: Draw the base shape (the "footprint") of your stairs or ramp as a face in SketchUp. : Select the face and click the desired stair type from the Extensions > Suforyou > Stair menu or the s4u toolbar. Refinement
In construction shorthand, usually means "Surfaced 4 Sides" (dimensional lumber). However, in the context of this defect, S4U often refers to a specific architectural detail or a user-defined location tag—specifically the top termination point of the stair stringer against a vertical drywall or partition. s4u stair crack top