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Adding Space and Framing Floors
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Any project that involves adding space, altering existing space, or putting a hole in the wall, floor, or roof will probably require some framing. Framing is a satisfying and dramatic stage of any project. It is fairly easy to do, although certain structural and code considerations must be satisfied.
Never undertake a project involving structural changes or additions until you have a proper permit and understand the function of each framing member involved. Otherwise, you may weaken the house without knowing it. You must also arrange for an inspection before covering up any of the new framing.
Types of framing. Some homes are framed with heavy timbers, but most wood-framed homes use 2-by lumber. Old houses may have a balloon framing system, in which long studs extend from the foundation to the roof. But by far the most common system is platform framing. This system, introduced at the beginning of the century, uses short wall studs that extend between floors.
Joists are the basic framing members for floors. Joists on the first floor bear on the foundation mudsills or short cripple walls, with large girders or beams supporting them at midpoint. Joists for upper floors rest on bearing walls. Joists are doubled wherever they carry concentrated loads, such as a bearing wall or bathtub.
The joists are set on edge. Rim joists (also called band joists) at their ends prevent them from turning over. They are strengthened by putting a line of blocking or cross-bridging over girders and every 8 feet along open spans. Sometimes blocking is used instead of, or in addition to, rim joists.
Never undertake a project involving structural changes or additions until you have a proper permit and understand the function of each framing member involved. Otherwise, you may weaken the house without knowing it. You must also arrange for an inspection before covering up any of the new framing.
Types of framing. Some homes are framed with heavy timbers, but most wood-framed homes use 2-by lumber. Old houses may have a balloon framing system, in which long studs extend from the foundation to the roof. But by far the most common system is platform framing. This system, introduced at the beginning of the century, uses short wall studs that extend between floors.
Framing Floors
Joists are the basic framing members for floors. Joists on the first floor bear on the foundation mudsills or short cripple walls, with large girders or beams supporting them at midpoint. Joists for upper floors rest on bearing walls. Joists are doubled wherever they carry concentrated loads, such as a bearing wall or bathtub.
The joists are set on edge. Rim joists (also called band joists) at their ends prevent them from turning over. They are strengthened by putting a line of blocking or cross-bridging over girders and every 8 feet along open spans. Sometimes blocking is used instead of, or in addition to, rim joists.
Sizing girders. These members must be sized according to the load they carry. Codes have formulas for sizes. A girder can be solid lumber or several 2-inch-wide boards nailed together: a built-up girder. Steel girders, such as I beams, will span long distances.
In all cases the posts are spaced according to the girder span and they are always supported on foundation footings.
In all cases the posts are spaced according to the girder span and they are always supported on foundation footings.
Installing joists. Codes specify size and spacing for joists. You can increase the span if you space the joists 12 inches instead of 16 inches on center. Select the straightest joists for the outside edges. Measure both diagonals, check for square, make any adjustments, then toenail joists to the mudsill or cripple wall.
Lay out joist spacings on the rim joists and across the top of the girders, starting from the same end. Facenail each joist to the rim joist and toenail it to the bearing surface. Install joists with the crown (or curved edge) up.
Splicing joists. Long spans require 2 joists, which must be joined at a girder. Connect them with metal braces on each side or lap them, extending past the girder an inch or two. If they overlap, they will not be continuous throughout the floor system, which must be taken into account if you install a plywood subfloor. An alternative attaching method is to use joist hangars.
Extending an existing floor. If the floor of a room addition will be at the same level as the existing floor and if you can install the new joists so that they run in the same direction as the old ones, use the same size lumber and rest the joists on the mudsill or wall plate that supports the existing floor.
Remove the rim joist, lap the new joists against the old, and install blocking. If the new subfloor or joists are a different dimension from the existing floor, attach a ledger and install the new joists with joist hangers taking the difference into account.
Installing blocking. Cut solid blocking out of joist material, measuring for each piece at the rim joists rather than where you will be installing it. Stagger the blocks so that you will be able to nail them at each end.
An alternative is to use metal cross-bridging. Attach the top, but wait until the subfloor is installed before nailing the bottom of the metal cross-bridging in place.
Lay out joist spacings on the rim joists and across the top of the girders, starting from the same end. Facenail each joist to the rim joist and toenail it to the bearing surface. Install joists with the crown (or curved edge) up.
Splicing joists. Long spans require 2 joists, which must be joined at a girder. Connect them with metal braces on each side or lap them, extending past the girder an inch or two. If they overlap, they will not be continuous throughout the floor system, which must be taken into account if you install a plywood subfloor. An alternative attaching method is to use joist hangars.
Extending an existing floor. If the floor of a room addition will be at the same level as the existing floor and if you can install the new joists so that they run in the same direction as the old ones, use the same size lumber and rest the joists on the mudsill or wall plate that supports the existing floor.
Remove the rim joist, lap the new joists against the old, and install blocking. If the new subfloor or joists are a different dimension from the existing floor, attach a ledger and install the new joists with joist hangers taking the difference into account.
Installing blocking. Cut solid blocking out of joist material, measuring for each piece at the rim joists rather than where you will be installing it. Stagger the blocks so that you will be able to nail them at each end.
An alternative is to use metal cross-bridging. Attach the top, but wait until the subfloor is installed before nailing the bottom of the metal cross-bridging in place.
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