the i is not pronounced in the combinations ci, gi, and sci when they are followed by a, o or u, unless the accent falls on the i
But what is the role in words like sciare [ʃiˈaːre]?
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Main law about exceptions (MLAE)
Every rule has its exceptions, including the main law about exceptions
You'll find several applications of the MLAE when dealing with grammar or orthography.
The trigraph sci is normally used for denoting the /ʃ/ phoneme in front of a, o or u (like for ci and gi). For etymological reasons, sciare has two syllables (sci-are). The Treccani dictionary makes this apparent by writing the entry as “scïare”, where the dieresis denotes a hiatus.
There's no hard and fast rule: after all, exceptions are cases that cannot be linked together in a rule or subrule.
However, this case is easy: I'm not aware of other words that share the same exception to the main sci rule. As far as I know, sciare and the other words derived from sci (English ski) are the only ones where one should consider the spelling as “(sc)iare” unlike in “(sci)arpa” (the parentheses denote a digraph or trigraph, for clarity).
There is also scia (English wake), where the i is tonic, so sci cannot be considered a trigraph.
See this forum (thanks to Charo for the pointer).
It's not rare, but not frequent either, to hear the word sciare pronounced as /'ʃare/ for analogy with all other similar words.
There are also exceptions in the other direction: an i might follow sc and not be pronounced at all:
scienza coscienza usciere
keep their i for etymological reasons, but there's no /i/ or /j/ sound in the pronunciation.