Construction moisture in concrete subfloors

Issue 24 – February 2011

It is common practice in new buildings to install floor coverings over concrete floors. Flooring failures attributed to excess moisture in concrete are a common occurrence and, because of the fast-track nature of a lot of today's construction projects, the installation may be rushed and failure can occur. It's important to know what to ask for and how to test whether the concrete is dry before flooring is installed.

Excess moisture in the concrete beneath floor coverings can create mould colonies that can affect people's health long before the mould can be seen or smelt. Reactions can range from headaches and asthma attacks to more subtle effects including concentration difficulties and chronic fatigue syndrome.

Introduction to concrete subfloors

Concrete is an ancient material that can last for thousands of years and floor coverings installed correctly on concrete floors can yield a lasting and durable wearing surface.

Firstly, using the right terminology is helpful because words like 'drying' and 'curing' are often misused.

The '28 day cure' is the approximate time a 100 mm slab takes to cure, and is often mistakenly used as a guideline to install floor coverings. After 28 days, the concrete is not dry because 'curing' and 'drying' are not the same.

'Curing' is the chemical reaction that bonds the ingredients (cement powder, sand, aggregate, and water) together to make concrete. 'Drying' refers to the evaporation of the excess water from the concrete after the curing period.

How long to dry?

The drying time before slabs are ready to receive floor coverings depends on atmospheric conditions and mix design. A 100 mm thick slab allowed to dry from only one side typically requires 4 months to achieve a relative humidity of 75 percent, although in New Zealand it can take even longer. The drying process only begins once the building is weathertight and the slab needs to evaporate about two thirds of the water in the mix before floor coverings can be safely installed.

A concrete floor slab on or below grade intended to receive floor coverings requires a damp proof membrane (DPM) to be installed below the slab.

Why does moisture cause floors to fail?

Often in new buildings the flooring is installed before the concrete has completely dried. Then, when the building is occupied and the interior conditioned air is dry, the floor covering blocks the movement of excess moisture that migrates upward through the slab. In older buildings, a missing DPM or external sources such as leaks, exterior ground levels, or unusual amounts of ground water, can cause moisture to pass through a slab.

When to test and why?

The short answer is, it is prudent to test for moisture levels in concrete slabs when installing floor coverings especially in new buildings. Standards AS/NZS 2455 and AS/NZS 1884, floor covering manufacturers, and adhesive producers recommend testing concrete slabs for moisture, regardless of age or elevation.

To say 'it looks dry', 'it feels dry', or 'it smells dry' isn't enough. Taping a plastic mat to the floor is also an inaccurate indicator. The electronic meters that test for moisture in concrete yield only a spot test, which isn't a sound basis for a decision to install flooring.

New Zealand and Australian Flooring Standards require concrete floors to have a relative humidity level of no more than 75 per cent before floor coverings can be installed. Anything over this can compromise the product or installation, not to mention void all warrantees.

For more information refer to NZ Flooring Industry Standards AS/NZS 2455 and AS/NZS 1884, New Zealand Building Code Clause E2 External Moisture, E2 AS1 and BRANZ Bulletins 330 and 515.


The good news is that there are reliable testing methods available to help you determine when the concrete is sufficiently dry for the installation of floor coverings.

The floor covering industry of New Zealand's standards recognise the Hair Hygrometer for testing moisture in concrete. This method captures the vapour emissions from the top of the concrete as the concrete is drying out.

In-situ probes are the latest technology, effectively measuring moisture inside the concrete slab, and are quickly being recognised by flooring manufacturers throughout the world as useful measuring tools in addition to hygrometers.

Summarised from Codewords November/December 2010, Department of Building and Housing.

Related Standards

  • AS/NZS 2455.1:2007 Textile floor coverings – Installation practice – General
  • AS/NZS 2455.2:2007 Textile floor coverings – Installation practice – Carpet tiles
  • NZS/AS 1884:1985 Floor coverings – Resilient sheet and tiles – Laying and maintenance practices

Published in building.