Multistory Additions: Two-Story Additions (How to Build Additions Guide)

Building a two-story addition involves the same techniques as a ground-level addition, except as de scribed below.

Like a ground-level addition, it disrupts the existing living space only when you remove or alter common walls between the structures, or if the upper floor of the new addition ex tends over part of the present house. The construction techniques for an overlapping section are the same as for a second-floor addition.

Building the Foundation

A perimeter crawl space foundation for a two-story home requires wider footings and thicker stem walls: generally 15 inches minimum for footing width, 8 inches for footing thickness, and 8 inches for stem wall thickness.

A slab foundation would have similar requirements for footings.

A full basement foundation or pier- and -grade-beam foundation would likely have the same dimensions for a two-story structure as for one story.

Techniques for digging trenches, building forms, setting rebar, and placing concrete are the same as for a ground-level addition.

In areas subject to earthquakes or high winds, plans may include “hold- downs.” One type uses straps em bedded in concrete and nailed to sheathing. Another system uses brackets and threaded steel rods em bedded in the foundation. The plans will indicate general locations for rods, but you must locate them precisely, aligning them directly beneath hold-down brackets attached to wall studs yet to be installed. To do this, you must plan your wall framing lay out at the same time that you are building foundation forms. You can then mark the forms so that you can later place the threaded rods accurately in the wet concrete.

The diameter of the threaded rods varies with the size of hold-down brackets, according to manufacturer’s specifications. They should be long enough to extend down at least 9 inches into concrete, up through the joist cavity, and out above the first floor at least 4 inches.

Framing Floor, Walls, and Roof

Check your plans to see if the mudsill is aligned with the outer edge of the foundation wall or set in to clear the sheathing. If a kneewall will be built on the mudsill, the studs must be 2 by 6s at 16 inches on center to support the two floors above.

Double any first-floor joists that support walls running parallel to them, spreading the joists 31/2 inches apart if plumbing will be run up through the wall. Support any bearing walls on a girder or foundation that runs perpendicular to joists of the first floor. Also provide foundation support for any posts or walls on the first floor that support beams for the upper floor joists.

Frame the walls the same as for a ground-level addition, except that header sizes may need to be in creased to bear the extra floor above.

Install sheathing or permanent diagonal bracing on the lower walls as soon as possible to stabilize the upper floor and roof.


In areas subject to lateral forces like earthquakes or high winds, plans may specify interior shear walls to help stabilize the upper floor. The requirement is usually for 3 ply wood nailed (and possibly glued) to wall framing before wallboard is in stalled. Like exterior wall sheathing, interior plywood panels are subject to strict nailing requirements: typically 8-penny (8d) common nails spaced

6 inches apart at the edges and 12 inches in the field. Stricter requirements may include nails spaced 4 inches at the edges and 6 inches in the field, double studs at each vertical joint, and use of 1/2-inch plywood. Plywood should always be extended from soleplate to top plate.

Floor joists for the upper story are typically 2 by 10s or 2 by 12s to span the downstairs rooms. For especially long spans they may have to be spaced 12 inches on center. You may also have to install solid blocking between the joists at the ends, rather than nailing them to a continuous rim or band joist. Check local codes.

The technique for framing a roof is the same as for a ground-level addition, except that you will not have to tie it into an existing roof if your house is one story.

The common wall between the house and addition may have to be demolished or altered before the shell is completed so that framing for the first floor can be finished before the second floor is framed.

Completing the Shell

The height of the second floor makes it more difficult to install sheathing, siding, and windows. For safety and convenience, erect scaffolding soon after framing is completed. You can hire specialists to install it, rent scaffolding sections and erect them your self, or build your own scaffold on the site from 2-by lumber. It is sometimes possible to install windows and ply wood siding on framed walls before raising them.

Plywood panels for roof sheathing can be hoisted up through open rafters. If roof slope is greater than 5 in 12, provide roof jacks and ropes for safety while nailing off sheathing. You can reach most of the panels while standing on the ceiling joists. To make access easier for roofing, don't install the last sheathing panel at the ridge until you have completed all the roofing to that point.

