Building Truss Roofs The Safe Way, continued

Installing the Corner Jacks

At this point I check the position of the hip truss, bracing it in place so it's equidistant from the girder and the nearest jack. I also check that the location of the tail of the hip matches the dimension on the truss plan (Figure 10).

Figure 10. Before installing the corner jack trusses, it's important to recheck the position of the end of the girder truss and the hip truss.

Before I mark the locations of the corner jacks, I need to know how they were fabricated. If I'm unsure, I call the truss fabricator. Some corner jacks are fabricated to the correct length and are designed to be installed as delivered. The top and bottom chords have a square plumb cut, not a beveled plumb cut, and those chords contact the hip only at the corners of the cut. Because there is not much contact, I think this is a cheesy way of doing it. If the trusses are delivered this way, I first nail the square ends to the hip truss. At the top chord, I then add a 2-by block with a bevel cut at the top end of the top chord to make a stronger connection with the hip truss. If I have any say when the trusses are ordered, I ask the fabricator to leave them long. That way the top and bottom chords of the corner jacks have the same style plumb cut, but since they're long, they can be beveled on site. It's easiest if the length of the top and bottom chords corresponds to the long points of the 45-degree bevel cuts. It's important not to get the right-hand bevels and the left-hand bevels confused. To keep things organized, I pile the corner jacks near each hip by size. Corner jack layout. Marking the positions where the corner jacks fasten to the hip trusses takes two people. The first step is to consult the "length hip or valley per foot run" line on a framing-square table. I follow the line across the table until I reach the roof pitch for the trusses I've got. The table is in inches and tenths of an inch per foot of run. Since roof trusses are generally set on 2-foot centers, I just double the number. For example, if the trusses have a pitch of 10/12, the framing-square table would give me 19.70 inches per foot of run. For 2 feet of run (trusses on 2-foot centers), I would double the number, yielding 39.4 — about 39 3/8 inches. Starting from the point where the hip touches the corner jack, I would mark down the top chord of the hip 393/8 inches. Then I would place the end of my tape on that mark and measure the position for the next corner jack — again 39 3/8 inches. Since my starting point would be the outer face of the last jack truss, my marks would indicate the locations of the short sides of the corner jacks. I use the same stepping method to locate the attachment points for the corner jacks along the bottom chord of the hip truss. But for the bottom chords, there's no need to look up any numbers on the framing-square tables, since all 24-inch-on-center trusses with 45-degree hip trusses have the same bottom-chord offset — about 33 15/16 inches.

I like to nail the corner jacks through from the hip truss as well as toe-nail them (Figure 11). After the top chords are tacked together, I check whether the bottom chords of the corner jacks and the hip truss are flush. If they're off by more than 1/8 inch, I probably made a mistake with the layout.

Figure 11. It's handy to have two people to help position the corner jacks on the hip trusses. The corner jacks are nailed at the top through the top chord of the hip truss.

Next I tack the tails of the corner jacks, spaced 24 inches on-center, to the 1x3 brace (Figure 12). Once the corner jacks are nailed in, I eyeball up the hip truss to see that it's still straight. If necessary, I use 1x3s nailed back to either the jack trusses or the girder trusses to straighten it out until the sheathing goes on. If the truss plan requires hangers for the corner jacks, I install them after I've nailed the bottom chords of the corner jacks to the hip truss.

Figure 12. To hold the tails of the corner jacks at 24 inches on-center, they're nailed to the 1x3 that was used to position the hip.

Subfascia and Sheathing

Although some builders don't install subfascias, I find them especially useful when preassembling hip trusses on the ground, because they lock the tails of the end jacks and corner jacks in place (Figure 13). Before nailing a permanent subfascia to the tails of the jack trusses, I check that the girder trusses and hip trusses are straight and that the trusses all line up fairly well along their tails. (I usually don't install the subfascia on the girder-truss tail side until the assembly is on the roof.)

Figure 13. A subfascia ties the tails of the trusses together and stabilizes the truss assembly for lifting.

On the ground, I sheathe only the end slope of the roof, not the return facets of the hip trusses. The lowest row of sheathing is either omitted entirely or tacked lightly in place, so it can be lifted up for toe-nailing the trusses to the wall plates. Sometimes I start with 24-inch rips of sheathing along the eaves, because they're easier to handle two stories up (Figure 14).

Figure 14. To facilitate fastening the trusses to the wall plates, the bottom course of sheathing can either be nailed loosely or be omitted.

On one job, I rushed the assembly of one of the truss assemblies; once the crane dropped the assembly in place, it didn't sit evenly on the wall plates. We ended up having to pop off several sheets of sheathing to make corrections. The lesson: It's important to take the time during assembly to make sure the trusses are square and straight.

Call in the Crane

Before the crane arrives, we mark the truss layout on the wall plates, paying special attention to the girder-truss location. We attach the fabric lifting straps to the hip-truss assembly through the top corners of the girder trusses, where the hip trusses are attached. We protect the lifting straps from the sharp edges of the truss plates by first wrapping the area with rags. To balance the weight of the assembly so it flies level during the lift, I run additional straps out from the crane hook to the subfascia or tails of the jack trusses. To make it easy to balance the assembly, I like to use adjustable straps or cable come-alongs. (There isn't nearly as much force on these straps or cables as there is on the primary straps; they're only needed to balance the load.) I have the crane operator lift the load until the girders are about a foot off the ground, and then I adjust the come-alongs until the system looks level. I always assign one crew member to use a tag line to control the assembly until it comes within reach of the crew on the staging (Figure 15). Once it settles onto the marks for the girder trusses, we slide the girders back and forth until the overhang is equal on both sides. The hip trusses should lie flat on the plates and should extend out from the corners of the walls. Once we're satisfied, we release the crane hook and nail the girders to the plates.

Figure 15. The truss assembly is easier to control if it flies level. Secondary straps extending to the subfascia can be used to help level the trusses during the lift.

An added benefit of assembling a hip system on the ground is that it gives a stable point from which to brace back the rest of the roof trusses, making installation of the remaining trusses faster and safer.