Metal-web vs I-Joists - what are the considerations?

Expert Insight | 29 January 2026

Metal-web or I-Joists play an important structural role in buildings – supporting floors and roofs to transfer their weight evenly to the walls and beams below. There are two popular choices for new builds – either Metal-web or I-Joists.

They both do the job that is required although some seem to be more popular for one-off houses and smaller sites whilst others seem to be favoured on larger housing developments.

When deciding which joist type may be most appropriate for your project, large or small, the starting point is clearly that each must deliver the required structural performance. However, as we shall explore in this blog, other factors such as ease of installation, accommodation for service penetrations as well as cost and availability may also be key considerations.

We will look at the differences between Metal-web and I-Joists to see what options and benefits they may bring to a construction project.

What are Metal-web and I-Joists?

Before comparing the two, it is worth touching upon the solid timber traditional joist that these engineered joists have replaced. Solid timber joists are, as the name suggests, made from a continuous length of solid timber. They have span limitations, require herringbone strutting/bracing for additional rigidity and their performance is dependent on the quality of the wood from which they were cut. Solid timber joists are prone to shake and wane and are rarely found in lengths greater than 6m.

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.

What are Metal-web joists?

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 are I-joists?

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.

This engineered combination creates a lightweight and strong product, that is available in lengths up to 13.5 m. This enables designers to take advantage of engineering principles that allow greater spans in dual-span conditions. Manufactured under controlled conditions and delivered at approximately 9% moisture content, i-joists offer superior dimensional stability, making them far less susceptible to warping or twisting than traditional solid timber joists, which typically have much higher moisture levels.

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
Availability
Cost

We will look at each in turn to see how Metal-web and I-Joists compare.

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.

Installation of Metal-web vs I-Joists

We will look at the installation of the joists as well as their effect on follow up trades fitting services and insulation.

What is the process for installation of joists and services?

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. Extra care must be taken to ensure that cables are protected, preventing damage when passing through the metal-web.

I-Joists are manufactured at volume in a controlled factory environment 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. This is a relatively simple process and can be done on site, however suppliers of I-Joists often offer a service where holes can be pre-drilled in appropriate positions to match each house type. This makes the I-Joist as convenient to use for later trades as the Metal-web joist.

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 as this may cause an area of cold bridging where heat can escape and condensation could form. 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.

What is the availability of Metal-web and I-Joists?

As Metal-web joists are made to order to fit the exact requirements of each individual building, they are more suited to single or smaller developments and may struggle to meet the lead time requirements of large-scale housebuilders. The large production runs of I-Joists make them more readily available to all types of construction including supplying at scale at short notice to mass market applications.

How do costs compare for Metal-web and I-Joists?

Due to their increased production and assembly time, metal web joists can have a cost premium compared to I-joists, which generally just need to be cut to their required length.

What projects are best suited to Metal-web and I-Joists?

As we have seen throughout this blog, both Metal-web and I-Joists can be considered for use in many of the same projects. They can both be specified to meet the necessary structural requirements and provide modern alternatives to the traditional solid timber joists.

The choice of which is best suited for a project will of course depend upon the type of dwellings being constructed and the scale of the development. For one-off houses or small developments both Metal-web and I-Joists have equal merit. However, for larger developments where many hundreds of joists are required for standard build types, the ability for I-Joists to be produced at volume can bring cost advantages to the project. This does not preclude I-Joists from being used on smaller projects and this makes them more likely to be considered by specifiers for a wider variety of projects of all sizes.

Where can I find out more information about I-Joists?

STEICO has years of experience of making STEICOjoist for floors, walls and roofs.

These engineered I-joists feature high-strength laminated veneer lumber (LVL) flanges and natural fibre board webs. The I-joist shape reduces thermal bridging and helps to improve energy efficiency. STEICOjoist is lightweight and offers excellent dimensional stability with minimal shrinkage or twisting^{^[1]}. The I-joists include defined zones for easy installation of services, and they are made from responsibly sourced, PEFC-certified wood.

To enquire about using our I-joists on your next project, contact us here and our expert team will be happy to help.

[1]https://www.steico.com/fileadmin/user_upload/importer/downloads/4028b6097384810e01749ff1e1ce608c/STEICOjoist_DoP_ETA-20_0995_04-0002.pdf

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