To cry from the house-top. To proclaim [it] from the house-top. To announce something in the most public manner possible. Jewish houses had flat roofs, which were paved. Here the ancient Jews used to assemble for gossip; here, too, not unfrequently, they slept; and here some of their festivals were held. From the house-tops the rising of the sun was proclaimed, and other public announcements were made.
“That which ye have spoken [whispered] in the ear ... shall be proclaimed upon the housetops.” —Luke xii. 3.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894