Subfloors, Bearers and Joists: What Goes Wrong Underneath

Plenty of older homes around Newcastle sit on a suspended timber floor. It is a good system and it lasts, provided the space underneath stays dry and ventilated.

How it works

From the ground up: footings or piers, then bearers spanning between them, then joists spanning across the bearers, then the flooring.

Each element has a span it can handle. Exceed it and you get deflection, which you feel as bounce.

What a bouncy floor means

Joists undersized or overspanned. Common in older homes and in DIY additions. The floor deflects under load.

Bearers overspanned or piers too far apart. Same problem one level down.

A pier that has dropped. Settlement, or the pier was never adequate. This produces a localised dip.

Rot or termite damage. Reduced effective section, so the timber no longer performs as designed.

Missing or failed blocking. Joists need restraint against twisting. Without it they roll under load.

A little deflection is normal. A floor that visibly moves when someone walks across it is not.

What a sloping floor means

Settlement. A pier or footing has moved down. Common on reactive clay, which is widespread across parts of the Hunter, where soil moisture changes cause ground movement.

Rot at bearer ends. Where a bearer sits on a pier and moisture has been present.

Original construction. Some old houses were simply never level and have not moved since. Worth establishing before spending money.

Take levels across the floor and mark them on a plan. That tells you whether it is a localised dip or a general slope, and it gives you a baseline to check against in twelve months to see whether it is still moving.

The three things that destroy a subfloor

1. Poor ventilation. The subfloor needs cross-flow ventilation through vents in the perimeter walls. Blocked vents, garden beds built up over them, or paving and decking that seals the perimeter all stop the airflow. Moisture then sits in the space and the timber stays damp.

This is by far the most common cause of subfloor problems, and it is usually created by later landscaping.

2. Drainage. Water running under the house from a downpipe discharging at the wall, a blocked stormwater line, or ground falling toward the building rather than away.

3. Ground clearance. Timber too close to the soil. Bearers should have meaningful clearance and never be in contact with the ground.

Fix the ventilation and drainage and most subfloors will look after themselves.

Termites

The subfloor is the primary entry route. Termites need moisture, and a damp poorly ventilated subfloor is ideal for them.

Inspect annually. Look for mud tubes on piers and walls, tap timber and listen for a hollow sound, and probe suspect areas. Keep timber, firewood and stored material off the ground under the house.

Repairs

Additional piers. Reducing bearer spans by adding intermediate piers is often the simplest fix for deflection. Relatively inexpensive.

Sistering joists. Bolting a new joist alongside a damaged or undersized one.

Replacing bearers. More involved because the floor above has to be propped, but routine.

Re-levelling. Jacking and packing at pier heads. Must be done slowly and evenly or you crack everything above.

Improving ventilation. Adding vents, clearing blocked ones, lowering ground levels, or installing a subfloor fan in difficult cases.

Costs

• Additional piers: $400 to $900 each
• Sistering joists: $150 to $350 per metre
• Bearer replacement: $250 to $500 per metre
• Subfloor ventilation improvements: $1,500 to $6,000
• Re-levelling a floor: $6,000 to $25,000 depending on extent

The practical advice

Get under the house, or have someone who will. Photograph what is there. Most subfloor problems are cheap to fix early and expensive to fix once the floor has dropped and every door in the house is binding.

We repair subfloors and re-level floors across Newcastle, Lake Macquarie and the Hunter.