90 Degree Angle Stairs . Slope for each of these types of stair or ladder is given in degrees. Using a level and speed square to find stair angle. tiling staircases partial rounds in the 90 degree angle Tiles from www.pinterest.com The angle where treads meet risers is simply a 90 degree angle. Keep the width even from top to bottom, then add rails where needed. Building codes mandate stairs should be at least 36 inches wide.