The horizontal area of such features, including the area enclosed by such features, that exceed the maximum permitted building height shall not exceed 20% of the horizontal area of that portion of the building having a height greater than 40 feet. This limitation shall be cumulative for all such features. [For example, the horizontal area of that portion of a sloped roof or roofs at an elevation greater than the maximum building height could not exceed 4,000 square feet in the case of a building with a horizontal area of 20,000 square feet (20,000 x 0.20 = 4,000).]