Timber flooring: why moisture decides whether your floor lasts

Timber is a natural material that takes up and gives off moisture as the air around it changes. A good floor is one that was laid at a moisture content close to where it will sit in service, over a base that stays dry. When flooring goes wrong, the usual causes are cupped boards, wide seasonal gaps, or boards lifting off the joists. Almost all of them trace back to moisture in the wrong place at the wrong time, rather than a faulty product.

This article covers solid timber strip flooring and the base beneath it. It draws on the Forest and Wood Products Australia design guide, Timber Flooring: Design guide for installation (WoodSolutions Technical Design Guide 09).

Start with the product and the standard

Australian hardwood flooring is milled to AS 2796 (Timber, hardwood, sawn and milled products), softwood to AS 4785, and seasoned cypress to AS 1810. Boards graded to these standards have defined defect limits and are supplied within a stated moisture range. Imported flooring may or may not meet Australian grading rules, so ask for the grade and the standard it is graded to.

The design guide states that flooring is generally supplied within a moisture content range of 9% to 14%, and that the average across a pack is typically around 10% to 12.5%. That range exists because it matches the moisture content most Australian floors reach in service. It is also the reason the next point matters.

Moisture content: check it, record it

The guide puts the responsibility squarely with the installer: it is the floor installer's job to check that the timber is at the right moisture content at the time of installation. Practically, that means:

• Deliver late and store dry. Flooring should not be delivered until it can go under permanent cover, and delivery during rain should be avoided.
• Measure with a meter. Readings should be taken from a sample of boards across the packs, and recorded.
• Compare with the expected in-service level. If the average supplied moisture content is already near the average it will reach in the finished building, acclimatisation is not necessary.
• Allow for the climate. Humid coastal areas, northern NSW, shaded gullies and forested sites can push in-service moisture higher, and extra expansion allowance may be needed. In consistently air-conditioned buildings or dry inland areas, boards can shrink after laying.

Acclimatisation is a tool, not a ritual. The guide notes it only works if each board is exposed to the in-service air (packs opened and restacked), it works at different speeds for different species (dense species like spotted gum respond slowly), and in dry conditions it does not remove the need to allow for expansion in wet weather. Pre-finished products may have manufacturer-specific advice, and thin overlay floors are often best left to adjust after laying.

Over a timber subfloor: ventilation and a dry ground

For houses on piers or stumps, the space under the floor is part of the floor system.

• Ventilation. The guide notes that the NCC subfloor ventilation requirements are mainly aimed at protecting the framing, which tolerates more moisture swing than a finished floor. For site-sanded-and-coated tongue-and-groove floors it recommends at least 7,500 mm² of ventilation per metre length of wall, evenly spaced for cross ventilation, which is more than the NCC figure of 6,000 mm² for higher-humidity areas it cites. Ventilation is covered by clause F1D8 for buildings under Volume One, and the equivalent provisions for houses.
• Ground cover. A polyethylene membrane over the soil reduces moisture entering the subfloor space, and the NCC allows it to reduce ventilation requirements. The guide cautions that if you go below its recommended ventilation, you must make sure the space stays dry in every season.
• Drainage and enclosure. Subfloors that are bricked in, damp, or subject to seepage need more than vents. Water has to be kept out of the space at the source. This is where site stormwater drainage and the ground interface covered in our envelope waterproofing article come in.
• Open undersides. Where the floor underside is exposed to weather, wind or fog, it can shrink or swell severely. Options include sealing the underside or lining it.

Over a concrete slab: assess before you lay

Slabs hold a lot of water for a long time. The guide says slabs on ground should be built with a continuous under-slab vapour membrane (the NCC requires this under F1D7 for floors on ground, referencing AS 2870), and that timber floors should not go down until the slab has been assessed. It notes the slab will generally need to have cured for at least three months after the roof and walls are in place and the building is enclosed.

Before gluing or fixing over a slab, check:

1. Moisture. Test the slab to the method set by the adhesive and moisture barrier manufacturers, which differ between products. An unassessed slab is the commonest cause of cupping.
2. Surface contaminants. Curing sealers, paint, oil and old adhesive can stop adhesion. If water droplets on the slab do not soak in, suspect a sealer. The guide describes mechanical removal, such as grinding, as most effective.
3. Flatness. Many adhesive manufacturers require no more than 3 mm deviation under a 3 m straight edge. Levelling compound or grinding fixes this, and should be done before any applied barrier is primed.
4. Soundness. Weak or patched surfaces fail under adhesive-fixed floors. Bond testing is an option when in doubt.
5. Construction joints. Unsealed joints let moisture rise into the boards above them, and a surface barrier will not stop it.

An applied moisture vapour retarding barrier is a safeguard against residual slab moisture. As the guide says, it is not a waterproof membrane, and it will not fix a slab that is wet from a leak.

Leaks are the other half of the story

Many flooring "defects" are plumbing or envelope leaks that happen to show up in the floor. A dishwasher hose, a leaking shower, a failed window flashing or a damp wall can wet boards for months. A floor that cups near a wall, a bathroom door or a slab joint is telling you where to look. Our posts on damp patches and mould and wet area waterproofing explain what to check first. Fixing the source comes before replacing a single board.

Apartments add approvals and acoustics

In a strata building, swapping carpet for timber often needs owners corporation approval, and the floor and underlay system usually has to meet an acoustic requirement. See hard floors in apartments and acoustic underlay and testing before you buy product.

Questions worth asking your builder or installer

1. What species and grade, and to which standard?
2. What was the moisture content on delivery, and where is it recorded?
3. How was the floor stored on site, and is acclimatisation needed for this climate and building?
4. For a timber subfloor, how much ventilation is there, and is the ground dry?
5. For a slab, what moisture test was done, what were the results, and which barrier and adhesive are used?
6. What expansion allowance is left at walls and fixed elements?
7. Who is responsible if gaps or cupping appear in the first year, and what does the warranty cover?

If something does go wrong after handover, our article on statutory warranties explains how defect periods work in NSW.

How we approach it

We treat flooring as the last step of a dry-building sequence. The envelope is closed, the slab or subfloor is assessed, moisture readings are recorded, and the floor is laid to the manufacturer's and industry guidance. The readings go in the handover record. If you are planning a new floor or have one that is cupping or gapping, get in touch.

Sources

• Timber Flooring: Design guide for installation (Technical Design Guide 09), Forest and Wood Products Australia, WoodSolutions
• NCC 2022 Volume One, Part F1 Surface water management, rising damp and external waterproofing, Australian Building Codes Board (F1D6 to F1D8)

This article is general information, not technical or legal advice. Moisture limits, ventilation rates and slab tests vary with the product, species, climate and building, so follow the manufacturer's instructions, the current standards and your installer's specification. Full website disclaimer