Except for slab foundations, most room additions have a floor structure consisting of a subfloor supported by joists, which rest directly on the mudsill or on a short kneewall built on top of the mudsill. For long spans, an intermediate girder supports the joists in the middle.
When building floors for a room addition, match the level of the new floor with that of the old unless your plans call for different levels. If the existing floor is not level, decide which point along the house floor to use as a reference for leveling the new floor. If the two floors will be connected only at a doorway, use the center of the doorway for the common leveling point. If the two floors are connected by two separate doors or by a wide opening, adjust the addition floor framing along its entire length to match the two floors.
Your preparations should include arranging for the delivery of lumber and plywood and for the following items as needed.
- For crawl space foundations with minimum access, have floor insulation on hand to install before laying subfloor.
- Have foundation vents on hand so you can cut openings as you frame.
- If you have a large girder, such as a 4 by 12 or steel beam, arrange for extra help to lift it.
- Have a bundle of shims handy. A girder over a basement may require a steel column or Lally column (steel filled with concrete) for support. Consult an ironworks early to find out what types are available and how long it will take to have one made. Unless it's adjustable, be very certain about your dimensions when you order it.
If there will be access to the new crawl space, plan your framing accordingly. Most additions provide access through a scuttle in a closet floor, existing basement wall, or exterior foundation wall.
If the ground level of the crawl space is too high for code-required clearances (12 inches to bottom of girder; 18 inches to bottom of joists), and you don't want to excavate any deeper, an alternative floor system increases crawl space clearance by eliminating joists. It uses 1 3/8-inch plywood or 2-by tongue-and-groove decking for subfloor. These materials can span longer distances than nor mal 3 subflooring, so they rest directly on girders spaced 4 feet apart rather than on joists. This system requires more girders and piers, but less excavation and no joists.
Anatomy of a wood-frame floor.