I N S TA L L AT I O N
Trowel choice makes a difference
Fred Gray from Laticrete Technical Service discusses the importance of trowel selection. It has a
direct influence on the outcome of the tile project.
F
or some time now, the tile industry has seen a
shift in the size of tile that is used to clad a wall
or floor. Historically, a tile was labelled as large
format if it was larger than 200mm x 200mm and
improvements in manufacturing technology have made it
possible to make a tile that is larger than 1m x 1m (not
taking into account the thin porcelain veneer panel) that
can weigh as much as 45kg.
In comparison, it would be difficult to consider
a 200mm x 200mm tile as being large, but proper
installation methods and materials can help to ensure
that any size ceramic or stone tile, masonry veneer, or thin
brick is installed in such a way as to meet the project and
industry requirements for adhesive coverage, distribution
and thickness.
While an increasing demand for greater surface
tolerances is critical to the success of the application,
having suitable adhesive coverage, distribution and depth
under the tile is as critical to the finished installation, as
the flatness and structural integrity of the substrate.
To this end, our own AS3958.1-2007 with caveats,
suggests acceptable levels of coverage for given service
requirements in Clause 5.6.4:
“The contact coverage of the adhesive should be
maximized. Minimum contact coverage should be:
• Internal residential walls 65%
• Residential floors 80%
• Commercial and industrial walls 80%
• Commercial and industrial floors 90%
• Wet areas 90%
• Swimming pools 90%
• External walls 90%
• Exterior floors, decks and roofs 90%”
Further to coverage, adhesive distribution and thickness
requirements are also commented on:
“The coverage should be sufficiently distributed to give
full support to the tile with particular attention to this
support under all corners and edges of the tile.
Expressing contact coverage as a percentage alone
presents some problems. For example, 80% coverage on a
floor tile is not generally acceptable if a significant portion
of the tile (e.g., one edge) is left without any adhesive.
“In conjunction with the amount of contact coverage,
the depth of adhesive is also seen as critical, as this is
an important point for stresses to be absorbed within
the installation. It is essential that the manufacturer’s
instructions be followed.”
So exactly how do you achieve these coverage,
distribution and thickness requirements when installing
ceramic tile and stone? If you are installing a tile that is
150mm x 150mm or less, the answer is easy.
36 | TILE TODAY #92 | www.tiletodaymagazine.com.au
1. An example of a poor attempt to bed a tile. Insufficient adhesive
coverage, distribution or technique in application
Typically, the adhesive coverage of a 150mm x 150mm
tile can be achieved with most likely a 6mm x 6mm square
notch trowel and the technique of moving the tile forward
and back, perpendicular to the notches, whilst pushing it
into the adhesive.
However, as tile or stone starts to become larger in area,
it becomes more difficult to achieve the adhesive coverage,
distribution and thickness required. So it becomes necessary
to take extra steps to ensure these are acceptable for the
proposed installation and meet industry standards.
One such method requires a combination of enough
time to do the work; back buttering the tile (applying a coat
of thin bed adhesive to the back of the tile to fill voids and
help assist in the transfer of the adhesive) as seen in Picture
2. Using a trowel size that will achieve adequate adhesive
coverage, distribution and thickness; trowelling out parallel
ribs perpendicular to the direction of the tiling; moving
the tile forward and back perpendicular to the notched
adhesive ribs during the laying process (to collapse the
adhesive notches and expel air); and using rubber mallets
for further tile embedment, adjusting and levelling in the
process of laying.
I N S TA L L AT I O N
The guidelines for trowel sizes, as stated in AS3958.1
are guidelines and cannot address every installation
circumstance. The substrate tolerances, type, size and
tolerance of tile, intended service requirements of the
installation, climatic conditions, interior or exterior use,
and individual manufacturer’s recommendations are all
factors the installer should consider in selecting the ideal
trowel for the job.
Picture 3 shows a 150mm x 150mm tile in a light duty
installation that was set using a 6mm x 6mm square
notched trowel without back buttering but using the
method of moving the tile forward and back. The 150mm
x 150mm tile falls into the range of good coverage
without back buttering the tile.
2. A back buttered large format tile. The tile adhesive is worked into the back
of the tile with the straight edge of a trowel to fill voids/depressions and cover
the back of the tile with a thin, uniform layer of adhesive just before it is
placed on the notched substrate.
So where service loads dictate; in immersed
installations; where guided by AS3958; or where
coverage is specified in the project specifications, back
butter each tile greater than 200mm x 200mm and select
a notched trowel sized to facilitate the correct coverage
and adhesive thickness.
Key/work the adhesive into the substrate with the
straight side of the trowel, and then comb with the
notched side of the trowel in one direction, parallel to
the progression of the work. Embed the back buttered
tile into the adhesive, pushing forward and back in a
direction perpendicular to the combed ribs and beat
with a rubber mallet to the required level and alignment.
With the correct sized notch trowel, this method
should produce maximum coverage with the corners
and edges fully supported. Systematically remove and
check a tile to assure that appropriate adhesive coverage,
distribution and thickness is being attained.
