Roof knocking is the practice of warning inhabitants of a building before it is bombed to give the inhabitants time to flee the attack. The practice has been used by the Israel Defense Forces (IDF) before attacking buildings that they suspect contain weapons. The term "roof knocking" (Hebrew: הקש בגג) or "a knock on the roof" has also been used to describe the IDF practice of firing a non-explosive missile at a roof to get people there to leave. As early as 2006 the IDF had the practice of warning the inhabitants of a building that was about to be attacked.
Roof knocking was used during the 2008–2009 Gaza War. In the six months prior to its use, Israel collected data on Hamas members, which they used to issue warnings. Typically, Israeli intelligence officers and Shin Bet security servicemen contacted residents of a building in which they suspected storage of military assets and told them that they had 10–15 minutes to flee the attack, although sometimes the attack occurred 5 minutes after the warning.
However, Hamas asked civilians to stand on the roofs of buildings to dissuade Israeli pilots from attacking. Some residents that were warned about an impending bombing climbed up to their roofs. When faced with this situation, IDF commanders have either called off the bombing or launched a newly developed, relatively harmless, non-explosive missile at empty areas of the roof, in order to frighten the people gathered on the roof into leaving the building. In several instances, the IDF has used a sound bomb to warn civilians before striking homes.
The Israeli Government reported that the warning systems were not perfect but still highly effective. Aerial video surveillance by IDF forces confirmed many times the departure of civilians from targeted areas prior to an attack, as a direct result of the warnings. When Nizar Rayan, a top Hamas military commander, was warned but didn't leave his home, he and his family of 15 were killed in the subsequent bombing.
According to the Israeli army, striking homes used for weapons storage, when sufficient warning is given to the residents, falls within the boundaries of international law and is legitimate. According to the Palestinian Centre for Human Rights, the warning of inhabitants by Israeli forces is psychological warfare and after the first week of the offensive, only 37 houses had been destroyed despite hundreds of warning calls.
See also 
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