Site-built scaffolding

Tying new walls to old roof; Roof-to-wall flashing, Header for opening to attic; Composition shingles; Header for opening to attic Roof sheathing, Wall sheathing.

Tying New Walls to Old Roof

Before framing second-story walls, re move existing gable wall, roofing, and roof sheathing to a point that lines up over the edge of the new floor framing. Then frame and raise the second-floor wall up against the old roof. Include a doorway for access to the attic. Cut a pair of 2 by 4 rafters and nail them to the attic side of the stud wall, up against the existing roof sheathing to support it. Nail sheathing into new rafters, either from above or with angle-framing clips from below. Cut and install 2 by 4 blocking between the studs to follow the roof line, just above the roofing. Install plywood sheathing on the exterior wall, butting the bottom against roof sheathing. If you are using plywood siding instead of sheathing, follow the next step before you install siding.

Re-shingle over the strip of bare roof sheathing before installing final siding, slipping step flashing against the wall sheathing at each course of roofing shingles. When you install siding, leave 1 inch of clearance between bottom of siding and roofing.

Building Stairs

Complex or ornate stairs are beyond the skills of most homeowners (and carpenters). But a conventional, straight-run stairway is something you may be able to take on if you are doing most of the construction your self. Build the stairs as soon as possible to make upstairs construction easier. Check local codes, which may include the following dimensions.

Maximum riser height: 7½ inches

Minimum tread width: 9 inches

Minimum distance between handrails: 30 inches

Height of handrail above tread nosing: 30-34 inches

Minimum headroom: 80 inches

Desirable headroom: 84 inches

Calculating Stair Dimensions

Measure vertical distance between floors. Allow for thickness of flooring materials so measurement is from top of finish floor to top of finish floor.

Divide distance by 7 to get approximate number of steps. Round answer up to next whole number and divide original measurement by that number. The answer will be 7 plus a fraction, which represents the height of each riser if stairs are all to be exactly the same.


108 inches ÷ 7 = 15.4 steps

108 inches ÷ 16 steps = 7.2 inches

Riser = 7 3/16-inches

To find tread width, subtract riser dimension from 17½-inches.


17 8/16-inches — 7 3/16-inches

Tread = 10 5/16-inches

Framing for staircase parallel to joists; Framing for staircase perpendicular to joists

Framing Stairwell and Installing Stairs

Frame opening in upper floor as shown above.

Cut stringers from straight 2 by 12 stock. To mark cuts for first stringer, set framing square so tongue (short leg) intersects top edge of board at riser dimension and blade does so at tread dimension. Scribe outline of square on board.

Slide square along board so it lines up the same way, intersecting original outline. Scribe again and continue until all steps are laid out.

Cut out stringers. To make bottom step the same height as the others, trim bottom of stringer by thickness of one tread. Then cut a notch for a 2 by 4 cleat.

Place stringer in stairwell and ad just it to fit. Then use the first stringer as a pattern to cut the second one. Stairways wider than 32 inches must have three stringers.

Install stringers, using ledger or joist hangers at top. If you install a stringer against a wall, nail a 2 by 4 to studs first to make a space for wall board and finish trim.

Cut risers from 1-by lumber or ¾-inch plywood and install them with glue and nails.

Cut treads from special tread stock with a rounded nose or from 1-inch plywood. Rip it to a width equal to tread dimension plus thickness of tread stock (usually 1¼-inches).

Attach treads with construction adhesive and ring-shank nails. Facenail treads into stringers and drive nails from back of risers into treads.

A handrail, which must be 30 to 33 inches above the tread nosings, can either be attached to the wall or supported by balusters and newel posts for open stairways. Allow at least 1 1/2 inches between the handrail and any wall. The ends of the hand rail should butt into the wall or be supported by a newel post.

Laying out a stair stringer: 2x 12 stringer stock; Notch for cleat; Framing square; Attaching handrail to newel post: Dowels; Attaching balusters: Dovetailed— cover edge of tread with molding; Stair framing: Joist hanger

Next: Second-Floor Additions

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