There were no casualties reported, but the grenade caused some damage to the building in the mountain resort town of Dhur Shwair.
The town is about 20km northeast of Beirut.
It was the fifth explosion to rock Christian areas of Lebanon since 19 March. Four previous explosions killed three people and wounded 24 and caused severe damage to the shopping, residential and industrial areas they targeted.
Fear has gripped Lebanon after the recent spate of bombs, which have been placed under or near cars.
Security forces are taking precautionary measures since the bombings started, conducting night patrols and deploying fire engines on major highways in case of any possible attack.
Lebanon's anti-Syrian opposition has blamed the government security agencies and their Syrian backers for the bombings, while the pro-Syrian camp blames saboteurs bent on destabilising the country in order to invite international intervention.
The explosions come as Syria withdraws its army from Lebanon under UN and US pressure that escalated in the wake of the 14 February assassination of former prime minister Rafiq al-Hariri in a massive bombing on a Beirut street.