Abstract: A window brace includes a vertically disposed pole to which is coupled one or more braces, each of which includes a pad bearing against a portion of a surface of unsupported glass. Each pad defines an internal chamber connected to ambient atmosphere by a check valve such that when the glass bows inwardly toward the pad, air is forced out of the chamber via the check valve that prevents air from returning into the chamber. Evacuation of the chamber, in this way, enhances adherence of the pad to the window glass and, thus, enhances effectiveness of the inventive device. For large pieces of window glass, a plurality of pads may suitably be employed. Alternatively, a valved port in the pad may be connected to a source of vacuum to evacuate the chamber.