I have installed this garage door and it's skewered in the finished opening.in the photos there is a picture of the upper left and right corners of the door,there is 3/4"difference side to side.the finished frame is plumb according to the level.corner to corner over a 9 ft span door opening is within 3/16 to 1/4 inch. The only culprit I can find is the slab,it slopes 1/2"over 9'.the bottom of the door itself seal across the 9' slope.If I stick a 1x4 under the low corner the door squares up in the opening and all measurements work.what do I do to remedy the problem? Obviously I cannot just leave a 1x4 under the door