Medieval Roof Outer Corners were implemented during the May 2017 launch update, together with the other Roof and Slope Corner blocks.
Medieval Roof blocks are part of the Colossal Castle blueprint and block-set. You can buy either the Colossal Castle bundle, which gives you the recipes, or the Medieval pack, which gives you a sample of each block, but not the recipes.
Outer Corners are created by putting Medieval Roof blocks into the processor, giving you 2 outer corner blocks per roof block.
The roof blocks can be rotated by pressing and holding R and moving the mouse while holding the left mouse button.