One of the main structural functions of a ceiling frame is to tie together the outside walls of the building. When located under a pitched roof, the ceiling frame also resists the outward pressure placed on the walls by the roof rafters (fig. 1-60). The tops of interior partitions are fastened to the ceiling frame. In addition to supporting the attic area beneath the roof, the ceiling frame supports the weight of the finish ceiling materials, such as gypsum board or lath and plaster.
JOISTS
Joists are the most important framing members of the ceiling. Their size, spacing, and direction of travel are given on the floor plan. As mentioned earlier, the spacing between ceiling joists is usually 16 inches OC, although 24-inch spacing is also used. The size of a ceiling joist is determined by the weight it carries and the span it covers from wall to wall. Refer to the blueprints and specifications for size and OC spacing. Although it is more convenient to have all the joists running in the same direction, plans sometimes call for different sets of joists running at right angles to each other.
Interior Support
One end of a ceiling joist rests on an outside wall. The other end often overlaps an interior bearing partition or girder. The overlap should be at least 4 inches. ceiling joists are sometimes butted over the partition or girder. In this case, the joists must be cleated with a 3/4-inch-thick plywood board, 24 inches long, or an 18-gauge metal strap, 18 inches long.
Ceiling joists may also butt against the girder, supported by joist hangers in the same manner as floor joists.
Roof Rafters
Whenever possible, the ceiling joists should run in the same direction as the roof rafters. Nailing the outside end of each ceiling joist to the heel of the rafter as well as to the wall plates (fig. 1-61) strengthens the tie between the outside walls of the building.
A building may be designed so that the ceiling joists do not run parallel to the roof rafters. The rafters are therefore pushing out on walls not tied together by ceiling joists. In this case, 2 by 4 pieces are added to run in the same direction as the rafters, as shown in figure 1-62. The 2 by 4s should be nailed to the top of each ceiling joist with two 16d nails. The 2 by 4 pieces should be spaced no more than 4 feet apart, and the ends secured to the heels of the rafters or to blocking over the outside walls.
Roof Slope
When ceiling joists run in the same direction as the roof rafters, the outside ends must be cut to the slope of the roof. ceiling frames are sometimes constructed with stub joists (fig. 1-63). Stub joists are necessary when, in certain sections of the roof, rafters and ceiling joists do not run in the same direction. For example, a low-pitched hip roof requires stub joists in the hip section of the roof.
Ribbands and Strongbacks
Ceiling joists not supporting a floor above require no header joists or blocking. Without the additional header joists, however, ceiling joists may twist or bow at the centers of their span. To help prevent this, nail a 1 by 4 piece called a ribband at the center of the spans (fig. 1-64). The ribband is laid flat and fastened to the top of each joist with two 8d nails. The end of each ribband is secured to the outside walls of the building.
A more effective method of preventing twisting or bowing of the ceiling joists is to use a strongback. A strongback is made of 2 by 6 or 2 by 8 material nailed to the side of a 2 by 4 piece. The 2 by 4 piece is fastened with two 16d nails to the top of each ceiling joist, as shown in figure 1-65. The strongbacks are blocked up and supported over the outside walls and interior partitions. Each strongback holds a ceiling joist in line and also helps support the joist at the center of its span.
Layout
Ceiling joists should be placed directly above the studs when the spacing between the joists is the same as between the studs. This arrangement makes it easier to install pipes, flues, or ducts running up the wall and through the roof. However, for buildings with walls having double top plates, most building codes do not require ceiling joists to line up with the studs below.
If the joists are being placed directly above the studs, they follow the same layout as the studs below (fig. 1-66, view A). If the joist layout is different from that of the studs below (for example, if joists are laid out 24 inches OC over a 16 inch OC stud layout), mark the first joist at 23 1/4 inches and then at every 24 inches OC (fig. 1-66, view B).
It is a good practice to mark the positions of the roof rafters at the time the ceiling joists are being laid out. If the spacing between the ceiling joists is the same as between the roof railers, there will be a rafter next to every joist. Often, the joists are laid out 16 inches OC and the roof rafters 24 inches OC. Therefore, every other rafter can be placed next to a ceiling joist.
FRAME
All the joists for the ceiling frame should be cut to length before they are placed on top of the walls. On structures with pitched-roofs, the outside ends of the joists should also be trimmed for the roof slope. This angle must be cut on the crown (top) side of the joist. The prepared joists can then be handed up to the Builders working on top of the walls. The joists are spread in a flat position along the walls, close to where the y will be nailed. Figure 1-67 shows one procedure for constructing the ceiling frame. In this example, the joists lap over an interior partition. Refer to the figure as you study the following steps:
- Measure and mark for the ceiling joists.
- Install the ceiling joists on one side of the building.
- Install the ceiling joists on the opposite side of the building.
- Place backing on walls running parallel to the joists.
- Install 2 by 4 blocks flat between joists where needed to fasten the tops of inside walls running parallel to the joists.
- Cut and frame the attic scuttle.
- Place strongbacks at the center of the spans.
The tops of walls running in the same direction as the ceiling joists must be securely fastened to the ceiling frame. The method most often used is shown in figure 1-68. Blocks, 2 inches by 4 inches, spaced 32 inches OC, are laid flat over the top of the partition. The ends of each block are fastened to the joists with two 16d nails. Two 16d nails are also driven through each block into the top of the wall.
Applying Backing
Walls running in the same direction as the ceiling joists require backing. Figure 1-68 (insert) shows how backing is nailed to the top plates to provide a nailing surface for the edges of the finish ceiling material. Lumber used for backing usually has 2-inch nominal thickness, although l-inch boards are sometimes used.
Figure 1-68 shows backing placed on top of walls. The 2 by 4 pieces nailed to the exterior wall projects from one side of the wall. The interior wall requires a 2 by 6 or 2 by 8 piece extending from both sides of the wall. Backing is fastened to the top plates with 16d nails spaced 16 inches OC. Backing is also used where joists run at right angles to the partition (fig. 1-69).
Attic Scuttle
The scuttle is an opening framed in the ceiling to provide an entrance into the attic area. The size of the opening is decided by specification requirements and should be indicated in the blueprints. It must be large enough for a person to climb through easily.
The scuttle is framed in the same way as a floor opening. If the opening is no more than 3 feet square, it is not necessary to double the joists and headers. Scuttles must be placed away from the lower areas of a sloping roof. The opening may be covered by a piece of plywood resting on stops. The scuttle opening can be cut out after all the regular ceiling joists have been nailed in place.
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