Bearers and Joists: How a Subfloor Frame Carries a Floor

Key points

• Bearers are the heavier beams that sit on the stumps, piers or posts. Joists run across the bearers at right angles and carry the floor.
• Load travels from the floor to the joists, from the joists to the bearers, and from the bearers down to the footings.
• Timber bearer and joist sizes for houses come from the span tables in the AS 1684 series or from the project engineer.
• Joist spacing is set by how far the floor material can span, which is why the floor sheet chosen changes the frame underneath it.
• Platinum “R” Floor has a recommended maximum of 800mm joist centres, compared with 450mm for 19mm yellow tongue particleboard and 600mm for 22mm red tongue.
• Platinum “R” Floor is fixed over the joists and works with the framing rather than replacing it.

Plenty of new Australian homes are built on a concrete slab, but many sites still call for a floor built above the ground. Sloping blocks, flood-prone land, reactive soils and highset Queensland homes all rely on a framed subfloor, and that frame is made of bearers and joists. Platinum Timber & Ply supplies the floor panel that sits on that frame, so this is how the frame looks from the builder’s side of the job, and how the floor on top changes what the frame has to do.

What Are Bearers and Joists?

Bearers and joists are the two layers of horizontal framing in a raised floor. Bearers are the heavier beams that sit on the stumps, piers or posts, and joists are the lighter members that run across the top of the bearers at right angles and carry the floor itself. Together they form the subfloor frame, the grid that collects the weight of the floor and everything on it and passes it down to the footings.

The simplest way to remember the difference is the order they go in. Bearers go on the supports, joists go on the bearers, and the floor goes on the joists.

How a Subfloor Frame Carries the Load

Each layer of the frame spans between the layer beneath it, so the load steps down through the structure in order:

1. The floor takes the weight of people, furniture and internal walls and spreads it to the joists.
2. The joists carry that load across the gap between bearers.
3. The bearers collect the load from every joist resting on them and carry it between the supports.
4. The stumps, piers or posts take the load down to the footings and into the ground.

Because each layer spans between the members below it, the layers are linked. Wider joist spacing asks more of the floor sheet, and a longer bearer span asks more of the bearer, which is why the parts that make up a subfloor, from footings to floor sheet are designed as one system rather than chosen piece by piece.

Bearers vs Joists at a Glance

What Decides Bearer and Joist Sizes

For timber framing in houses, bearer and joist sizes are normally taken from the span tables in the AS 1684 series for residential timber-framed construction, or set by the project engineer. The tables account for the span, the spacing, the stress grade of the timber, the loads being carried and the wind classification of the site, which is why two houses on the same street can end up with different framing.

Steel subfloor framing is sized from the manufacturer’s design data or by the engineer, and panel floors laid over steel joists are fixed and finished in the same way as over timber.

Span tables aren’t reproduced here, because the right figures depend on the timber grade, the loads and the site’s wind classification. The current standard or the engineer’s design is the source to use for any specific frame.

Joist Spacing Is Set by the Floor on Top

Joist spacing is measured from the centre of one joist to the centre of the next, and it is decided by how far the floor material can span between supports. A thinner or less stiff floor sheet needs joists closer together, while a stiffer one can bridge a wider gap. The frame is planned around the floor, not the other way round.

Particleboard figures are the manufacturer’s published limits for general residential flooring. The Platinum “R” Floor figure is from its published specifications. Actual spacing is always confirmed by the project engineer.

The difference shows up in the joist count. On a floor 7.2 metres long, joists at 450mm centres means 17 joists, at 600mm centres it means 13, and at 800mm centres it means 10. The engineer still sizes the joists for the spacing chosen, so this is an illustration of the count rather than a design, but fewer joists means fewer members to cut, lift, level and fix.

The same comparison runs through the rest of the floor. How Platinum “R” Floor compares with yellow tongue and particleboard on insulation and installation stages is part of the same decision, along with the main floor sheet materials set side by side.

