In more than 20 years of building inspections across South East Queensland, subfloor defects are one of the most consistently underestimated problems I encounter. Bearers and floor joists are out of sight — tucked under the house where most buyers never look — and that’s exactly why problems go unnoticed until they become expensive.
This article explains what bearers and floor joists are, how they work structurally, and — most importantly — what defects I look for during a pre-purchase building inspection. If you’re buying a home in Queensland, understanding these elements could save you tens of thousands of dollars.
What Are Bearers and Floor Joists?
Bearers and floor joists are the primary structural members that make up a suspended timber floor system — the most common floor construction type in older Queensland homes and many contemporary builds.
Bearers
Bearers are the large horizontal timbers that span between the stumps (posts) supporting the house. They run perpendicular to the floor joists and carry the full load of the floor, walls, and roof down to the stumps and into the ground. Bearers are typically hardwood — species like spotted gum, ironbark, or tallowwood in older Queensland homes — and are sized according to the span they cover and the load they’re expected to carry.
Standard bearer sizes range from 100×75mm for short spans to 200×75mm or larger for longer spans in heavier-loaded areas. The relevant standard is AS 1684 — Residential Timber Framed Construction — which sets out the span tables, spacing requirements, and minimum species/stress grades for all structural timber.
Floor Joists
Floor joists are the secondary members that sit on top of the bearers, running at right angles to them. The flooring — whether hardwood boards, particleboard sheet, or plywood — is fixed directly to the top face of the joists. Joists are typically spaced at 450mm or 600mm centres, depending on the flooring material and span.
In Queensland hardwood construction, floor joists are commonly 100×50mm or 150×50mm. In more recent construction, MGP (Machine Graded Pine) is widely used for joists. Where MGP pine replaces seasoned hardwood, correct sizing becomes critical — pine has lower structural values than hardwood, so an undersized pine joist in a high-load area is a genuine defect.
How the System Works Together
The load path is straightforward: floor loads transfer from the flooring into the joists, from the joists into the bearers, from the bearers into the stumps, and from the stumps into the footings and ground. Any failure or deficiency anywhere in this chain affects everything above it — floors, walls, and roof included.
This is why seemingly minor subfloor defects — a notched bearer, an undersized joist, a deteriorating stump — can have structural consequences well beyond what a buyer would expect from looking at the floor surface above.
What Does a Building Inspector Look For in the Subfloor?
This is where the real value of a thorough inspection lies. A visual check of the floor surface will tell you almost nothing — a structurally compromised subfloor can feel solid and look perfectly fine from above. You need someone who will actually get under the house, assess what they’re looking at, and interpret it correctly.
Here’s what I look for in every subfloor inspection:
1. Timber Decay and Rot
Timber decay is caused by fungal attack, and fungal attack requires moisture. When bearers or joists are subject to persistent dampness — whether from poor subfloor ventilation, drainage issues, plumbing leaks, or rising damp — the timber gradually loses its structural integrity. Early-stage decay can be hard to detect visually; I use a probe to test for soft spots, and a Tramex moisture meter to measure moisture content in the timber itself.
A moisture content above 18–20% in structural timber is a red flag. Above 25%, active fungal decay is almost certain. Severely decayed bearers lose most of their load-carrying capacity and can fail under normal floor loading. Replacement is the only remedy — there is no structural repair for deeply decayed timber.
2. Borer Damage
Queensland has several species of timber-boring beetles that attack seasoned hardwood and softwood structural timbers. The most common in subfloor environments is the Queensland pine beetle (Calymmaderus incisus), which attacks softwood (including pine joists), and the common furniture beetle (Anobium punctatum), which affects hardwood and softwood alike.
Borer damage appears as small round exit holes (1–2mm for most species) in the timber face. The presence of fresh frass — the fine powdery residue left behind — indicates active infestation. Heavily bored timber can be structurally compromised even when it looks solid from the outside, because the tunnelling destroys the internal wood fibres. This is one reason a dual-licensed building and pest inspector is valuable: structural assessment and pest identification happen simultaneously, in context.
3. Termite Damage
Subfloor timbers are prime termite territory. Subterranean termite species — particularly Coptotermes acinaciformis, the most destructive species in Australia — enter from the ground through the stumps, through earth-to-timber contact points, or through concealed leads along pipes and wiring. Because they avoid light and open air, termite workings in bearers and joists can be extensive before any surface sign appears.
I check every accessible bearer and joist with a probe and torch, paying close attention to earth contact points, pipe penetrations, and areas of high moisture. Thermal imaging can detect the temperature differential that termite workings create — moisture-laden workings inside hollow timber are measurably cooler than the surrounding structure. This is a standard inclusion in every Admired inspection.
4. Undersized or Incorrect Members
One of the defects I see most often in older homes — particularly those that have been renovated or extended — is undersized or incorrectly specified structural members. This happens when builders or handymen substitute whatever timber is available rather than what AS 1684 requires for the span and load, or when an extension has been added without proper structural consideration of the existing floor system.
The symptoms are often subtle at first: a slightly springy floor, a door that sticks at the top corner, a small ridge in the flooring. By the time the defect is obvious, the structure has usually moved enough to cause secondary damage to walls and finishes. I assess member sizes against the span tables for the species and grade on-site, and flag any discrepancy as a structural deficiency requiring assessment by a structural engineer.
5. Illegal Notching and Drilling
Plumbers and electricians routinely need to run services through the subfloor framing, and when they do, they notch or drill through the bearers and joists. The problem arises when those cuts are in the wrong location or exceed the permitted dimensions set out in AS 1684.
