Engineering and Construction

Although complex in its details, the platform frame is simple in concept  (Figure 5.3). A floor  platform  is  built. Loadbearing walls are erected  upon it. A second-floor platform is  built upon these walls and a second  set of walls upon this platform. The  attic and roof are then built upon  the second set of walls. There are,  of course, many variations: A concrete slab that lies directly on the ground is sometimes substituted for  the ground-floor platform; a building may be one or three stories tall  instead of two; and several types of  roofs are frequently built that do not  incorporate attics. The essentials,  however, remain: A floor  platform  is completed at each level, and the  walls bear upon the platform rather  than directly upon the walls of the  story below.

Figure 5.3 The concept of platform framing, shown in cross section, reading from left to right: A foundation wall is constructed. A ground-fl oor platform is framed and sheathed. Ground-fl oor wall frames are assembled horizontally on the platform, then tilted into their fi nal positions. A second-fl  oor platform is constructed on top of the ground-floor walls, and the process of wall construction is repeated. The attic floor and roof are added.

The advantages of the platform  frame over the balloon frame are several: It uses short, easily handled lengths of lumber for the wall framing. Its vertical hollow spaces are  automatically Þ restopped by the  platform framing at each floor.  Its  platforms are convenient working  surfaces for the carpenters who build  the frame. The major disadvantage  of the platform frame is that each  platform constitutes a thick layer of
wood whose grain runs horizontally.  This leads inevitably to a relatively  large amount of vertical shrinkage  in the frame as excess moisture dries  from the wood, which can cause dis- tress in the exterior and interior finish surfaces.

A conventional platform frame is made entirely of nominal 2-inch  members, which are actually 1½  inches (38 mm) in thickness. These are ordered and delivered cut to  the nearest 2-foot (600-mm) length,  then measured and sawn to exact  length on the building site. All  connections are made with nails,  using either face nailing, end nailing, or toe nailing as  required by the characteristics of  each joint. Nails are driven either by  hammer or nail gun. In either case,  the connection is quickly made be- cause the nails are installed without  drilling holes or otherwise preparing the joint.

Each plane of structure in a  platform frame is made by aligning a number of pieces of framing  lumber parallel to one another at specified intervals, nailing these to crosspieces at either end to main- tain their spacing and flatness, and then covering the plane of framing with  sheathing, a facing layer of  boards or panels that join and stabilize the pieces into a single struc- tural unit, ready for the application of finish materials inside and out (Figure 5.4). In a floor structure, the parallel pieces are the floor  joists,  and the crosspieces at the ends of the joists are called headers, rim joists,  or  band joists. The sheathing on a floor is known as the  subfl  oor. In a wall structure, the parallel pieces  are the  studs, the crosspiece at the  bottom of the wall is the sole plate or  bottom plate, and the crosspiece at the  top (which is doubled for strength if  the wall bears a load from above) is  called the top plate. In a sloping roof,  the rafters are headed off by the top  plates at the lower edge of the roof  and by the ridge board at the peak.

Figure 5.4 The basic components of platform frame construction. (a) Walls are framed with repetitive vertical studs that are connected at the top and bottom by horizontal plates. (b) Floors are framed with repetitive joists that are connected at their ends by headers. (c) Roofs are framed with rafters. Every surface is sheathed with wood panels of either plywood (as shown) or, more commonly, less expensive OSB.

Openings are required in all  these planes of structure: for windows  and doors in the walls; for stairs and  chimneys in the floors; and for chimneys, skylights, and dormers in the  roofs. In each case, these are made  by heading off the opening: Open- ings in  floors are framed with headers and trimmers, which
usually must be doubled to support the higher loads placed on them by  the presence of the opening. In walls,  sills head off the bottoms of window  and door openings, while trimmer  studs on the sides provide support to loadbearing headers across the tops.

Sheathing, a layer of wood panels nailed over the outside face of  the framing, is a key component of  platform framing. The end nails that  connect the plates to the studs have  little holding power against uplift  of the roof by wind, but the sheath- ing connects the frame into a single  strong unit from foundation to roof.  The rectilinear geometry of the parallel framing members has no useful  resistance to wracking by lateral forces  such as wind, but rigid sheathing  panels brace the building effectively  against these forces. Sheathing  also furnishes a surface to which  shingles, boards, and ß  ooring  are  nailed for finish surfaces. In build- ings constructed without sheathing,  or with sheathing materials that are  too weak to brace the frame, such  as insulating plastic foam, diagonal  bracing must be applied to the wall  framing to impart lateral stability.