A bay window will droop like wet cardboard if there's nothing to hold it up.
Foundations stop sags, for a hefty price, but they work only on first story bays.
With the steel-cable suspension system, introduced in 1987, a bay can be installed just about anywhere. Interior trim nailed over the mullions hides the cables, disguising the fact that anything is holding the bay aloft. This Old House and the This Old House window and This Old House Online are trademarks of WGBH Educational Foundation. Neither WGBH Educational Foundation nor PBS endorses any product or service mentioned within This Old House Online or any web sites linked to or from This Old House Online.