The Main Components of Floor Trusses

Posted on: 17 August 2018


Theoretically, you could just place planks or sheets of plywood between joists to create a floor, but if you do that, the timber won't be strong enough to support the weight on the floor. Ultimately, a floor built like this will bow in the middle and eventually fall apart. To prevent that from happening, you need to use floor trusses. Similar to the designs that hold up bridges, floor trusses contain the following essential elements.

Top and Bottom Chords

The top and bottom chords are the straight pieces on the top and bottom of the floor joists. The bottom chord makes up the lower border of the truss, and it takes the bending stress. The top chord is the part that actually creates the surface of the subfloor, and you attach the flooring directly to this part of the truss.

The Diagonal Pairs

Between the top and bottom chords there are a series of small pieces of wood, arranged in an X shape. These two pieces are referred to as the diagonal pairs. In some cases you may hear one side referred to as the diagonal pair, and the corresponding side of the X called the "counter" element. The angles created by these X pieces lend stability to the truss and allow it to hold a lot more weight than it would be able to hold without these shapes.

With some floor truss designs the diagonal pairs don't make the X shape. Instead, they just run at a 45-degree angle from the chord, and when you look at the two pieces next to each other, they create a V shape.


There are a series of rods that run between the top and bottom chord of most floor trusses. The rods are straight lines, oriented perpendicular to the chords, and they sit in between each X shape. Although the majority of the floor truss is made of metal, this component may be made of metal.

Angle Blocks

Angle blocks can also be made of metal. They sit at the base of the rods, and they stabilise both the rods and the diagonal pairs. With a standard floor truss, these three components all extend from the angle blocks. The angle blocks simply support everything and keep it together.

In contrast, an I-joist skips those shapes. Instead, it just wedges a small piece of plywood into groves created on either side of the top and bottom chords. Although this is not as efficient as a truss, it can save money and work in some situations. To learn which trusses or joists are best for your project, contact a supplier near you.