Publications Article

Metal Plate Connected Wood Trusses For Residential Roofs

© 1996 Stephen Smulski, Ph.D.
Wood Science Specialists Inc.
Shutesbury, Massachusetts
413 259-1661 fax-1610

Name the roof -gable, hip, saltbox, mansard, gambrel- and it can be framed with metal plate connected wood trusses. Precision-made from dimension lumber and metal connector plates, pre-fabricated trusses have revolutionized residential roof framing over the last three decades. Today, over 75 percent of all new homes are constructed with trusses. Lightweight and needing no on-site assembly, trusses give builders a bigger bang for their buck. Truss-framed roofs can be erected faster and with less skilled labor than stick-built roofs. Often, trusses go up and sheathing down on the same day, so closure against the weather comes sooner. Trusses' long, clear spans offer greater flexibility with floor plans. And since interior bearing walls aren't needed, their expensive underpinnings aren't needed either. Highly efficient in their usage of lumber, trusses help conserve forest resources. Most often made of 2x4s and spaced 24 in. o.c., a truss-framed roof uses less wood than one stick-built from 2x6 or 2x8 rafters and joists 16 in. o.c.

Shapes and sizes
The outline traced by a truss' chords determines its shape. Triangular, mono pitch, dual pitch, scissors, stub, and hip shapes are common. Trusses with the same shape are distinguished by the pattern of the webs inside. King post, Fan, Fink, and Howe trusses, for example, are all isosceles triangles, yet each has its own signature web layout.

Residential roof trusses range from 15 to 50 feet long, and from 5 to 15 feet high. Roof pitch and span plus cantilever, if any, determine truss height and length. Tall trusses are sometimes made as two separate trusses so that they can be shipped over the road. Called piggyback trusses, the two parts are joined on site with plywood or metal gusset plates during erection.

Special trusses
Where common trusses can't be used or aren't appropriate, special trusses fill the bill. Master and split truss sets, for example, are used to frame roof penetrations that exceed truss spacing, like chimneys and skylights. Truncated in mid-span to form the opening, split trusses are then headed-off to a full-span master truss on either side.

With all webs oriented vertically, gable end trusses are a unique breed. Riding atop a building's endwalls, they're usually supported along their entire length, functioning more like a wall than a truss. Shorter than the last common truss by the width of its top chord, a drop top gable end truss makes ladder-framing wide overhangs a snap. Other gable end options include drop bottom chord trusses for use with brick veneer and trusses with framed openings to accept a triangular louver.

"But you don't have any attic storage space with trusses" is an often-heard, but unfounded concern. Perfect for steeper roofs and garages, attic frame trusses are designed with a room-size central opening for use as storage or living space.

Girder trusses consist of two or three trusses factory- or field-fastened side-by-side. In L-, T-, H- and U-shaped buildings, girder trusses eliminate the need for a bearing wall where an ell joins the main building. Here one end of each main building common truss is clipped flush and hung with a metal hanger from the bottom chord of the girder truss. A series of step down valley trusses installed on top of the common trusses extends the ell roof back to the main roof. Hip roofs are framed in a similar fashion with common, girder, and step down hip and jack trusses.

Scissors and vaulted trusses give instant cathedral ceilings. With single and double cantilever trusses, porches, entrance roofs, and wide overhangs are simply extensions of the truss. Common trusses are fabricated with a variable top chord overhang and a variety of soffit return details for box and closed cornices. And where thick ceiling insulation that extends to the outside of the top plate and an air space above are needed, raised heel trusses do the trick. They also allow steep roofs with wide overhangs that don't interfere with doors and windows.

How trusses work
Triangles are naturally rigid geometric shapes that resist being distorted when pushed on. In the upright position, a truss is rigid for the same reason. Regardless of its overall shape, all its chords and webs form triangles, or triangulate. Stick-built roofs operate on the same principle, with rafters, ceiling joists, and collar ties forming the triangles.

Under the weight of sheathing and roofing, a roof truss as a whole is stressed in bending. Its chords and webs, however, are stressed principally in either tension or compression. Top chords, which are in compression, push out at the heel and down at the peak. The bottom chord, firmly fastened to the top chords, is stretched in tension to resist the outward thrust. The result is a stable, self-balancing structure.

One important difference between stick-built and truss-framed roofs is that ceiling joists rarely span the width of the building. Instead, they bear on interior partitions, as well as on exterior walls. Trusses are almost always designed to bear only on exterior walls, with the webs connecting the top and bottom chords providing intermediate support. That's why webs, depending on their location, are stressed in either tension or compression.

Specifying trusses
With so many choices, specifying trusses might seem daunting. Not so. A basic specification begins with a list of each type of truss needed. Then, for each type, you state the number needed; span; roof pitch; top chord overhang, end cut, and soffit return details; gable end options; and any special loadings (slate roofing or HVAC equipment, for example). The specs for my 22-foot-square garage read: 10 common trusses; 2 gable end trusses; 8/12 pitch; 22-0-0 span; 12 in. overhang, plumb cut, no soffit return. Many of the larger truss makers carry an inventory of stock trusses in the sizes and styles most popular in their area. With no production lead time, you can order trusses on Monday and be closed in by Friday.

