Under the floor

Many timber-framed floor problems can be traced to moisture levels, excessive loads or borer.

Under the floor hero

Houses with timber-framed floors can be at risk of having poor underfloor ventilation. This can lead to rising damp, dry rot, timber decay, fungal growth and borer. Simple and regular maintenance can prevent serious problems developing.

Underfloor access

Buildings with concrete perimeter foundations should have been constructed with an access point. If this is not the case, then one will need to be put in the floor inside the house or through the foundation wall (if it is high enough).

Not all homes are built with sufficient ground clearance, so to get access under your floor you may need to create a crawl space by digging out the ground.

For all homes, check the underfloor for construction waste, as this may reduce ventilation.

Wooden underfloors

Signs of dampness can include: rot, musty smells, borer, persistent condensation, mildew, mould, sagging floor joists, sagging and swelling particleboard floors, bubbling under vinyl flooring, carpet rot and corroding fasteners and metal fittings.

You can check timber for rot by pressing a screwdriver into it. If it gives easily, the timber is rotten and will need to be replaced. This is work for professionals. A building consent will be required for re-piling and replacement of structural timbers such as joists.

Dampness may be caused by leaking pipes, blocked drains, surface run-off going under house, a high water table and/or inadequate underfloor ventilation.

Pipes/drains: Check downpipes, plumbing and drains. If there are damp patches beneath the floor where there is a bath or shower above, this could indicate a leaking pipe or waste pipe. Repair any leaks. Make sure stormwater or sewer drains or gully traps are not blocked – this should be an annual check.

Built-up gardens: Look for raised/built-up gardens against the foundations, this can effect ventilation and can divert surface water under the home.

Run-off: You may need to create channels or intercept drains to divert water away from the house, paths and driveways. Do not direct water towards or under the house.

Inadequate ventilation: A poorly ventilated space will allow moisture to evaporate up into the structure above.

Some steps to take are:

  • clear the underfloor of obstructions such as stored timber or rubbish and ensure vents are uncovered
  • check the clearance of wall claddings above the adjacent ground as this can also contribute to a lack of ventilation
  • if ground levels outside the house are higher than under the house, lower them to allow increased ventilation and reduce the chance of water ponding under the house.

You may need to cover the ground with heavy-duty polythene to prevent moisture evaporating from under the house and making its way into your home.

Make sure the polythene is taped at the joints and trimmed neatly at perimeter walls and around piles to ensure it is a snug fit. Weigh it down to stop it from moving.

Movement in the joists or bearers: You can spot this if the floor flexes or vibrates when it is walked on. This can be caused by borer, rot, bearers or joists that are spaced too widely or not substantial enough, or movement of the piles. Work to remedy rot or borer will need to be carried out by professionals.

If there is movement of less than 20mm between bearers and the top of the pile, you can pack the gap with bituminous felt wedges, but this still might be masking a bigger problem. It’s best practice to call in the pros to give it the once over.

Loose wiring/pipes: Secure wires to the framing or battens with clips. If wiring has frayed, call an electrician to repair it. Add strap supports for water pipes.


Rotten timber: This can indicate high underfloor moisture levels or that the piles have reached the end of their life.

Test by pushing a screwdriver into the pile just below ground level. The wood will give easily if it is rotten. The piles will need to be replaced with H5 treated timber or pre-cast concrete piles.

See moisture and rot above for tips on reducing underfloor moisture levels.

Corroded fixings: Corrosion is caused by moisture or a corrosive atmosphere such as close to the sea or in geothermal areas. Corroded fixings, such as nails and screws, will eventually fail.

Address the cause of moisture.

Replace badly corroded fixings with hot-dipped galvanised or stainless steel fixings. Stainless steel fixings are required in some situations, such as within 500m of sea spray, on decks and in some sub-floor situations. Grease applied to hot-dipped galvanised fixings will extend their corrosion resistance.

For less severe corrosion, remove rust and apply zinc-rich primer, metal primer and finish coat.

Loose bolts: This could be due to timber shrinkage after bolt installation. Re-tighten the nuts.

Concrete block foundations

Cracks in concrete block work, especially in corners, party walls and over openings, can indicate:

  • Movement in clay as moisture content changes.
  • Ground subsidence.
  • Undermining from excavation.
  • Tree roots adjacent to the foundation.
  • Corrosion of the reinforcing steel, or
  • Damage due to frost.

Establish the reason for the cracks. Monitor the crack. If it is stable, seal it with an exterior-grade sealant. If it is still moving, consult a structural engineer.

Structural Steel

Sagging beams: This is usually caused by overloading. Check with a professional and install additional support if necessary.

Corrosion: Corrosion is generally caused by damage to the hot-dipped galvanising or a lack of galvanising.

Remove surface rust with a wire brush, sand and treat with rust-passivating solution, then prime with a zinc-rich primer and metal primer and repaint. In future, for external structural steel, wash it down frequently and re-treat at the first sign of rust reappearing. In the case of significant corrosion, you’ll need professional help.

Where the beam is inside, determine and eliminate where the moisture is getting in to prevent further corrosion.

Cracked welds: This is caused by overloading of the member or poor welds. Consult a professional to check that the loads are acceptable. If they are, have the joint rewelded by a professional.


Foil underfloor insulation is no longer recommended. It doesn’t work as well as an insulator as modern replacements and it can be a safety hazard.

WARNING: Before you touch foil insulation get a competent person to test that it’s not electrically live. People have died when they touched live underfloor foil.


Borer loves damp or untreated timber. Address the cause of moisture and treat for borer. If the borer infestation is severe, get in the professionals. See Borer and other pests for more information.

This page was put together with the help of BRANZ.

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