The requirements for current large and future extremely large telescopes as well as the quick development of IR instrumentation demand a proper characterization of precipitable water vapour (PWV) above astronomical sites. A comparison of PWV estimations from a photometer and a GPS (Global Positioning System) above the Observatorio del Roque de los Muchachos (ORM, La Palma, Canary Islands, Spain) was carried out and it was found a linear relation between both measurements. Such relationship will allow the calibration of the GPS measurements recorded at ORM for the period June 2001- December 2008. These large time series of PWV estimations from GPS were used to perform a statistical analysis of water vapour content above this astronomical site. Average annual value of night-time PWV of 4.86 mm was found. It was also found a clear seasonal behavior of the PWV above ORM, with smaller water-vapour columns during winter nights (average 3.36 mm). The largest values of PWV are reached in the summer nights (average 6.75 mm). The data indicate that a significant percentage of nights (~38%) are well suited for thermal infrared observations (with PWV < 3 mm), and 71% of nights present a "fair" or better IR observation opportunity at ORM.