Specifying trusses for complex roofs is even easier. Take the framing plan to your building materials supplier or to one of the nation's 1500 or so truss fabricators, and they'll do the take-off for you. The system works like this. Most truss fabricators use the machinery, plates, and engineering services of one of about 8 metal connector plate manufacturers. Plate makers actually engineer and design the trusses, while truss fabricators assemble and sell them.

Design
Once the specs are set, trusses are designed by computer, with building-code-required roof, ceiling, wind and snow loads, as well as any special loading conditions, taken into account. An engineering drawing detailing the forces that develop in each chord and web under the design loads; lumber species, size and grade for each chord and web; gauge, size, and orientation for each metal connector plate; truss dimensions and pitch; and the location of permanent bracing is generated for each truss type. Engineering drawings are supplied to the truss fabricator, who passes them on to the erection contractor. If you're the erection contractor, and you didn't get them, be sure to ask. A cutting list for chords and webs, and a shop drawing to guide factory assembly of the members, are also prepared.

Fabrication
Trusses are made mostly from southern pine, Douglas fir, and the woods of the spruce-pine-fir group: eastern and sitka spruce; lodgepole, red, and jack pine; and western and balsam fir. With each of the two common truss assembly methods, chords and webs are first crosscut to the precise length and angles needed with computer-controlled saws. The kind, size, and grade of lumber for each chord and web on the cutting list is based on how great a force each has to resist while under load. Highly stressed as a rule, chords are made from machine stress-rated lumber that has been nondestructively tested to ensure performance. Usually subjected to lower stress, webs are more likely to be No. 2, No. 3, or even Stud grade.

A truss' integrity depends on the integrity of its metal connector plates. Stamped from 16-, 18-, and 20-gauge structural steel coated with zinc, plates have numerous integral teeth 5/16 in. to 3/8 in. long. With about 8 teeth per square inch, plates are sized during design according to the level of stress they have to transfer between members. At panel points in tension members, plates may be stressed in a combination of compression and shear, or tension and shear in the plane of the plate. Here loads are transferred between members from wood-to-metal-to-wood. In top chords and other members stressed in compression, loads are transmitted across joints primarily by wood-to-wood bearing.

In one assembly method, clamping pedestals with electromagnetic bases are arranged in the shape of the truss on a floor of steel plates, with one pedestal at each panel point. After chords and webs are laid on the pedestals, their ends are tightly butted and clamped in place. Connector plates are then carefully positioned on both faces of the joint. A hydraulic C-clamp suspended from a gantry is slipped into the top of the pedestal, squeezing the teeth of both plates into the wood simultaneously.

With the other technique, trusses are assembled inside jigs fixed to metal or wood tables. Chords and webs are placed in the jig, then panel points are lifted, and a connector plate slipped underneath. Another plate is set on the exposed face, and its leading edge lightly hammered in place. Both plates are pressed into the wood at once by a mechanized roller that travels the length of the table. The truss is then passed between nip rollers for a finish pressing.

To complete the process, and to assist builders during erection, brightly colored Caution!, Warning!, and Danger! tags are affixed to trusses at critical locations like cantilever bearing points and permanent lateral bracing sites. Don't ignore them.

Completed trusses are stacked, banded and stored in the truss yard, either vertically or horizontally. When stored on their side, trusses are elevated off the ground on stringers spaced to minimize lateral bending.

Delivery and handling
Trusses are transported by truck either lying on their side or cradled vertically in a special frame. Ideally, they're unloaded at the jobsite with a forklift or crane, but the reality is that most are gingerly dumped on level ground. Trusses should always be elevated off the ground on stringers, and protected from the weather under a loosely draped tarp. When unloaded on their top chord, the bundle should be braced on both sides to prevent it from falling over, and especially to keep trusses from toppling when the band is broken.

Erecting trusses
Depending on their span and the height of the building, trusses are erected either by hand or by crane, and occasionally, by forklift. With one-story buildings, trusses under 30 feet can usually be raised manually, while longer trusses should be hoisted by crane. A crane is a must for buildings over one-story, regardless of truss length. Whether carried or hoisted, trusses should always be held in their upright position. When held horizontally, lateral flexing and bouncing can overstress the connections, causing plates to loosen or pop out. Long trusses are especially vulnerable.

A hand erection sequence on a one-story building might go something like this. With its peak pointing down, the heels of a gable end truss are carefully positioned on top of the sidewalls. Then, using Y-shaped lifting poles, the truss is rotated until it's upright. To prevent damage during lifting, two poles are used. Each is positioned at the panel point closest to the quarter-points of the span. If only one pole is used, it's placed at the peak. After making sure that the overhang is correct, the bottom chord is toe-nailed to the endwall top plate with 16d nails.

