Choosing the subﬂoors
Floating and direct adhesive ﬁx applications a wide range of subﬂoors can be put
over provided the subﬂoor is in an appropriate condition to accept the ﬂooring as
outlined below. In the case of direct adhesive ﬁx applications the structural integrity
of the subﬂoor must be adequate to withstand forces associated with board expansion.
In most cases subﬂoors are going to be concrete slabs or sheet ﬂoors of
plywood or particleboard. Underlay, which could be acoustic, can provide an intermediate layer.
Other subﬂoors suitable to some product manufacturers also include existing timber ﬂoors, Masonite,
resilient ﬂooring and ceramic tiles. Some of products can be ﬁxed direct to joists or battens.
For the speciﬁcs concerning to the preferred installation method, the product supplier’s
installation recommendations need to be viewed and then recommendations adhered to.
According with the Building Code of Australia, engineered ﬂooring is not to be installed
in wet areas such as the bathroom, toilet and laundry. Kitchen and food preparation areas
are not deemed to be wet areas.
Engineered flooring -:Subﬂoor construction, ﬂatness and cleanliness
Subﬂoors need to be sound and structurally agreed with related Australian construction standards.
Any determined issue with an existing subﬂoor or overlaid product such as squeaks in an existing
timber or sheet subﬂoor or tile ﬁxing integrity that may affect the performance or appearance of
the installed engineered ﬂoor need to be adjusted earlier to ﬂoor installation.
All subﬂoors need to be sufﬁciently ﬂ at to accept the ﬂooring system
(ﬂoated or direct adhesive ﬁ x). For ﬂoating ﬂoors this generally does not exceed 3 mm
beneath a 1 m long straight edge. For direct adhesive ﬁ x applications 3 mm beneath a 3 m
long straight edge applies.
Excessive variation in the sub-ﬂoor may lead to excessive deﬂection of a ﬂoating ﬂoor panel
with foot trafﬁc. Such movement may also lead to excessive noise, therefore proper preparation is
critical in order to achieve a solid and quiet ﬂoor underfoot.
Individual ﬂooring products having speciﬁc recommendations or as recommended by adhesive
manufacturers may be tighter than this and in such cases will apply. Sometimes concrete
subﬂoors are not sufﬁciently ﬂat. In such cases we need to level the subfloor using levelling
compounds, grinding etc. When there is a timber subﬂoors, packing of joists and sanding of sheet
subﬂoors may be essential.
The surface on which the ﬂooring is to be adhesive ﬁxed must be free from all
contaminants that could stop or reduce the efficiency of the adhesive. In certain
the surface required to free from any waxes, grease, paint, sealers and other similar
substances. Concrete slabs may be required Sanding or grinding to provide a suitable surface.
For ﬂoors direct adhesive ﬁxed to concrete subﬂoors the following additional provisions apply:
Any intermediate layer between the ﬂooring and the subﬂoor (e.g. ceramic tiles over a slab)
needs to be removed or if permitted to be laid over by the engineered ﬂooring manufacturer,
it needs to be tested to ensure it is adequately adhered to the subﬂoor. If adhesive ﬁxing to
an intermediate layer the ﬂatness provisions outlined above apply
On a concrete slab we apply levelling compounds according to the manufacturer recommendations.
Also care is needed to ensure sealers used in conjunction with the product are completely covered
by the levelling compound. Sealers prevent adhesion. The levelling compound also
needs sufﬁcient tensile strength to accommodate the expansion forces from the adhered ﬂooring.
Levelling compounds with low tensile strength are not sufﬁcient for adhesive ﬁxed ﬂoors.
The levelling compound in the photo can be gouged with a steel ruler
Concrete slab subﬂoors protection from moisture-:
Regardless of whether the ﬂoor is ﬂoated or direct adhesive ﬁxed, phases need to be taken to prevent possible
moisture uptake into the ﬂooring from the concrete subﬂoor. Moisture absorption from beneath the ﬂoor can effect
in larger levels of expansion. It is resulting in buckling, adhesive bond failure and a cupped or crowned appearance.
