Roof joists are horizontal or pitched structural members, typically made of solid timber or engineered wood, that span between load-bearing walls or ridge beams to support the weight of the roof and the loads applied to the roof. Roof joists spread the load of the roof evenly to prevent stress points that could lead to structural damage and help to hold the walls together by preventing them from moving apart due to the weight of the roof itself.
The size of roof joists and spacing between them will depend on the expected loading of the roof and the distance the joists need to span.
In this blog we will look at some of the key functions and characteristics of roof joists that determine their specification within buildings.
How do I know what size of roof joist is required?
The size of roof joist you need will depend upon these key factors:
- Roof joist Span – this is the distance between the load bearing elements that the roof joist needs to cover.
- Strength of the roof joist – this will depend upon the material used. In the case of timber its strength is determined by its grade such as C16 or C24. For other materials such as I-joists the strength characteristics can be determined by contacting the manufacturer.
- Loading of the roof – the total loading that the roof is expected to experience. This includes live loads such as wind and snow as well as the dead load of the weight of the roof.
- Roof joist spacing – this is the distance between each roof joist.
Taking all these factors together, along with the structural requirements of building regulations will allow the correct size of roof joist for a project to be determined. However, deeper joists can often be specified due to the thermal performance requirements of the roof rather than purely their structural performance.
What is the difference between a roof joist and a floor joist?
The key difference is that floor joists are only installed horizontally whereas roof joists can also be pitched members. Roof joists support the dead and live loads associated with the roof. Floor joists serve the same structural purpose but support the loads imposed on the floor. These include anything placed upon the floor such as furniture as well as the people who use the room. Floor joists also create a base for the floor of the room whereas roof joists are joined to other structural members like rafters and trusses to help create a robust roof space.
What is the difference between a roof joist and a beam?
The key difference between a roof joist and a beam is the scale of the load they need to support. A roof joist forms part of a sequence of evenly spaced joists that span between the load-bearing elements of the building. Together roof joists each play their individual part in sharing and supporting the loading of the roof.
In contrast, a beam is the primary load-bearing element in a roof supporting the roof joists and beams usually run the entire length of the roof space so need to be designed to cover longer span lengths than roof joists without bending. A beam needs to be able to bear a much greater load than individual roof joists and for this reason is typically thicker and stronger than roof joists.
What are roof joists made of?
Although roof joists can be made from many different materials, there are three main types of roof joist typically considered for specification in dwellings:
Solid timber roof joists
These are traditional roof joists made from solid timber. They can be produced from various species, including spruce, pine, and fir. The timber is strength-graded for quality, stiffness, and structural performance, and is assigned a classification from C14 to C24. The “C” denotes coniferous timber, while the numbers represent its strength class—the higher the number, the stronger the wood.
C24 timber offers excellent strength, stiffness, and durability. Because it is typically sourced from slower-growing trees, it is denser and more robust, making it well-suited for use as roof joists. However, lower grades such as C16 may also be appropriate, depending on the specific loading and span requirements for the roof design.
Roof I-joists
I-joists, or engineered wood roof joists, are shaped like the letter ‘I’. They have the top and bottom flanges made from laminated veneer timber (LVL) or solid wood. These are connected by an OSB or natural fibre board vertical web, creating a lightweight yet strong roof joist. The structural design of the roof I-joist helps resist warping, shrinking and twisting and delivers a consistent performance over the span width.
Roof I-joists are manufactured at volume in a controlled factory environment and can be pre-cut to the length required before being delivered to site.
Metal web roof joists
Metal web joists are engineered joists combining solid timber flanges joined together with a network of pressed steel webs. They are made to order to fit the exact requirements of each roof.
Can roof joists be replaced?
Where roof joists have been damaged by rot, they can be fully replaced. If the damage is extensive replacing the entire joist can be a major structural undertaking. It can sometimes require removing parts of the roof or even lifting the whole roof to get enough access to replace the entire roof joist.
If the damage is not too severe a roof joist can be repaired from the inside by a suitably qualified professional tradesperson rather than replaced. Depending on the extent of the damage, the fix may involve reinforcing a single joist with a new section and appropriate connectors, or attaching a new rafter alongside the existing one to reinforce it
Are there differences between roof joists for flat vs. pitched roofs?
The roof joists used for flat and pitched roofs are the same. They may differ in the exact size and strength required, as this will depend on the structural loading requirements of the individual flat or pitched roof.
The rafter for a flat roof will form part of a flat deck with a slight slope for drainage purposes. A pitched roof joist will combine with rafters or trusses to form a framework that will give the roof its pitch. In both cases the basic functionality of the roof joist will be the same – to support the loading of the roof – it is just the shape of the roof above the roof joists that will differ.
Where can I find out more information about roof joists?
STEICO has years of experience of making STEICOjoist an I-joist system for roofs, walls and floors.
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]www.steico.com/fileadmin/user_upload/importer/downloads/4028b6097384810e01749ff1e1ce608c/STEICOjoist_DoP_ETA-20_0995_04-0002.pdf