Name the roof type - gable, hip, 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 roof framing over the last three decades. 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.
How roof 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. Roof 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.
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 1 to 65 feet long, and any height. 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 roof trusses:
Where common trusses can't be used or aren't appropriate, special trusses fill the bill. Girder 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 girder 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 trusses with framed openings to accept a square or triangular louver vent.
"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 field-fastened side-by-side. In L-, T-, H- and U-shaped buildings, girder trusses eliminate the need for a bearing wall where an "L" joins the main building. Here one end of each main building common truss is 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 "L" 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 and 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.