Abstract: Method and apparatus suited for forming retrofit detectable warning surfaces on existing surfaces or new warning surfaces. Apparatus includes a tool in the form of a stamp having one or more dome-shaped volumes that may be truncated domes. Dome-shaped volumes may have interior beginning on a bottom side of a stamp and the truncated portion may be defined by a hole provided in a top side of the stamp. Thanks to the hole, when the stamp is removed from the unhardened concrete, the hole functions as a pressure-relief or pressure-equalizing valve eliminating undesirable distortion of the unhardened concrete and ensuring formation of desired dome-shaped concrete projections.