CAPÍTULO 5: EVALUACIÓN ECONÓMICA
5.1. CONSIDERACIONES PARA UTILIZAR
Purlin installed plumb 2x4 post notched
to support purlin
Rafter
2x4 kicker
Purlin i nstalled square to rafters
Block nailed to kicker
I nterior bearing wall Joists parallel to rafters I nterior bearing wall
Rafter
1 x6 rafter tie
Collar tie
Five 8d nails in each end
Joists at right angle to rafters
with five 8d nails at each end. Rafter ties form the bottom chord of the truss triangle, preventing the load on the roof from pushing down, bowing walls outward and creating a sag in the ridge. Rafter ties are usually made from 1x4 or 1x6 stock and are in stalled every 4 ft. If the roof is going to be covered with heavy tiles, 2x ties may be called for.
Purlins are required when rafter spans are long. They should be placed as close as possible to the middle of the rafter span and run the length of the building. Purlins can be toenailed to the rafters ei ther plumb or square. If there's a load-bearing inte rior wall beneath the center of the rafter span, in stall the purl in plumb, directly over the wall, and support it with several 2x4 posts that bear on the double top plate of the wall. The 2x4s are notched so that they support the purlin and can be nailed to the sides of the rafters.
If there isn't a wall beneath the center of the span, toenail the purlin square to the rafters and in stall 2x4 kickers up from the nearest load-bearing parallel wall at an angle not exceeding 45°. A block nailed to each kicker below the purlin will help hold it in place. Kickers and posts are typically installed
every 4 ft. Larger purlins, such as 2x1 2s, require fewer supports. When several lengths are needed to form the purlin for a long roof, lap them at least 4 ft. and nail them together with five 1 6d nails.
In some parts of the country, rafters must be tied to the top plates or blocking, or even down to the studs, with framing anchors or hurricane ties for added protection against earthquakes or high winds. Check your local code.
Gable-end studs
Gable ends are filled in with gable studs spaced at 1 6 in. or 24 in. on center. If a gable vent is called for on the plans, place the two center studs 14 in. apart to allow room for the vent directly under the ridge. Measure the length of these studs to the long point, then calculate the common difference of the gable studs, that is, the difference in length between suc cessive studs. Once you know the common differ ence, you can quickly determine the lengths of the remaining studs without measuring each one sepa rately. A pocket calculator makes this task easy.
To calculate the common difference, divide the rise by the run and then multiply the result by the
To lay out gable studs, lay the stock flush against a bottom plate. Measure and mark the longest studs (one on each side of a gable end), then subtract the common difference and mark the next set, and so on. The studs then need to be cut at the angle of the roof pitch.
on-center spacing. For a 4-in- 1 2 roof with gable studs spaced 16 in. o.c., the equation goes like this: 4 + 12 x 16
=
5.33. In other words, the difference in length between successive studs is 5.33 in., or about 5% in. Another way to calculate the common dif ference is to divide the rise by three and add the re sult and the rise together (4+
3=
1 .33+
4=
5.33). When gable studs are 24 in. O.c., double the rise to find the common difference.Cut all the gable studs at the same time. If there are two gable ends, two sets of gable studs will be needed to fill them in, so cut four of each length. This is a good time to use up some 2x4 scrap. Lay the pieces out on edge, flush on one end. Mark the length of the first set, then mark each successive set by subtracting the common difference. Don't bother cutting any studs shorter than 1 6 in.
For the angle cuts, set your saw at the angle of the roof pitch ( 18Y2° for a 4-in- 1 2 roof). Nail the gable studs plumb using your eye as a gauge. There is no need to lay out the double top plate or align the gable studs with the wall studs below. Be careful not to force a crown into the end rafters when nailing in the gable studs.
When installing gable-end studs, nail through the rafter into the stud, rather than vice versa, to keep from putting a crown in the rafter. When you're working along the gable end, be sure to keep your body well balanced at all times.
Barge rafters
The next step is to install the barge rafters, if the plans call for them. These are rafters that hang out side the building line, creating an overhang on the gable end. Barge rafters are usually cut from the same size material as the fascia (see pp. 1 5 6- 1 58), which they tie into at the corners. A barge rafter doesn't require a bird's-mouth cut because it doesn't land on the double top plate. For this reason it has