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.