The ornamental design for the support surface for flooring, as shown and described.
FIG. 1 is an isometric view of a support surface for flooring comprising my new design; and,
FIG. 2 is a bottom view thereof.
The support surface for flooring is shown with three sides broken away to indicate the claimed design has no specific length or width. It is understood that the support surface repeats uniformly throughout the length and width of the flooring.