Abstract: An improved joist hanger tool suitably adapted to be placed over a standard joist hanger which is in turn placed over the end of a joist, to shape the joist hanger to its proper installation configuration and to hold the joist hanger securely to the joist so that the installer has both hands free to support the joist and/or to fasten the joist hanger to a header; said joist hanger tool being a substantially “U”-shaped, monolithic clip fashioned out of a flexible, resilient, and substantially rigid material, having a wider bottom, an intermediate narrowest point, and a flared open end.