making a rafter plate.
• Snap a chalk line parallel to the ridge through the mark made on the shingles in Step 2.
• Cut rafter-plate pieces of 0.5-inch plywood as wide as the length of the rafter ridge cut so that the assembled rafter plate extends 0.5 inch beyond the doubled rafters at both ends.
• To create the rafter plate, position the plywood strips along the chalked line and nail them to the rafters under the sheathing.
4. Cutting the bird’s mouths.
• Set the ridge end of the rafter board on the rafter plate. Have a helper hold the board against the top plate and mark for a bird’s- mouth cut. Mark the uncut rafter end for the appropriate cornice.
• Cut the rafter and use it as a template to mark and cut other rafters.
• Toenail a rafter to each end of the rafter plate and attach it to the dormer top plate with rafter anchors, placing the rafters flush with the ends of the rafter plate and top plate.
• Attach the other rafters directly above the rafters of the old roof.
• Remove the bracing between the corner posts and the roof.
5. Adding the side-wall plates.
• Along the roof, measure between the rafter plate and the corner post and cut a 2-by-4 to that length to serve as a side wall plate.
• Bevel the ends of the plate to fit against the post and rafter plate.
• With 4-inch nails, nail the plate through the roof into the doubled rafter.
• Toenail the plate to the corner posts with 2.5-inch nails.
• Cut and attach the other side-wall plate in the same way.
6. Setting the side-wall studs.
• Select a 2-by-4 longer than the distance between the roof and the tops of the dormer rafters, measured at a point 16 inches from the corner post.
• Have a helper hold the board plumb against the side-wall plate’s in side edge, 16 inches from the post.
• Mark lines across the board at the side-wall plate and along both edges of the rafter.
• Cut the 2-by-4 at the bottom and topmost marks, then add a notch 1 inches deep to fit the rafter.
• Nail the finished stud to the rafter and toenail it to the plate.
• Mark, cut, and install studs at 16-inch intervals along both side walls in the same way.
INSTALLING CEILING JOISTS
1. Opening the roof.
• Level a 2-by-6 laid between the top plate and the roof surface, then slide the level along the top until it touches the shingles.
Mark them at that point.
• Repeat the process at the other side of the dormer.
• Snap a chalk line between the marks.
• Cut through the shingles and sheathing along this line and along the insides of the side-wall plates.
• Pry the roof from the rafters; for easier handling, remove it in sections by cutting between rafters.
2. Nailing the ceiling joists.
Install 2-by-6 ceiling joists from the out side edge of the dormer top plate to the sheathing on the opposite side of the ridge. Proceed as follows:
• Cut one end of a joist to match the roof slope. Cut the other square to fit flush with the outside of the top plate. Use this joist as a template to cut others.
• Working from ladders on the attic floor, level one of the outermost joists and nail it to the house rafters on both sides of the ridge with six 3.5-inch nails; if a rafter is doubled on the inside, add blocking to the rafter on the attic’s other side. Secure the joist to the top plate with a rafter tie. (For clarity, parts of the roof have been omitted from the illustration.)
• Add blocking between the joist and the dormer rafter; nail the joist to the blocking. Attach the other outer joist.
• install the remaining joists in the same manner, but without blocking; they can be face-nailed directly to the corresponding dormer rafters.
• Complete the dormer construction by sheathing the walls and roof, leaving room for ventilation.
3. Extracting the old rafters.
• Working within the dormer, use a crosscut saw or a reciprocating saw to cut the old rafters flush with the floor joists. To avoid cutting into studs, you may have to finish the job with vertical cuts. (For clarity, wall sheathing has been omitted from this illustration.)
• Have a helper support each house rafter as you saw it along the bottom edge of the dormer ceiling joist nailed to the rafter.
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Friday, April 4, 2014 20:43 PST