The Development of Overlay Flooring
The option of overlay flooring that developed in the 1990’s is an extremely important concept in timber floor surfaces. The reason for this relates to floor heights. Floor heights within houses often have some disparity. So for example, carpet with an underlay may be 8 or 9 mm high whereas tiles on a good bed of adhesive may be 14 mm high. If you introduce a flooring type to a house that abuts some existing floor and that is considerably higher it can readily be appreciated that there will be difficulties. At the very least there will be some sort of potential tripping hazard introduced or some form of ramp or covering strip to delineate between the two surfaces. This has several issues not the least of which is the fact that it is not an attractive option let alone a requirement for some sort of alien cover strip that doesn’t exactly match either flooring.
The solution to this, developed in the 1990s, was direct stick flooring using flexible polyurethane adhesive and it was readily seen that it would be important to develop flooring of an appropriate thickness to match other floor heights within the house. Testing of a number of species showed that timber flooring between 80 mm in width through to 105 mm in width could easily be manufactured from most Australian species and could be directly applied over the concrete subfloor. (So long as the sub floor has a moisture resistant epoxy applied to the surface then this form of flooring it has been extremely successful. The primer should have both water resistant and concrete consolidation features and these take around 12 hours to dry as apposed to quick drying temporary measures.)
Overly flooring between 12 and 14 mm in thickness has become the standard timber installed in houses. On an appropriate bed flexible polyurethane adhesive this timber performs very well and can be installed and finished to an appropriate floor height based around the overall height of 14 mm. Thus overlay flooring directly addresses a particular need relative to other floor heights within the house.
The testing of the overlay principle reveals certain important findings.
Firstly, it was demonstrated that where appropriate moisture barrier and glue were used any thickness of flooring could be successfully applied to a concrete surface so long as that flooring size was within the ratio of dimensional stability. So for example a 130 mm wide board by 19 mm thick board of any Australian species could easily be applied over concrete so long as the right adhesive and moisture membrane were used.
The second thing that was discovered was that where the ratio of size to thickness was not maintained with a number of eastern states species of timber, the process was extremely prone to failure. There is a huge difference between Western Australian species of timber and New South Wales, Queensland & Victorian species when it comes to dimension stability. It was therefore found that while it was easily possible to produce a 135 mm x 14 mm thick timber in Western Australia Blackbutt, Western Australia Jarrah et cetera it was not possible to do this with many other species from other states for use in the direct stick situation. This is not to say the other thicknesses of timber cannot be used in flooring but that these wider boards manufactured in the eastern states should be applied over plywood. The plywood acts to separate the timber from the concrete and thereby works effectively to counteract the slight moisture flow effect to that occurs in connection with epoxy primers.
Another feature of testing that is important to remember occurred when very thin overlay timbers were applied over concrete. Some timbers were cut to 10 mm thickness and 8 mm thickness and applied over a primer and flexible polyurethane adhesive. It was discovered that these timbers, while they stayed in situ very well tended to follow the uneven curvature of the concrete subfloor. Therefore the sanding process effectively levelled the floor by taking away almost the entire wear layer of the timber at high points while maintaining the entire wear layer at low points. Effectively what this means is that the timber floor could never be re-sanded. Thus it was recommended that timbers at 8 to 10 mm in thickness were installed over plywood. Large plywood sheeting applied over the concrete would automatically introduce better levels to the overall surface of the concrete and therefore when the thinner timber was applied over the surface of the plywood it would result in a floor that was much more level and would in fact be able to be re-sanded subsequent to its initial use.
Based on these initial test results which we undertook in the early 1990s we made the following recommendations.
Firstly, that as a general rule care should be taken to ensure dimensional stability for the installation of timber as overlay flooring dependent on species.
Secondly, that thin timbers 8 to 10 mm in thickness should be installed over plywood.
Thirdly, that it would be a good idea to promote an overall floor height of 14-19 mm in order to ensure that floor heights were appropriate to or at least close to the level of adjoining floorcoverings.
Finally, that Mills be encouraged to produce timber of an appropriate thickness for use as overlay flooring and that where the dimensions fell outside what would be deemed to be appropriate for direct stick they be encouraged to provide instructions directing the installer to use plywood.
Recommendations For Australian Solid Timber*
Recommendations For Australian Solid Timber*
- 8mm All Species OVER PLYWOOD
- 10mm All Species OVER PLYWOOD
- 12-14mm up to 105mm wide All Species
Direct Stick or Plank On Ply - 12-14mm over 105mm wide WA Species
Direct Stick or Plank On Ply - 12-14mm over 105mm wide ES Species
Plank On Ply [Unless Size Limited]