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.


Galvanized steel Unistrut channel would work, you just have to be creative with which connectors you need. It's available at most big box home improvement store, usually in the electrical section.

Unistrut fitting examples

  • The Unistrut is exciting. I will use it for the undercarriage of the shelf. I will need to find the right junctions that will allow me to attach the channel sideways, with the mounting holes against the house, and the side channels flush with it. Does that sound correct?
    – Jorabi
    May 30 '19 at 14:42

Pressure-treated 2x4 lumber. Nail it together as shown to make two rails. Probably not the prettiest solution, but definitely the cheapest and the quickest.

support rails


A couple ideas come to mind...

  • Two-by lumber wrapped in metal or vinyl
  • Aluminum box tubing or U-channel (2x2 or larger) resting on the support brackets with synthetic decking spanning the other direction
  • Synthetic decking resting on the support brackets with floating Aluminum angle extrusion bolted to the underside between the brackets

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.