The Light Steel Frame (roof structure) is very similar to the wood/timber-framed construction in principle. That is, the wooden framing members are replaced with light steel sections. The steel sections used in Ghana are called cold-formed sections, meaning that the sections are formed, or given shape at room temperature.

Currently, there are three categories of materials used for the manufacture of roof structures in Ghana. They include timber, structural or heavy steel and light steel frames (LSF).

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Timber Trusses

Timber trusses are the most common roof structures for the residential market. Timber is an easily accessible material in Ghana, hence, its popularity. Recent restrictions from the Forestry Commission of Ghana on the felling of good timber; and the activities of illegal mining (galamsey) has led to the shortage of well-treated timber for roofing.

Heavy Structural Steel

Heavy structural steel is classified as sections that are hot rolled into required shapes and sizes. A wide variety of sections are available for selection by the engineer to optimise strength against the cost.

Light Steel Frames (LSF)

Light steel frames (LSF) are cold rolled sections which come in a variety of shapes, sizes and thicknesses (such as lipped channels, Z purlin) and are used for mainly commercial and residential buildings. The material thickness and grade are optimised to be engineered and produce light steel frame structures.

LSF materials have been used to manufacture structures for roofing, walls, ceilings, floor joists etc.

Uses of LSF Trusses

Due to the unique nature of light steel trusses, they can be used in a variety of ways that other timbers or wood trusses are unable to.

Large Span Roof Structures

Clear spans in excess of 20 metres are possible with LSF trusses. LSFs have the capability to construct complex curved roofs and hip roofs with lofted ceilings.

Medium Span Roofs

Many residential roof structures are supplied in LSF. It is a cost-effective structure that provides the basis for a perfect straight roof.

Short Span Roofs

LSFs are specified for a growing number of Affordable Housing projects. The fact that the truss is very light in comparison to other types of trusses, makes it ideal for manual handling on-site and speedy erection. Steel trusses, in most instances, offer a cost advantage over other roofing structures.

Roof Cover and Other Loads

It is critical that all loads be identified and stipulated when a roof structure is designed. LSF roof structures can be designed for all common roof cladding types. Concrete tiles, sheeting, slate and so on are all materials extensively used on LSF trusses. Engineers should always demand engineering reports of trusses designed, since not all truss systems provide for this option.

The truss engineering reports show the areas of deflecting, bending and the critical failure points.

The placement of solar panels, air conditioning units and acoustic ceilings must be prominently indicated on layout drawings to ensure the loads are incorporated into the truss design.

Strength of Steel Truss and Corrosion Protection

The steel used in truss manufacture is graded in terms of its yield strength. The most commonly used steel grades are between G350 to G550 MPa depending on the type of load.

In Ghana, the EN 1993 Eurocode is mostly used to address the design criteria.

The steel is protected by a layer of corrosion-resistant material. This is, normally, a zinc or aluminium/zinc alloy.

The coating applied in the final stages of the steel manufacturing environmental conditions will determine what the required coating thickness should be.

The required coating in Ghana is AZ150 for Aluzinc or Z275 for Galvanised steel.

The Advantages of Light Steel Trusses

1. Lightweight, but durable, allowing for easy handling.
2. Steel does not warp with age and a high-quality finish is maintained over the full life-span of the building which enhances the aesthetics of the roof design.
3. The installation of the LSF roof structure is quicker than the use of timber.
4. Trusses are engineered to be accurate.
5. Steel frames are termite-proof.
6. No wastage on-site, since all the trusses are designed and engineered to fit a structure.
7. As in most cases, light steel is a more cost-effective option as compared to other roofing frames.

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Your roof should be designed to last you a lifetime so always make sure to invest in the right roofing frame to make your building strong and aesthetically pleasing.

Guest Post by Fred Kwofie

Fred Kwofie is the Managing Director of Domod Roof Limited. Domod Roof fabricates, markets and installs long-span 100% Precoated-Aluminium and Aluzinc Roofing Sheets as well as Light Steel Frames commonly referred to as Steel Trusses over the past 15 years.