A board with preformed seals. In an exemplary embodiment, a sealing material is applied to at least one edge of a board to form a preformed seal on that edge. The board is configured such that when the board is attached to a frame adjacent to a another board, at least a portion of the sealing material applied to the edge of the board seals against at least a portion of an edge another board. In one exemplary embodiment, the boards are configured to seal against edges of substantially identical boards. A plurality of the sheathing boards may be installed on a building frame to form a sheathing system that provides a moisture and/or gas barrier between and interior and exterior of a building.