Floor joists are horizontal structural members, typically made of solid timber or engineered wood, that span between loadbearing walls or beams to support a floor. The size of floor joists and spacing between them will depend on the expected loading of the floor and the distance the joists need to span.
We will explore some of the key functions and characteristics of floor joists that determine their specification within buildings.
What is the purpose of floor joists?
Floor joists run parallel to one another to form a framework between loadbearing walls. The top of each floor joist provides a supporting surface upon which floorboards can be fixed. The primary function of floor joists is to bear and distribute the load imposed on the floor. These can be split into the dead load and the live load.
The dead load is the weight of all the permanent materials that make up a floor — like joists, boards, fixtures and fittings. This load is considered by the floor designer when sizing the floor joists for a specific span condition.
The live load refers to the total load being carried by the floor for items which can be regarded as temporary and moveable. These include the contents of the room such as beds, furniture and of course the occupants themselves.
Floor joists also perform another important function by tying the walls together to give structural stability to the building. In addition, they provide a space to run pipes and wiring for building services.
What are the different types of floor joist?
There are three main types of floor joist typically considered for specification in dwellings.
Solid timber floor joists
These are traditional joists made from solid pieces of timber. They can be cut from various types of wood such as spruce, pine and fir. The timber used is graded based on its strength, stiffness and quality and will be assigned a code from C14 through to C24. The ‘C’ stands for ‘coniferous’ and the numbers indicate the strength class. The higher the number, the stronger the wood. C24 timber is known for its superior strength, stiffness and durability. Sourced from slower-growing trees, it is denser and more robust and is suited for specification as floor joists. However, lower grades such as C16 can also be suitable – it all depends on the loading and spanning requirements of each floor specification.
I-joists
I-joists, or engineered wood floor joists, are shaped like the letter ‘I’ and have the top and bottom flanges made from laminated veneer timber (LVL) or solid wood. These flanges are connected by an oriented strand board (OSB) or natural fibre board (NFB) vertical web.
This design creates a lightweight yet strong floor joist that helps resist warping, shrinking and twisting and delivers a consistent performance over the required span. Due to their design the installation of services through the web material is significantly easier and more flexible than with solid timber joists.
I-joists for floors 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 joists
Metal web joists are engineered joists with solid timber flanges and a metal web between them. Their design allows the routing of plumbing, electrical and other services through the joist. They are made to order so that they fit the exact requirements of each floor.
What determines the size of floor joist required?
The size of the floor joist can only be determined by looking at the following key specification parameters.
Span of the joists
The distance the joist needs to span between its supports is a critical factor, with longer spans requiring larger sized joists.
Spacing between the joists
The space between each joist when measured centre-to-centre affects the joist size. The closer together the joists are, the smaller they can be. Conversely, the wider apart the centres are the larger they need to be to cope with the extra loading that they will have to bear.
The strength of the floor joist
Each type of floor joist – whether it is a solid timber, I-joist or open web joist will have its own load bearing capacity, and this will alter with the size of the joist. When determining the size of a floor joist needed for a project, the strength of each joist declared by the manufacturer must be included in any structural calculations.
The loading of the floor
The dead and live loads for the floor will also affect the sizing of joists – the higher the total load the more likely it is that larger joists will be required.
Once all the data has been gathered the optimum size and spacing of the floor joists (to deliver the required loadbearing capacity) can be calculated by a suitably qualified structural engineer or floor design specialist.
Where can I find out more information about floor joists?
STEICO has years of experience of designing, refining and manufacturing STEICOjoist an I-joist for floors, walls and roofs. These engineered I-joists feature STEICO LVL (laminated veneer lumber) 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]. Our 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.