Just read this from an aussie online mag
"Decks are subjected to surface
temperature changes that can
be as high as 95°C depending
on their location and surface
colour. Dark charcoal colour tiles can
heat up to 85°C in summer and be as
low as -10°C on a clear winter night.
These extremes of temperature result
in significant temperature-induced
movements in the surface tiling and
the substrate sheeting. Cement-based
sheeting and installed tiling generally
have coefficients of movement with temperature
around 10 X 10-6 per degree
Celsius. Plywood has a much lower coefficient
as it is constrained by low expansion
along the grain which is around a
third of this expansion coefficient. For a
temperature change of 80°C — taken as
a mean of the temperature change within
the body of the tiles — the 10 X 10-6
per degree Celsius equates to 0.8mm of
movement for each metre of tiling.
The installation detailing for membranes
precisely explains how to cater
for movements across joints in the substrate
material. Bond relief is employed
to prevent fracturing of the membrane
over the joints in the substrate sheeting.
But with tiles being directly adhered
onto the membrane it is essential
that these movement joints in the substrate
are continued through the tiling.
The James Hardie Company realised
this some time ago. Their installation
instructions for the use of their compressed
sheet on decks stipulate that
the joint around the edge of the sheeting
panels be carried through to the surface
of the deck. However, the maximum
size of the sheet width — 1200 mm —
does not correspond with spacing control
joints in tiling of 4.5m as given in
AS 3958.1 – 2007 ‘Ceramic tiles - Guide
to the installation of ceramic tiles’. This
usually results in the lack of continuation
of the joint to the surface. Not having
joints in the tiling above joints in the tiles as shown in Figure 1.
To prevent this reflection cracking,
it is essential to make sure that the
control joints in the tiling are directly
over the movement joints in the substrate
sheeting. When smaller sheeting
than the design spacing of joints chosen
in the tiling are specified, they need to
be structurally joined so joint spacing
will coincide with the tile joints. This
requires careful forward planning. The
fixer of the substrate sheeting should
be supplied with the tile layout prior
to laying the substrate. This requirement
is rarely followed in the industry.
In fact, when the substrate is being laid
the overlaying tiling has not even been
selected.
Some of the substrates used have
brittle characteristics. With brittle
material, it is essential that they have
roughly square dimensions, otherwise,
if they are restrained they will crack.
This is due to them having strong
strength characteristics in compression,
so in heating up they will expand
without any trouble. However, in cooling,
due to their weak tension characteristics,
if restrained they will crack if
they are not roughly square. Cracking
can be caused by restraint if the joints
in the tiling do not correspond with the
substrate joints. This is the reason for
cracking in the cement sheeting shown
in Figure 2.
In this case the membrane on top of
the sheeting failed over the sheet cracks
with resulting leakage as illustrated.
If the control joints in the tiling do
not line up with the joints in the substrate
sheeting, and there is no evidence
of reflective cracking as shown
in Figure 1, there will be considerable
shear stresses in the tile adhesive as it
will have to accommodate the different
temperature movements between the
tiles and the substrate. If these shear
stresses become critical they will lead
to fracturing of the bond between the
tiles and the substrate, resulting in
excessively drummy tiles. This lack of
alignment of joints is a common source
of drummy tiles.
Therefore, with direct adhesion of
tiling over a membrane on a sheeting
substrate, it is essential the whole job
is well co-ordinated between substrate
installer and the tiler. With joints in
the tiling being located directly over
those in the substrate, the waterproofing
membrane will also be better able
to accommodate the temperature movements
in the deck.
