Calaio converted from the spot in the second half after Simon Kjaer brought down Mattia Destro. Victory for the Tuscan club is their first since January 4 and sees them go five points clear of the drop zone to give their survival hopes a major boost.
Roma dominated possession but it was the hosts who looked the more dangerous on the break, with Destro, twice, and Calaio wasting good opportunities in the opening period.
Erik Lamela’s volley from outside the box was Roma’s best effort of the first half and just six minutes after the break, Siena took the lead.
Roma keeper Maarten Skelenburg guessed the right way but Calaio’s penalty kick to was too good.
Daniel Osvaldo, Roma’s top scorer, was introduced from the bench on the hour mark to make his first appearance of the year after injury but he could do nothing about the result. Roma, who had the chance to leapfrog Internazionale into fifth, stay sixth after the loss.
Napoli moved a point behind the Giallorossi after they eased to a 2-0 home win over Chievo. That victory came courtesy of first-half goals from Miguel Britos and Edinson Cavani. Chievo are 14th, two places above Siena, and were made to rue their failure to pull 12 points clear of the relegation zone. Britos opened the scoring on 15 minutes, the Uruguayan defender powering a header past Stefano Sorrentino in the Chievo goal. And that lead was doubled in the 38th minute after Sorrentino brought down Cavani, who dusted himself off to take the kick.