It's essential that the gable end truss then be securely braced to the ground, since all other trusses will be braced against it. The common trusses are then raised sequentially in the same manner, with each secured in place with temporary lateral bracing that traces back to the gable end. It's important that the 24 in. o.c. spacing be maintained at the heel and peak of each truss, and that each goes up square and plumb.

Good rigging practice is essential in preventing damage when setting trusses by crane. Trusses up to 20 feet long can usually be lifted with a cable looped around the top chord at mid-span. A tag line lashed to one heel is used to guide the truss into position. Trusses up to 40 feet are typically hoisted at two symmetric lifting points separated by one-half of the span. Again, cable ends are secured around the top chord. A tag line is needed as well. Lifting 40- to 50-foot long trusses without lateral flexing generally requires a spreader bar with three cables. Typically one-half to two-thirds of the truss' length, the bar is centered over the truss. Cable ends looped around the top chord should toe-in slightly. A tag line attached to both heels increases control. Never attempt to lift a truss by its webs.

Once in place, trusses are customarily toe-nailed to the top of the wall with 16d nails through slots in the heel plates. While adequate in most instances, toe-nailed fasteners can withdraw under the uplift forces exerted by high winds. As seen in the aftermath of hurricanes, if you want uplift resistance you've got to use metal framing anchors or straps for truss-to-wall connections. Scissors trusses are an exception. Because these trusses have a significant horizontal thrust by nature, one heel has to be free to move. The solution: a framing anchor with a horizontal slot. Never rigidly attach trusses to interior partitions; this could induce bending forces that trusses weren't designed to carry. It could also cause cracks to open at wall/ceiling junctions, or partitions to be lifted off the floor due to the well-known truss rising phenomenon.

Bracing, bracing, bracing
After heels are nailed, the top chord of the truss must be secured by temporary lateral bracing. Starting at the heel, 2x4 bracing is usually installed at about 8-foot intervals along the top chord. Bracing should span four or five trusses and be fastened to each truss with two 16d nails. Its ends should overlap on at least two trusses. Bottom chords need to be braced too, at intervals of about 15 feet across the span.

While helping to maintain on-center spacing, lateral bracing won't prevent connected trusses from tipping over as a unit. To prevent this catastrophe, trusses must be braced diagonally, either across the top chords or through the webs, about every 30 feet starting at the gable end. With the first option, bracing is laid at 45( across several trusses between lateral bracing on both sides of the peak. When run through the webs, bracing starts beneath the top chord against the web closest to the center of the gable end truss. Descending at 45(, it crosses several trusses, terminating above a bottom chord. Through-the-web diagonal bracing is sometimes left in place, becoming part of the system of permanent bracing. Bottom chords are braced on the diagonal in each corner of the building, with other diagonals placed below those on the top chords. In all cases, 2x4 bracing is fastened with two 16d nails to every truss it passes. Like those found in the brochure truss makers provide to the erection contractor with every shipment of trusses, the guidelines here are based on the recommendations of the Truss Plate Institute. Don't ignore them. Inadequate temporary bracing is the number one cause of truss collapse during erection.

Closing in
Once the all the trusses are in place, temporary bracing is removed truss by truss as sheathing is laid. Ideally, each panel is fully nailed with the proper size fastener at the recommended spacing before moving on. The reason: a panel tacked in place with a few nails may not provide the same resistance to lateral movement as the bracing just removed. With a crane on site, you may be tempted to hoist all the sheathing or shingles to the roof at once. Don't do it. Trusses can be damaged or broken under the concentrated load exerted by such heavy weights.

The size, location and attachment of permanent bracing is the responsibility of the building designer. When designed correctly, permanent bracing works in unison with the building's other structural elements to achieve total structural integrity. Trusses' top chords are usually assumed to be permanently braced by the sheathing. But long webs, and bottom chords not braced by a ceiling, as in a garage or over a suspended ceiling, for example, may need to be permanently braced to prevent lateral buckling.

Damaged trusses
What should you do about the odd truss with a broken web or a popped plate or two? The logical thing is to sister a 2x over the break, or pound the plate back in. But the right thing, and the smart thing, is to contact the truss fabricator for advice. Why? First, once a truss is damaged, it no longer acts like a truss. Secondly, whoever does the repair assumes responsibility. And in today's litigious society, that's no small matter. For the same reasons, never cut, notch, drill, or in any way modify a truss without first seeking engineering advice. More likely than not, the truss engineer will come up with a workable repair scheme.

For more information, contact the Truss Plate Institute at (608) 833-5900 or the Wood Truss Council of America at (608) 271-1176.

Contact Information: Dave Damery, Director Building Materials and Wood Technology 120 Holdsworth Natural Resources Center University of Massachusetts, Amherst, MA 01003 Tel: +1 (413) 545-1770