Question 1159942
Assuming the roof ends flush with the walls of the 20ft x 15ft building, you need enough to cover the slanted roof surfaces of {{{ (15/2)*sqrt(2) }}}ft each.

There are two such slanted surfaces, each 20ft long, so  {{{ 20*(15/2)(sqrt(2))*2 }}} sq ft or  {{{ highlight( 300sqrt(2) ) }}} sq ft,  approximately 424.26 sq ft.

In reality, the answer depends on the depth of the eaves.