Solidify Faces
Reference
Mode:
Edit Mode
Menu:
Face ‣ Solidify Faces
This takes a selection of faces and solidifies them by extruding them uniformly to give volume to a Non-manifold surface. This is also available as a Modifier. After using the tool, you can set the offset distance in the Adjust Last Operation panel.
Thickness
Amount to offset the newly created surface. Positive values offset the surface inward relative to the normals direction. Negative values offset outward.