Where the Floor Panel Sits on the Frame

A panel floor is laid over the joists as a platform, with each sheet fixed down to the joists beneath it. Platinum “R” Floor works the same way: it is fixed over timber or steel joists and doesn’t replace the bearers or joists. What changes is what arrives on the frame, because the panel combines the structural floor and the insulation, so the floor is insulated as soon as it is laid, with published values from R1.2 to R2.5.

The panel’s framing guidance is a recommended maximum of 800mm joist centres, a minimum joist thickness of 45mm and blocking under any load-bearing wall, with spans and load cases confirmed by the project engineer. The panel’s published specifications set out sizes, thicknesses and R-values, and the installation sequence on site follows the same order as any platform floor.

The panel also changes one of the fiddlier framing jobs on a raised floor. Wet areas normally need joists notched, dropped or run at a different height to form a set-down for the tiles. With Platinum “R” Floor, a strip of fibre cement flashing between the panel edge and the 19mm wet-area flooring creates the step down, so the joists under bathrooms and laundries stay at one height.

Bearers and Joists on Sloping and Elevated Sites

A framed subfloor is what makes a level floor possible on a slope. The stumps or piers are set to different heights, the bearers run level across them, and the joists and floor follow. The same frame lifts a home clear of flood-prone ground or reactive soil, and builders across Queensland use the suspended floor options a sloping block allows for exactly these sites.

A raised frame also leaves an open, ventilated space under the floor, and that space is where a floor loses heat. Whether that floor needs insulating comes down to the climate, the subfloor type and the energy assessment, and the answer for a floor over an open void is almost always yes.

A Frame Built Around a Better Floor

Bearers and joists do the same job they always have: they carry the floor and pass its load to the ground. What has changed is the floor that goes on top. A panel that spans further, arrives insulated and simplifies wet-area set-downs lets the frame be planned around a better floor rather than around the limits of a thin sheet. The frame stays the same. The floor does more.

To check joist spacing and panel thickness for a project, contact the Platinum Timber & Ply team with your plans.

Frequently Asked Questions

What is the difference between bearers and joists?

Bearers are the heavier beams that sit directly on the stumps, piers or posts, while joists are the lighter members that run across the bearers and carry the floor. Load travels from the floor to the joists, from the joists to the bearers, and from the bearers down to the footings. Platinum “R” Floor is fixed over the joists and works with both timber and steel framing.

Which way do joists run on bearers?

Joists run at right angles to the bearers and parallel to each other, usually sitting on top of the bearers and sometimes fixed to their sides. The floor sheets are then laid across the joists. Platinum “R” Floor panels are laid over the joists in the same way as conventional sheet flooring, so the framing layout doesn’t change.

How far apart should floor joists be?

Joist spacing is set by how far the flooring can span. Particleboard sheet flooring is published at a maximum of 450mm centres for 19mm yellow tongue and 600mm for 22mm red tongue. Platinum “R” Floor has a recommended maximum of 800mm joist centres with a minimum joist thickness of 45mm, and the project engineer confirms the final spacing for each design.

What size bearers and joists do I need?

For timber framing in houses, sizes come from the span tables in the AS 1684 series or from the project engineer, based on span, spacing, timber grade, loads and the site’s wind classification. Steel framing is sized from the manufacturer’s data or by the engineer. When the floor is Platinum “R” Floor, the joists should be at least 45mm thick, with spacing up to the recommended 800mm maximum where the engineer’s design allows.

Does Platinum “R” Floor replace bearers and joists?

No. Platinum “R” Floor is installed over engineered timber or steel joists and does not replace structural framing. It combines the structural floor and the insulation in one panel, which removes the separate underfloor insulation stage, and it removes the need to notch or drop joists to form wet-area set-downs.

Do bearers and joists need to be insulated?

The insulation belongs to the floor rather than to the framing. In a raised floor it sits between the joists, under the floor or inside the floor panel, and the level required depends on the climate zone, the subfloor type and the energy assessment. Platinum “R” Floor puts the insulation inside the panel, from R1.2 to R2.5, so the floor is insulated as soon as it is laid.