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Building Truss Roofs The Safe Way, continued

Jacks and Kings to Open

Trusses with bottom chord overhangs or raised heels can be set up on the ground, because any unevenness can be corrected by blocking up the girder or jacks during assembly. But if we're assembling top-chord-overhang trusses, we have to block up the trusses on stacks of extra floor joists or wall plate stock, to keep the tails from digging into the ground. With the first girder truss upright at the chosen spot, I install the rest of the jack trusses to the first girder truss. I use a framing gun to nail off the jacks from behind, through the girder truss webs. When all the jack trusses are attached, I eyeball the bottom chord of the girder truss to see that it's still fairly straight, pushing it in and out until it's within 1/4 inch. Next, I mark out a 1x3 brace to match the layout of the jack trusses, and I nail it across the bottom chords of the jacks near their tails (Figure 5).

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Figure 5. Once the jack trusses are fastened to the girder, a long 1x3 holds the tails of the jacks in position.

Beveling the tail. Hip trusses are fabricated in one of two ways: The tails are either cut to length or left long. In both cases, the tails are cut square by the fabricator, so they need to be beveled at 45 degrees in the field. If the fabricator has left the tails long, I always calculate the desired length of the hip's bottom chord. This chord is the hypotenuse of a right triangle; the other two sides of the triangle are the bottom chord of the abutting jack truss and the fascia length between the jack truss and the hip. I use the Pythagorean theorem (a2 + b2 = c2) to calculate the length of the hypotenuse, which is the measurement to the tip of the trimmed tail. If the hip truss turns out to be too long, I trim it as necessary. The square corners of the 2-by web are designed to nestle into the corner between the girder truss and the first jack (Figure 6). I insert the hip into the hanger, and holding the truss at roughly a 45-degree angle, I nail it home at the top, through both the girder and the jack. Again, I drive only a couple of nails through the hanger to hold the base of the hip; most of the hanger nails are installed after the second girder truss layer is assembled. To hold the tail of the hip in position, I tack a 1x3 across the tails of a couple of adjacent jack trusses.

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Figure 6. The hip truss is installed tight to the corner where the outside jack truss meets the girder truss. Although the vertical web in the corner should not be trimmed with a bevel, the tail of the hip truss needs to be trimmed on site with two 45-degree bevels.

Doubling the Girder

Before installing the second girder truss, I need to straighten the first girder, so that I don't inadvertently brace the girder while it's crooked. I eyeball the top and bottom chords of the girder truss and then either lift or lower the ends of the jack trusses and hip trusses until the girder is fairly straight. Next, I square up the position of the jacks, using two tape measures to pull diagonals (Figure 7). A 1x3 diagonal brace across the top of the jacks keeps things in place.

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Figure 7. The jack trusses are squared to the girder truss by checking the diagonals across the top chords.

Now it's time to tack the second girder truss to the first, using just enough nails to close up any gaps between the top and bottom chords. One more time, I check to see if the girder trusses are straight. I use 1x3s and scrap blocks kicked into the earth to brace the bottom chords straight to a string (Figure 8). It's inevitable that the steel truss plates will hold the chords somewhat apart when the two girders are sandwiched together, so I ignore these slight bulges when lining the girders to the string.

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Figure 8. The second girder truss is paired with the first after the jack trusses and hip trusses have been installed. Before the two girder trusses are nailed together, they have to be straightened to a string. Diagonal 1x3s and kickers driven into the dirt are used to persuade the girder into position.

Double-check the fastener schedule. Before I start nailing off the girders, I always reread the truss documentation. Roof trusses are an engineered system; if the documents specify nailing or bolting "by others," I know that means me. The girders on the job shown in the photos required spikes 4 inches on-center staggered along the top and bottom chords, and 9 inches on-center staggered along all the webs. Once the girders are nailed together, I finish nailing off the hangers. The universal hangers for the hip trusses and adjacent jack trusses usually have slots rather than round holes for nailing, making it possible to angle the nails so you can swing a hammer in the tight quarters (Figure 9). A pneumatic palm nailer is handy in these locations.

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Figure 9. Once the two girder trusses are nailed together, the rest of the hanger nails can be installed.

If the house has only one story, at this stage I'd call the hip system complete enough to raise. Installing the corner jacks to the hip trusses is fairly easy when you can work off a ladder, and the same goes for sheathing.

Keeping things square. After nailing the girders together, I recheck the strings and make adjustments as necessary. I verify that the jack trusses are square by checking the diagonals from the end of the bottom chord of the outermost jack truss where it meets the girder truss to the tail of the opposite outer jack truss. Once everything looks good, I nail a 1x3 diagonally across the top of the bottom chords. To prepare for sheathing, I replace the diagonal strap that I had nailed across the top chords of the jack trusses with one on the underside of the top chords.