It’s common in many regions to build houses directly on a concrete slab. When this is the case, wall building can begin once the sill plates are down. Other builders use a system of posts and girders to support joists so that a floor can be nailed to them. Still others use manufactured joists (called I-joists) that span a basement from sill plate to sill plate without any interior support, leaving a room large enough for a dance hall. [7]
length of the building. Houses frequently have an exterior stemwall foundation with girders supported by piers set on concrete footings (see the drawing above).
Posts in a basement will be quite long, while posts in a crawl space will be shorter—only about 1 ft. to 2 ft. But to keep wood dry and away from termites, make sure that no wood is within 6 in. of the ground. To determine the exact length of each post, pull a chalkline directly over the tops of the concrete piers that will support them, from foundation wall to foundation wall (see the top drawing on p. 91). Then place a scrap piece of girder stock (like a 4×6) on a pier. The distance between the string and the top of the girder stock is the length of the post for that pier. Make a list and note the length of every post before beginning to cut them.
Posts are usually cut from 4×4 stock. In some areas, they may need to be pressure treated. They can be cut to length with a circular saw or with a chopsaw. Leave the string in place to help align the posts as you nail them to the top of the piers. Toenail three 16d nails (two on one side, one on the opposite side)—or four 8d nails—down through the post into the wooden block on top of the pier (for more on toenailing, see the sidebar below).
Once the posts are nailed in place, it’s time to cut and nail on girders. Again, use straight stock that isn’t twisted. Pier posts are often 6 ft. apart, so cut the girders to break in the middle of a post, which will ensure solid bearing for all girders. Secure the girders to the posts with three 16d or four 8d nails. Take some simple steps to strengthen the
girder frame structurally, especially if you live in earthquake country: If the posts are over 3 ft. long, nail a 1x brace diagonally (45°) from the bottom of the post to the girder with five 8d nails in each end (see the bottom drawing on the facing page). Unite the joints with a metal strap or a plywood gusset. I prefer the gusset because it ties girder to girder and girders to the post.
INSTALLING JOISTS
Joists are placed on edge across the sill plates to provide support and a nailing surface for the subfloor and a platform for the walls (see the drawing on p. 92). The joists need to be strong enough to support your grand piano without having it wind up in the basement.
Joist systems are made from either standard 2x lumber or from manufactured joists. The weight these members can