It is important to guarantee that slabs have under slab moisture retarding barriers
that comply with AS 2870 - Residential Slabs and Footings. These barriers distinct
the concrete from possible sources of moisture that may delay or could stop the concrete
from drying sufficiently. Provided they are installed correctly, water vapour transmission
through them is minimal. It has been shown that such barriers form close contact with the slab
avoiding moisture movement between the barrier and slab. Puncturing, gaps or inground piers can result
in localised areas of higher moisture and slab edge dampness also needs to be considered
At the time of engineered ﬂoor installation the installer will usually not be aware of what the actual water-cement
ratio was , how well the ground moisture vapour barrier was installed or
how well the concrete was placed. The presence of beams also requires to be considered. The weather, including wind, temperature and humidity
variations, also inﬂ uences drying. Due to that, regardless of how old is the slab, its moisture levels need more assessment before laying an adhesive
ﬁxed, engineered ﬂoor.
Slab moisture can be assessed with concrete moisture meters as well as in-slab relative humidity tests.
Such measures along with assessments of the risks outlined above are necessary for all slabs.
When moisture meters are used new slabs may give readings with a concrete capacitance moisture meter of
about 6% a few days after placement. Within 3 months the readings may be down to about 4% and after two
years readings may settle to about 1.5%. Once a slab is known to be reducing in moisture content like this
and at least 3 months old, other means of protecting against possible slab moisture can be employed if
considered necessary. Some engineered ﬂooring manufacturers will leave the onus on the installer while others
recommend an applied moisture retarding barrier or similar as an added safeguard.Concrete moisture meters used
to asses moisture as well as in-slab relative humidity tests.
There are some of the limitations of concrete moisture meters. They measure moisture near the
top of the slab and once a ﬂoor is laid moisture levels usually increase toward the top of the slab.
In-slab relative humidity measurement is a method of slab moisture assessment that is increasing
in popularity throughout the world and is considered to provide a more accurate assessment
of the potential for slab moisture to affect a ﬂoor. The test takes into account that moisture
in a slab increases toward the top surface of the slab once an engineered ﬂoor is laid. In-slab
relative humidity remains relatively high in all slabs and information from overseas suggests that
in-slab relative humidity’s of about 80% are at a level where timber based ﬂooring products can be
considered. In slab relative humidity requires holes to be drilled in the slab, the holes plugged and
readings with a hygrometer taken some time later. Again other means of protecting against possible
slab moisture may also be part of engineered ﬂooring manufacturer recommendations and should be followed.
Therefore before laying a direct adhesive ﬁx ﬂoor the level of moisture in a slab needs
to be assessed and when down to suitable levels an applied moisture vapour barrier,
if recommended by the ﬂooring product manufacturer, is to be used. If the ﬂoor is
being laid on battens a 0.2 mm polyethylene plastic over a slab should be used.
With ﬂoating ﬂoors an underlay and moisture retarding layer is a standard recommendation
of engineered ﬂooring manufacturers. Most of engineered ﬂooring products have speciﬁc
underlays that have to be used and contain an integral moisture retarding layer.
Engineering ﬂooring product supplier recommendations concerning the desired system are
to be followed and underlay or applied moisture vapour barrier products are to be applied
in accordance with manufacturer recommendations.
Timber and sheet subﬂoors – Protection from moisture
Engineered ﬂooring can be laid over particleboard or plywood subﬂoors on joists
and often on solid timber ﬂooring on joists as either a ﬂoating ﬂoor or an adhesive
ﬁxed ﬂoor. It is essential to guarantee possible moisture in either the sheet or timber
subﬂoor and the subﬂoor space beneath does not affect the ﬂooring being laid
Other subﬂoors and those needful acoustic rating
Engineered ﬂooring may also be installed over subﬂoors not speciﬁcally
outlined above, but as these are less common the ﬂooring product manufacturer needful
regarding the product and installation system to be used.
A degree of acoustic isolation is achieved with either ﬂoating installation on
underlay or with some adhesive ﬁxed systems. Always requirements speciﬁc underlays and installation
practices need to be employed to achieve the required sound isolation. Features relating to the thickness
of concrete subﬂoors play a signiﬁcant role and thinner timber ﬂoors generally result in less sound transmission
than thicker ﬂooring. Underlay performance relates more to the design of the underlay rather than the thickness.
Timber ﬂooring systems including acoustic underlays often provide about 10 to 20 dB attenuation.
Hereafter with such applications advice from the ﬂooring product manufacturer and others will likely be necessary.
As engineered ﬂooring differs considerably in its construction between manufacturers not
all products are suitable for installing over heated subﬂoors. Therefore if installing a ﬂoor
over a heated subﬂoor it is necessary not only to choose the correct product but also to follow
the speciﬁc manufacturer installation instructions.