I will be building a shelf on the outside of the house to hold a 120lb (18,000 BTU) air conditioner. The worrisome constraint is that the two shelf supports have to be 38" apart**. The 24" wide AC will sit in the middle of the 38" span.
I cannot find any information in layman's terms on how to know if the shelf will support the weight using various composite materials for the shelf. Does anyone have a suggestion for what to use, thickness, etc? I would like to use something that does not need painting.
** the reason for the wide span is the AC will be in the top pane of a double-hung window. The bottom pane still needs to be usable via its tilt-in ability, without obstruction from the shelf supports. The shelf will be right at the top of the bottom pane.