The actual process of framing an addition -- nailing together and erecting framing members -- isn't particularly difficult. However, proper framing that results in a safe and stable structure requires some carpentry skills, the ability to understand basic math and engineering concepts, and knowledge of and adherence to local building codes. Framing a basic addition may be within the grasp of an experienced do-it-yourselfer, but the involvement of professional contractors is always a good idea.
Building Codes and Permits
Any home addition project must conform to local building and fire codes. You'll be required to obtain a building permit from your local building authority. Framing codes are intended to ensure the structural stability of the addition, as well as such details as the size and the placement of windows and doors, and the height of ceilings. The terms of permit generally require an inspection of the framing during construction.
The tools you'll need for a framing project include measuring tools such as a framing square, a speed square, a carpenter's level, a plumb bob, a chalkline and a measuring tape. You can cut framing members with a circular saw, and a reciprocating saw is best for cutting openings in sheathing. A heavy framing hammer is adequate for most nailing tasks, but a compressor-powered pneumatic framing nailer will be a big help on large projects.
An addition's framing begins with the subfloor. Sill plates are horizontal boards, usually two-by-six or two-by-eight pressure-treated lumber, as specified by local codes, that are bolted directly to the top of the foundation. Rim joists sit vertically on the sill plates around the perimeter of the addition, aligned with and nailed to the outside edge of the sills. Floor joists, usually spaced 16 inches on center, span the width of the addition and are nailed to the rim joists, either directly or via metal joist hangers. Depending on the length of the joist span, the floor may require support from intermediate beams and support posts. After the joists are in place, the subfloor is completed by plywood decking nailed on top of the joists.
Exterior Wall Framing and Sheathing
Exterior wall framing usually consists of two-by-four studs spaced vertically, 16 inches apart, between horizontal two-by-four bottom and top plates. A second top plate laid on top of the first strengthens the top of the wall, and overlapping the top plates at corners ties adjacent walls together. Plywood or oriented strand board sheathing covers the outside surface of the wall framing. The easiest way to frame walls is to lay them out and nail them together on a horizontal surface, then raise them into place after they're constructed.
Door and Window Framing
Window and door openings in walls require additional framing to replace the support of the studs removed to make the opening. Full-height studs, called king studs define the outside of the opening and extend from bottom plate to top plate. Trimmer studs or jack studs attach to the king studs and stretch from bottom plate to window header. A horizontal header, the size of which will be specified by local codes, rests on top of the trimmer studs and spans the top of the opening. Partial-height cripple studs fill in the spaces above and below the opening.
Basic gable roof framing consists of rafters that slope upward from the top plates of the exterior walls to a ridge board at the peak of the roof, and horizontal ceiling joists tie the exterior walls together. In most modern construction, however, roofs are framed with preassembled engineered trusses, which are designed to provide maximum structural stability while using the minimum amount of lumber. Roof framing, particularly where an addition's roof ties into an existing roof, is complex, and a framing plan is best designed by a structural engineer.
Partition Wall Framing
Interior partition walls are framed much like exterior walls, with two-by-four studs spaced 16 inches on center. Non-load-bearing walls need not be as robust as load-bearing walls, though, and they usually can be framed with single top plates and smaller headers over door openings. You may need to frame partition walls in place, since there may not be room to raise the walls into place after you've framed them on the floor.
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