The upper joists of a terras roof are cut to form a peak in the center of the beam while the lower joists have a V-shaped groove plowed into their top surfaces.  This eccentric framing system allows the wooden sheathing boards that are laid over the joists to form the ridge and furrow pattern that defines a Jeffersonian terras roof.  Additionally, because the joists are also cut to slope downward from the roof’s centerline, the furrows essentially form gutters that shed water away from the middle of the roof and outward towards the eaves.