The standard specifies exactly where notches can be cut (within the middle third of a joist’s depth), how deep they can be (no more than one-third of the joist depth), and where holes can be drilled (in the neutral axis of the member, within defined zones). A notch at the wrong location — particularly at mid-span where bending stress is highest — can reduce the effective structural capacity of a joist by 50% or more. I photograph and document every non-compliant cut I find.
6. Inadequate Bearing and Stump Condition
A bearer must bear on its support — the stump or wall — by a minimum distance as specified in AS 1684. Where bearers have shifted, settled, or been cut short, inadequate bearing results in a stress concentration at the support point that can cause splitting or crushing of the timber. I also assess the condition of the stumps themselves: concrete stumps for cracking and spalling, hardwood stumps for decay and termite attack, and adjustable steel stumps for corrosion and correct engagement of the thread.
Settlement in stumps is extremely common in Queensland clay soils, particularly in properties that have experienced extended dry periods followed by heavy rainfall. Uneven stump settlement causes the floor to rack out of level, which stresses the wall frames and can crack internal and external finishes. In severe cases it affects structural connections at the roof level.
7. Subfloor Ventilation and Drainage
AS 3600 and AS 1684 both require adequate cross-ventilation of the subfloor space to prevent moisture accumulation. The minimum ventilation area is 6000mm² per linear metre of external wall — in practice this means evenly spaced ventilation openings around the full perimeter of the subfloor.
Blocked or inadequate vents cause moisture to build up in the subfloor, raising timber moisture content and creating conditions favourable for fungal decay and termite activity. I check both the quantity and positioning of vents — vents on one side of the building only don’t create cross-flow — and assess whether the ground surface in the subfloor shows signs of water ponding after rain.
Why These Defects Are So Easy to Miss
Subfloor defects are genuinely difficult to detect without access, experience, and the right tools. A floor surface that feels firm and looks level can conceal a bearer with 40% section loss from decay, a joist with active termite workings, or a notch that has reduced structural capacity to the point of risk. None of these will be visible in a building inspection that doesn’t include a proper subfloor assessment.
There are also inspectors in the market who are licensed as pest inspectors only — they can identify termites and borer, but they are not licensed to assess structural implications. Equally, some building inspectors carry a building licence but lack real-world experience interpreting structural defects in the field. A QBCC-licensed builder who also holds a pest controller licence can do both at once, in context, and give you a genuinely complete picture of what’s happening under the house.
In 20+ years of inspecting properties across Brisbane, Ipswich, Logan, and surrounds, I’ve found structural and pest defects in subfloors that directly affected the purchase decision — defects the buyers had no idea existed and that would have cost them $20,000 to $80,000+ to rectify. A $550 inspection that identifies an $80,000 problem is not an expense. It’s the best money you’ll spend in the entire purchase process.
What Happens When Defects Are Found?
Finding defects isn’t the end of the process — it’s the beginning of a better negotiation. Every defect I identify goes into the same-day report with photographs, a description of the issue, its likely cause, its severity classification, and a recommended course of action. That report gives you three things:
- Negotiating leverage. Quantified defects are grounds for a price reduction. Sellers regularly reduce the purchase price or agree to rectification works when presented with a detailed, professionally prepared inspection report.
- A rectification roadmap. You know exactly what needs fixing, in what order, and what category of tradesperson you need. There’s no guessing, and no nasty surprises after settlement.
- Peace of mind either way. If the property is structurally sound, the report confirms that. You settle with confidence. If it’s not, you have the information you need to walk away or renegotiate.
Common Queensland Subfloor Scenarios
Pre-1980s Hardwood Homes (Highsets and Lowsets)
Older Queensland hardwood homes are built with species that are naturally durable — spotted gum, ironbark, and tallowwood are extremely resistant to decay and termite attack when dry and undamaged. However, age, moisture ingress, and pest activity can compromise even the best timber. I pay close attention to areas near the ground, around pipe penetrations, and at the base of internal walls where moisture tends to accumulate. These homes also tend to have lower subfloor clearances, making access difficult and defects harder to find without the right equipment.
These homes make up most of what I inspect around Ipswich - Booval, Newtown, Silkstone, Eastern Heights and Brassall are full of them, and a full subfloor assessment is included in every Ipswich building and pest inspection I do.
Post-1990s Pine Frame Construction
Treated pine floor systems became common in Queensland from the late 1980s onward. H2 and H3 treated pine is the standard for subfloor joists in non-termite-risk zones, with H4 or H5 treatment required in high-risk areas. I check the treatment marks on any accessible pine framing to confirm the correct hazard level has been used. Untreated or under-treated pine in a termite-active area is a critical defect — termites will consume it within a few years.
Renovated and Extended Homes
Renovations are where I see the most non-compliant work. Extensions added without proper structural consideration, load-bearing walls removed without adequate beam replacement, and subfloor alterations done without permits are all common. I assess the original structure and any additions or alterations separately and note where work appears to have been done without approval. Unapproved structural work is a liability for the buyer — it’s uninsured, may not comply with current standards, and can affect the ability to insure or resell the property.
Book a Building and Pest Inspection — Same-Day Report Guaranteed
Every Admired building and pest inspection includes a full subfloor assessment as standard — not an add-on, not a checkbox item. I get under the house with a torch, probe, and Tramex moisture meter, and I assess every accessible bearer, joist, stump, and connection point. Drone roof inspection and thermal imaging are also included in every inspection.
QBCC Licence 1313905 covers both builder and pest controller qualifications. That dual licence means one inspector who can assess structural defects and identify pest activity in the same visit, in the same report, with the technical background to interpret what they actually mean for your purchase.
Your report is delivered the same day as the inspection — guaranteed. 300+ five-star Google reviews. Lachlan personally attends every inspection — no subcontractors, no franchises.
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Related reading: moisture in the ground is behind more defects than any other single cause in this region — see what we’re finding in Redbank Plains homes at ten years.
