I have seen this on the Schluter website on the data sheet.
Balconies / terraces
As an uncoupling membrane, Schlüter
®-
DITRA 25 neutralises the stresses between
the substrate and the tile covering that are
caused by the frequent and extensive temperature
changes occurring on balconies.
Furthermore, Schlüter
®-DITRA 25 serves as
a bonded waterproofing assembly together
with the tile covering (see notes on waterproofing).
The substrate (concrete, screed)
must be sufficiently sloped.
For refurbishments, the existing surface
assembly can generally remain, provided it is
sufficiently sloped and load bearing. Prior to
the application of Schlüter
®-DITRA 25 over
existing and cracked tile floors, the loose
tiles must be removed and the remaining
cavities filled using an appropriate mortar.
The preparations also include ensuring a
proper slope in the substrate.
We recommend Schlüter
®-DITRA-DRAIN
(see product data sheet 6.2) for coverings
with side lengths exceeding 30 x 30 cm
Is it me or does it read that you can on to existing tiles or is it different because it is on a balcony or terrace ??????????????