Apart from the GR20 (which the most people climbing Rotondo surely use) and the approach via Manganello Valley as mentioned one also can access the Monto Rotondo south side and Petra Piana hut from the west.
From the Golf of Sagone it is a one hours drive via Vico and Guagno=les=Bains to Guagno (Gîte d'Etappe). The road continues several km upwards and changes into a gravel road just behind a garbage dump. Several hundert meters ahead the trail starts and a signpost mentiones the Bocca Mangenello. Altitude about 1000m.
The path decends close to the river, several serpentines can be cut short. Contouring at about 900m meters above the river it leads through beautiful forests to an abandoned bergerie under big lariccio pines. The Bocca is already visible and reached through open terrain, i.e. sun-exposed. 3-3.5h from the start.
Here one joins the GR20 an the hut is visible shortly afterwards. A half hour of scrambling with little up and down and one stands in front of Refuge Petra Piana.