Choosing the right size notch trowel
AS3958.1 in clause 5.6.2 (a) (1) suggests the following,
and is a good start:
• 200mm × 200mm — 8mm notched trowel
• 250mm × 250mm — 10mm notched trowel
• 300mm × 300mm —12mm notched trowel
• 400mm × 400mm and above — 12 mm notched trowel
and buttering of the tile
Irrespective of the various recommendations for
trowel size, no installation of tile, stone veneer, or thin
brick is ever exactly the same – much like the fact that
substrates, jobsite conditions and tile flatness will vary.
With this said, there is no universal rule on the proper
size notched trowel required that will fit every application
based simply on tile size.
38 | TILE TODAY #92 | www.tiletodaymagazine.com.au
3. A 150mm x 150mm ceramic tile being installed using a 6mm x 6mm
square notch trowel. Sufficient coverage has been achieved without back
buttering for light duty service requirements.
But in the tile industry, some job site failures or tile
breakage have been due to the lack of adhesive coverage on
the back of the tile or stone. A common cause for this type
of failure is an inadequately sized notch trowel selection,
incorrect adhesive application techniques, and neglecting
to check coverage on a regular basis as the work progresses.
(Picture 1 highlights a tile that delaminated due to these
omissions.)
The installation of a 450mm x 450mm tile on a good
surface will probably require a 12mm x 12mm or larger
trowel with back buttering. Initially the process should be
one of trial and error to select the right trowel.
I N S TA L L AT I O N
4. A 450mm x 450mm tile installed using a 10mm x 10mm square
notch trowel and no back buttering – resulting in insufficient coverage
As Picture 4 clearly indicates, a nominal, minimum
coverage of 90% is not being met for a commercial
floor, as an example, as required by AS3958.1. Obviously
insufficient coverage may lead to potential missteps such
as loss of bond, cracking/breaking under load (due to
lack of support), or a hollow sounding tile, especially on
installations subject to impact and point load.
A 10mm x 10mm trowel is not the proper size for a
450mm x 450mm tile where 90% coverage and bedding
is required. For better coverage and bedding – back
buttering – a 12mm x 12mm trowel and tile movement
forward and back is the minimum starting point to
begin to assess what will be required to meet installation
expectations.
Keep in mind that using or specifying larger trowel
sizes does not always guarantee sufficient coverage.
Factors that could alter the recommended size of the
notch trowel used include tile warpage; the angle at
which the notch trowel is held during installation; wear
(age and use) of the trowel; and swirling of the adhesive
during application. All these elements can play a role in
affecting adhesive coverage.
40 | TILE TODAY #92 | www.tiletodaymagazine.com.au
5. A 450mm x 450mm tile installed using back buttering; a 12mm
x 12mm square notch trowel and the forward and back movement –
sufficient coverage and thickness “may” have been achieved.
I N S TA L L AT I O N
Also, back buttering is not some sort of magic panacea
and does not guarantee sufficient coverage or void free
installation. It is just one part of the installation method
for large format tile or critical installations that ensures
the adhesive is worked well into the tile surface; provides
wet on wet contact in hot, dry and/or windy conditions;
and fills minor depressions or keys in the tile.
Picture 6 shows coverage has not been achieved even
though the tile was back buttered and manipulated
forward and back. Note the cross hatched tile ribs that did
not make contact with the adhesive bed on the substrate
resulting in long voids along the length of the tile. The layer
of adhesive back buttered to the tile can often conceal the
real level of “effective” coverage for the unweary eye!
Tile contractors are now faced with tile sizes from
mosaics to large, heavy tiles so becoming familiar with the
appropriate tools and techniques to help make sure that
the tile or stone is properly bedded is important.
Conduct test areas, check coverage during installation,
be consistent when mixing batches of adhesive, back butter
large format tile, clean tools frequently, make sure the
substrate is flat prior to installing the tile or stone, make
adjustments as necessary to achieve maximum coverage,
and make sure the adhesive is of suitable consistency to
“wet out” the back of the tile/stone and the substrate.
finishes&
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42 | TILE TODAY #92 | www.tiletodaymagazine.com.au
6. Shows coverage has not been achieved even though the tile was back
buttered and manipulated forward and back. Note the cross hatched tile
ribs that did not make contact with the adhesive bed on the substrate.
Although these are only guidelines, they can provide a
great place to start.
Correct methods should be followed throughout the
installation and tiles should be checked periodically to
ensure that the tile is consistently achieving acceptable
coverage rates.
As an aside, an interesting format that can present
installation challenges is the large, thin porcelain panel
with sizes in excess of 3m x 1m. With these products,
surface preparation and flatness, adhesive application
and panel laying techniques are even more critical due
to, amongst other things, the product thickness and the
very large areas of single modules. In particular, thinner
product on floor applications are vulnerable.
The current AS3958 guidelines do not address this
new product category and it is hoped the next release will
offer some guidance on these products. In the meantime,
there is more of an imperative to observe the panel
manufacturers’ installation instructions and specification.
For instance, many of the manufacturers require not just
back buttering of the panels but back notching as well. In
a horizontal direction and parallel to the notchings on the
substrate to encourage better mating of the two surfaces.
Larger sheets are laid with the aid of handling frames and
in some instances, vibrators. Q