Almost a month after a ceasefire agreement between Israel and Hamas came into effect, Israel is refusing to lift its blockade and allow vital building material into the enclave. The UN says Israel's onslaught destroyed more than 20,000 homes, leaving much of Gaza's infrastructure in ruins. But despite an agreement between the UN, Israel and the Palestinians on how materials will be used, Israel says Hamas might use cement to rebuild tunnels. Al Jazeera's Charles Stratford reports from Gaza.