Before I describe the principle types of trusses, let's define what exactly a truss does.

First, a truss withstands gravity and holds up the roof. That may sound obvious, but it's the single most important criterion. The diagonal rafters of the truss do most of this gravity-defying work. But as gravity pushes down on the roof, the walls supporting the rafters will want to fall outward.

The horizontal tie beam is what keeps the whole thing rigid. Over time this tie beam will have a tendency to weaken and sag. That's where the post comes in, lifting and supporting to prevent the aging effects of gravity.

The King Post Truss

King post trusses are just about the most common truss type. They're so named because there's one post, the king, in the center that keeps the bottom horizontal tie beam from sagging.

Elements of a king post truss:

• Rafters: diagonal members that lie against and are parallel to the roof
• Tie beam: at the bottom of the truss, spanning from wall to wall (or rafter to rafter)
• King post: connects the tie beam to the point where the two rafters meet
• Struts: diagonals that connect the tie beam to the rafters

How to use it: A king post truss is often used in vaulted spaces where the designer wants to simultaneously achieve the spaciousness of the vault and, by establishing a lower horizontal line of tie beams, the intimacy of a lower ceiling.

The Scissor Truss

A scissor truss eliminates vertical and horizontal elements. It creates a more spacious feeling, because there's no horizontal element defining a lower "virtual" ceiling line.

How to use it: A scissor truss is appropriate when the ceiling pitch, or slope, isn't very steep. A shallow pitch, like in this example, can still be quite spacious when a scissor truss is used to support the roof.

How to use it: Hammerbeam trusses really come into play in large spaces, where their scale and quality resonates. A hammerbeam truss is appropriate for baronial-style halls and barns, or some combination of the two.

Mixing It Up

An important point to consider in truss design is that just about anything goes once you understand what each truss component's job is. A king post doesn't have to be made of wood, for example. Sure, that's the traditional material, but a metal rod will work just as well. In fact, a metal rod serving as a king post is visually more appropriate when the truss components are visually articulated, as in this example.

Whether the truss starts as a king post, queen post, scissor or hammerbeam, it can end up being some kind of hybrid. In fact, these days it's rare to see a truss design that's simply just one basic type.

Again, the trick is understand what each truss component does and design accordingly. Metal rods, which are better at withstanding tension, are more appropriate than wood for tie beams and king or queen posts. And why not pull the tie beam, or in this case the tie rod, higher to open up the space, not unlike the struts in a scissor truss?

As mentioned at the outset, the principle function of trusses is to hold up the roof by withstanding gravity. But by being a device that establishes a rhythm and scale in a space, trusses also have an important visual job to do. In fact, there are many instances when the trusses are used solely for their visual impact, with something else doing all of the structural work.