Roofing Support Structures

The Third Estimate

May 3, 2022

Roofing support structures are essential for the stability of your roof. Knowing the differences between them can help you choose the right ones for your project.

What Are Roofing Support Structures?

Rafters, Purlins, King posts, Underpurlins, and underplates are all common types of roofing support structures. If you're wondering what each type is and how they function, this article will provide a brief explanation. To learn more, continue reading. We'll also discuss the differences between them. Roofing support structures are essential for the stability of your roof. Knowing the differences between them can help you choose the right ones for your project.


The roof is supported by rafters. Rafters are long beams that span the length of the roof, usually two by tens or twelve by twelve feet. The rafters are typically attached to a central support beam at the ridge, which connects the roof to the house's exterior walls. Rafters also support the sheathing on the roof. Rafters are also referred to as "stick framing" since they are created before arriving at the construction site.

Rafters are part of the roof structure, composed of dimensional lumber joists that support the ceiling and floor below. These members span from the outside wall of the structure, and their spacing determines how far they can span. Rafters support the ceiling on one side, while the roof frame rests on the other. Rafters generally connect to the ridge board at the peak and are connected at the collar tie.


Purlins are horizontal, channel-shaped structural members that span the rafters in a building. They support the weight of the roof deck, which is typically made of concrete and insulated. Depending on the application, purlins can replace the closely spaced rafters in a wood-frame structure. These structures are also commonly used in the construction of pole buildings and pre-engineered metal building systems.

While purlins play a pivotal role in maintaining the position of roof sheets, they are not the only important parts of a roof. They also act as the basic framework for supporting the sheeting itself and the overall structure. Hence, it is essential to use high-quality purlins and other components for the construction of a roof. This is why builders always prioritize the use of quality purlins in their structures.

King post

No matter whether you are renovating an older home or constructing a new one, you've probably seen a King post roofing support structure. The name is indicative of its two inclined posts, and the king post truss can span eight meters or more. King post trusses use tension to support the beam above them, and they're versatile enough to be used in many different types of roofs. They're especially useful for roofs with simple, angled rooflines, as well as auditoriums.

A king post truss is a special truss that supports the peak vertically, thus preventing sagging. They're typically supported by concrete, stone, or supporting walls and are often left exposed for aesthetic purposes. Regardless of its use, a King post truss is a versatile and beautiful roof support structure. The king post truss is lightweight and easy to install, making it a popular choice for roofs that span five to eight meters.


Underpurlins are horizontal beams that run around the middle of the roof, which support the rafters in a conventional roof frame. These rafters typically carry half of the roof's weight, though they can support more depending on the framing method used. The underpurlins can be wood, steel, or concrete and may be re-aligned as necessary.

Underpurlins are fixed horizontally under the rafters and at right angles to the roof slope to provide additional support. When used on metal buildings, they may replace closely spaced rafters. Wood frame buildings may use intermediate supports such as wall frames. In addition to providing additional support, underpurlins can improve the strength of the roof structure. The underpurlin is a critical part of the roofing system. It helps stabilize the roof and prevent the roof deck from sagging.


When designing and building roof trusses, one key structural element to consider is the strut. This long, thin, curved member is integral to the load-bearing structure and serves to resist longitudinal compression while also providing outward-facing support in lengthwise directions. Roof struts may be used to separate two components of a truss that is not necessarily connected, as in the case of barns. Experts recommend replacing automotive shocks and struts every 50,000 miles for safe performance. Original equipment gas-charged shocks and struts begin to degrade at that point.

Struts are also necessary for the roof frame, which transfers loads from the underpurlins and other beams to the load-bearing walls of the building. Struts are often supported over timber walls by hangers or props. While struts may seem unnecessary, they're a vital part of a roof structure. They must be properly installed to prevent collapse. However, struts should never rest on ceiling joists.

Collar ties

When installing collar ties in roofing support structures, you'll need to keep three things in mind. The first thing is that you need to know the length of the tie. Collar ties can be four or eight inches long, and they'll need to be angled to match the angle of the rafters. You can measure the length of one collar tie by measuring the other three, then cut them to size.

Second, you'll need to know the materials used for the collar ties. Most are made of pine wood. If you're doing it yourself, you can assume that different types of wood will be stronger than the rest. But in reality, the same kind of material will work for all collar ties. This is because collar ties are made of the combined strength of the entire roofing support structure, and the more stable it is, the more secure it will be.


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