Sappada can be reached in different ways:
Coming from Veneto you have to take the motorway Mestre-Belluno until Pian di Vedoia and then you have to continue, following the indications of Pieve di Cadore. Then you have to carry on Auronzo and Comelico; near Auronzo there is a crossroads and you have to turn on the right, following the indications, for about 30 km.
coming from Friuli Venezia Giulia you have to take the motorway Palmanova-Tarvisio until the way out of Tolmezzo-Carnia, then you have to take the main road 52 which through Ovaro and Rigolato brings to Sappada.
From Veneto you can go with the train until Calalzo di Cadore and then take a bus of Dolomitibus of the line Callazo-Sappada. From Udine there is a bus route of Saf which brings until Sappada, or with the train until Carnia and then take the same bus.