Bowens - The Builders Choice

Building Advice
Frank DiStefano

Q: Dear Bill, My friend who is an apprentice carpenter wants to put hardwood timber flooring down in their house over an existing concrete slab. The ceiling height is a minimum of 2.4m high. My initial thought is to lay like this:

  • 1. clean concrete
  • 2. moisture barrier (plastic sheeting)
  • 3. 12mm ply
  • 4. 19mm floor timbers Is this what you would recommend?

Thanks. (Anthony, Caulfield South)

A: Hi Anthony, there are different ways to lay timber flooring depending on the installer, but in this case, I would consider the direct stick method which is different from the traditional fixing method over concrete (what you’ve outlined).

So you know, the Australian Standard does state boards over 85mm shall be face fixed. However, some manufacturers are now producing wide boards (greater than 85mm) that can be secretly fixed. In addition, some adhesive manufacturers are also warranting direct stick for wider boards. If you choose to use these methods, please do so in accordance with their instructions.

On the other hand, direct stick refers to the floorboards being glued down directly to the concrete substrate. It is long-lasting, quiet to walk on and affordable. Before you do anything, first ensure the concrete slab is level and to industry regulations – this applies to sheet flooring as well as concrete slabs. There can be no more than a 3mm deviation over or under a 3m straight edge (refer to precise regulations set out by the Building Code of Australia). Just as important, the adhesive you use must include a moisture barrier, like Bostik Single Step2, which also includes an acoustic layer. Because you will be laying over an existing concrete slab, it is most likely dry with moisture content less than 5.5%.

Even so, you could have seasonal changes affecting the moisture content later on so making sure the slab is sealed with a moisture barrier is important. New concrete is considered “green” with high moisture content and will always require a moisture barrier prior to installing timber flooring. If you are worried about the un-evenness of your concrete, self-levelling compounds can be used to fill voids or grinding high points can be an option. With a level substrate and a suitable adhesive, engineered timber flooring, such as our exclusive Arbre range, can be a better choice than solid flooring when direct sticking to the concrete. Engineered flooring is much more stable than solid timber flooring.

When the flooring is laid on a levelled substrate like concrete (or particleboard), Bostik single step2 flooring adhesive is screeded on with a 6mm vee notched trowel. Not only does this make sure the correct amount of glue is used (with the correct sized trowel), but the floor will feel and sound more ‘solid’ to walk on (and without that springy feel you get with floating). Always make sure you allow an expansion gap of 10mm at all wall perimeters as per the flooring Installation guidelines. As your project is a renovation then I suggest you consider acclimatising the flooring before laying it. Direct sticking solid timber and engineering flooring to concrete require an understanding of concrete properties, levelling compounds, moisture barriers and adhesive performances and should not be attempted without understanding the installation guidelines.

At Bowens, we offer full installation advice. You can download more information here. Good luck Anthony!

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