It 's become another military euphemism , along with `` collateral damage '' and `` friendly fire . '' The `` knock on the roof '' is the Israeli military 's warning for civilians before it fires on a building and is being used extensively as Israeli airstrikes target Hamas sites in Operation Protective Edge . The Israeli Air Force developed the technique in 2009 as a way to warn civilians in Gaza to leave buildings it has identified as locations where Hamas keeps ammunition , a rocket stash or command post . But it is a controversial policy that has been criticized by human rights groups . The procedure generally begins with a phone call to the occupants to leave a building , according to Relik Shafir , a retired brigadier general in the Israel Defense Forces ( IDF ) and a former fighter pilot . Such places , he says , are often under constant surveillance , and the IDF has a sense of how many people live there , and how many leave . If it is still unclear whether a building is occupied , a missile that carries little or no explosive load is aimed at the roof of a building . The impact is felt , but it rarely causes casualties . `` It 's meant to get people to take us seriously , '' says Shafir . There is no standard gap between the delivery of the `` dummy '' missile and fully armed missiles , says Shafir . It can be minutes or even hours . It depends on how important the target is . But there are hundreds or even thousands of such places in Gaza , chosen by Hamas precisely because they complicate targeting . One dramatic example of the `` knock on the roof '' appeared on social media Sunday . Distributed by the Gaza-based Watania news agency , it showed at first the impact of an unarmed rocket or missile on the roof of a house . Smoke rose from the roof ; curtains billowed from an open window . Some time later -- Watania says about 15 minutes -- two missiles slammed into the building , blowing out the facade and sending debris and thick smoke across a wide area . It 's not clear whether the occupants of the house had heeded the warning -- it may have been empty -- but no one was hurt . But human rights groups have condemned the technique -- despite the precautions . What you need to know about the country of Israel . A warning or an attack ? `` In some but not all cases , families receive telephone calls from the Israeli military in advance , '' said Philip Luther , Middle East and North Africa Director at Amnesty International . `` There is no way that firing a missile at a civilian home can constitute an effective 'warning . ' Amnesty International has documented cases of civilians killed or injured by such missiles in previous Israeli military operations on the Gaza Strip , '' he said . Human rights groups in Gaza agree . `` The sending of a missile can not be considered a warning . It is the targeting of civilians with a weapon , regardless of how small , and it is a violation of the Geneva conventions , '' said Mahmoud Abu Rahma of the Al Mezan Center for Human Rights . `` Imagine you are in Gaza and there are airstrikes everywhere , and many families are in the bottom floor of their home , '' Abu Rahma added . `` Families miss the sound of the 'warning ' missile because it sounds like just another explosion . '' CNN 's Ben Wedeman , reporting from Shabilya Friday , visited one targeted house whose residents said no warning had been received . At another property that was also destroyed , a warning had been received . But the man living next door , Raouf Abu Odeh , told CNN he did n't get any warning -- and now half of his house is also in ruins . `` When I heard there was danger , we ran away with my family . We have no bomb shelters , we have nothing . All we can do is escape , '' Odeh said . It 's not clear whether any warning was delivered by any means to the home of Gaza 's police chief , Tayseer al-Batsh , on Saturday night . The missiles that hit his house killed 18 members of his extended family , including six children . When Wedeman and his team visited the ruins of the compound Monday , they found what appeared to be the reason it was targeted : evidence of reinforced tunnels . Shafir says the precision of such attacks has improved as crews have gained experience . `` The number of strikes has more than doubled compared to Operation Pillar of Defense ( the 2012 conflict ) , but the number of casualties is about the same , '' he said . Dense area , deadly damage . But even in an era of precision targeting , the impact of missiles ca n't be restricted to one house in such a densely-populated area . Many of those injured in the strike on the al-Batsh compound were hit by shrapnel as they left an adjacent mosque . In other instances , people have gathered only a short distance from a building designated for attack -- not far enough away from flying concrete , wood and metal . And the United Nations says some 70 % of those killed in the current conflict have been civilians . And there has been at least one deadly miscalculation . On July 8 , eight civilians -- all members of the Kaware family -- were killed when their home in Khan Yunis was hit . According to the IDF , the family left the house after a phone warning but had returned home prematurely after a `` knock on the roof . '' Perhaps they mistook it for the explosive missile . The property had been targeted because it belonged to Odeh Kaware , a senior official with the Izzedine al-Qassam Brigade . A senior air force officer told the Israeli newspaper Haaretz that it was too late to prevent the airstrike that followed the warning . `` There was nothing to be done , the munition was in the air and could not be diverted , '' he said . Some in Gaza have even put themselves in harm 's way . Ten people were killed July 9 when they formed a human shield on the roof of another property belonging to the Kaware family , according to Palestinian medical and security sources . The `` knock on the roof '' technique was first used in the 2009 conflict in Gaza . But it was criticized by the United Nations-appointed Goldstone Commission that investigated the conduct of both sides in that fighting . The commission said it was `` not effective as a warning and constitutes a form of attack against the civilians inhabiting the building . '' `` The fact that a warning was issued does not relieve a commander and his subordinates of taking all other feasible measures to distinguish between civilians and combatants , '' the commission 's report said . Oren : A smart way out of the Gaza confrontation . Phone calls and leaflets . The Israeli military also uses telephone messages and leaflets to warn people in Gaza to leave neighborhoods where their forces intend to target alleged Hamas facilities . On Saturday , the air force dropped thousands of leaflets across several neighborhoods in northern Gaza , warning people to leave their homes ahead of military operations against Hamas targets . `` Whoever does not adhere to these instructions and does not vacate their house immediately is subjecting their lives and the lives of their children and family to danger , '' it read . In past campaigns against Hamas , the Israeli military has also commandeered television and radio signals to issue warnings . Leaflets and phone messages were used in the 2008-09 and 2012 conflicts . They were partly political ; blaming Hamas for the violence , and partly an attempt to clear areas where Israeli forces intended to focus strikes . During both conflicts , leaflets were dropped warning people not to come within 300 meters of the border . In 2012 , leaflets warned residents that terrorist organizations were hiding among them and represented a direct threat to their lives . In 2008-09 , according to an IDF report , some 2.5 million leaflets were dropped . One warned that `` anyone having ammunition and/or weapons in his home is risking his life and must leave the place for the safety of his own life and that of his family . '' The same message was delivered in some 165,000 phone messages . The Goldstone Commission acknowledged `` significant efforts made by Israel '' in 2008-09 to alert Gaza residents to planned attacks , but said `` the lack of specificity and thus credibility '' of some of these messages had diminished their value . The leaflets can also have the effect of worsening an already difficult humanitarian situation . As people leave their homes in the current fighting , thousands end up in the care of the United Nations Relief and Works Agency , in crowded schools . They must leave their livelihoods behind . Others say they have nowhere to go . `` These are more threats than warnings , '' says Abu Rahma of the Mezan Center . `` Telling 150,000 people to leave their homes when there are airstrikes all along the route and nowhere to go because every home in every place is a target . '' Abu Odeh in Shabilya tells CNN , `` There 's no escape . '' `` God has sealed our fate , whether we stay or leave . '' Cease-fire effort collapses as rockets , airstrikes continue . Running from death in Israel , waiting for it to come in Gaza . Trapped in war : 'This is tyranny '
Israel 's military tries to warn civilians in Gaza ahead of a strike . Comment: