My invention relates to an improvement of a siphon head for use on a flat roof for a building having a recessed drain. The drain includes an inverted cup-shaped drain member with peripherally spaced elongated drain slots. The siphon head is comprised of a soft pliable elastomeric material including a relatively flat foot flange adapted to rest flatwise on a roof to be drained, and a dome shaped head portion centrally located and integrally formed with the relatively flat foot flange. The relatively flat foot flange extends outwardly in a relatively flat plane beyond the head portion and surrounding the head portion in all directions. The head portion is open on its top side providing an annular edge for nested engagement with the drain with a portion of the elongated slots lying below the edge, and other portions of the elongated slots lying above the edge. The edge is in snug nested engagement with the drain while the relatively foot flange is supported on the flat roof.