Metal-web and I-Joists are two popular choices for new builds and both play an important structural role in buildings – supporting floors and roofs to transfer their weight evenly to the walls and beams below.
Metal-web and I-Joists are engineered joists that offer greater span lengths, improved strength and rigidity and more consistent quality when compared to their solid timber counterparts.
When deciding which joist type may be most appropriate for your project, the starting point is that each must deliver the required structural performance.
I-Joists, also known as engineered wood joists, consist of top and bottom flanges made from laminated veneer lumber (LVL) or solid wood. These are connected by a vertical web made of Natural Fibre Boards (NFB) or oriented strand board (OSB). They form a shape like the letter ‘I’ when viewed end-on that gives the joist its name.
Metal-web joists are made of two wooden flanges joined together with steel webs. This metal and wood combination creates a structural joist that offers an alternative to traditional solid timber joists.
What characteristics are important for joists and how do Metal-web and I-Joists compare?
The key characteristics for specifiers when considering the most suitable joist for a project are:
- Structural performance
- Ease of installation
- Cost
Structural performance of Metal-web vs I-Joists
Metal-web and I-Joists both offer very similar structural performance to each other, and both offer alternatives to traditional solid timber joists. For standard housebuilding projects, a popular specification is either a 253mm Metal-web or a 240mm I-Joist. Both will provide similar structural performance.
Ease of Installation and effect on follow up trades fitting services and Insulation
Metal-web joists are constructed to meet the exact requirements of each dwelling, so take longer to manufacture. The two wooden flanges are selected from graded timber and cut to size with stop blocks at each end. A metal web of the correct size is then fixed in place. This metal web has gaps that allow following trades to pass wires and pipes through the roof or floor void for lighting and building services equipment.
I-Joists are manufactured at volume offsite in a variety of depths and widths. They can be cut to exact lengths to fit each house type and are lighter in weight than both solid timber joists and Metal-web joists. Unlike Metal-web joists, holes need to be drilled to accommodate service penetrations and can be done on site. Some suppliers of I-Joists offer to pre-drill holes in appropriate positions to match each house type.
This can be an advantage where large ducting is required for ventilation systems situated within the structural zone. Where ducting runs through metal-web joists it has to be fed through the metal struts and with I-joists it can often run in pre-drilled holes up to 300mm wide.
How is insulation installed between Metal-web and I-Joists?
Where insulation is installed between joists it is important that no areas are left uninsulated. Metal-web joists have a gap between the two wooden flanges that should therefore be insulated. However, as the gap is covered by metal webbing it makes it a difficult task to fill that gap with insulation on-site.
I-Joists do not have this gap, and it is easier for insulation to form around the ‘I’ shape of the joist. Some manufacturers produce an insulated I-Joist to make this insulating task even easier. Insulated I-Joists may be used by manufacturers of the prefabricated and insulated timber panels that form roof cassettes.
Costs for Metal-web and I-Joists?
Metal web joists tend to have a cost premium due to increased production and assembly time. I-joists generally just need to be cut to their required length.
What projects are best suited to Metal-web and I-Joists?
For one-off houses or small developments both have equal merit. Larger developments allow I-Joists to be produced at volume and can bring project costs down. However, this does not preclude I-Joists from being used on smaller projects and specifiers can consider both products for a wide variety of projects.
To find out more about I-Joists contact STEICO’s expert team for advice.