I'm going to install a through-wall air conditioner using a purpose-built sleeve. The instructions from the manufacturer say to install it such that it angles slightly downwards to the outside (about six degrees) so any condensation or rainwater will run off and drain outside the house.

All the instructions and how-to guides I've read show the framing (header, supporting studs, etc.) installed level and at right angles. The only thing I've seen that gives any indication of how to properly arrange the frame to support the downward installation is to install the header 1/4" above the actual size needed for the sleeve opening so that it falls back and down to the outside a bit.

Is this the correct way to do this? Should the sleeve be shimmed at the bottom or should the horizontal studs be installed at a slight angle so the sleeve sits flush with the header and horizontal support stud?

I haven't done this before so any insight on how to get the installation angle correct and still maintain a proper safe and secure installation is appreciated.

  • For simplicity, I prefer the shim approach rather than trying to angle the whole support. I appreciate the question because it answers a question I had - Should the whole A/C be tilted to the outside. Your info implies yes. Many/most have drains but i'm sure the drains might plug, especially if left in place permanently.
    – user54415
    May 25, 2016 at 16:39

2 Answers 2


It's much easier to check that something is level than to check that something is 6 degrees or 1/4 inch higher.

Build it level. Then add a 1/4 inch strip of wood (a length of lath or trim) across the inside edge of the horizontal framing member.

You are correct that the sleeve will be sitting on two edges, the added trim strip and the outside edge of the exterior horizontal member. But the air conditiner is quite rigid and the sleeve sufficiently so to resist flex. The weight is not so great that the boards will be compressed or the machine flexed.


It's quite easy to check that something is at a particular angle. Just put the appropriately sized block on your level at the appropriate distance, and make your level (with the block) level.

Your off-level surface makes a right triangle with respect to an imaginary (or provided by your level) level surface. The level is the "adjacent" side of the angle, the surface is the hypotenuse, and the opposite side is the block. tan is the function that gives you "opposite over adjacent" so it's the one to size a block on the level. This is what your teacher should have answered when someone asked "if there was a use for trigonometry in real life."

For 6 degrees, tan=0.105104235265677 (-ish - and be sure to use a calculator in degree mode, not radians mode) so you measure out the level an appropriate distance (for the size of what you are building, or the length of the level if it's shorter than what you are building) and install a block of the correct size.

If 6", 0.63" (5/8" for most intents and purposes)

1.26" if you have 12", etc...

1/4" only gives you 6 degrees if it's 2.37" or so from the other support.

It's handy technique to have in the bag. 1/8" per foot plumbing is a 1/2" block on a 4 foot level. 1/16" per foot plumbing is a 1/4" block on a 4 foot level.

You can use the same technique or a speed square if you need to mark angled cuts.

  • Thank you for the reply but the question isn't at all about how to calculate angles. It's about proper construction technique when inserting an object through a wall at an angle.
    – par
    Apr 8, 2014 at 17:58
  • As noted, if you offset by 1/4" over a distance of more than 2.37", you will not achieve a 6 degree slope. The answer is, in fact, about how to frame an angled opening, at an angle, properly. But do feel free to ignore that, this is only the internet. It's no more difficult than framing it level...
    – Ecnerwal
    Apr 8, 2014 at 19:34

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