You can reach Piuro:

from South (Lecco - Milano)
covering the state road n° 36 that skirts the lake of Como on the Lecco side;

from East (Switzerland - St. Moritz)
cross the Maloja Pass and the Bregaglia valley;

from North (Switzerland - Chur - Madesimo)
cross the Spluga Pass and the S. Giacomo valley.