Can I shorten a prehung exterior door?

My husband and I are trying to replace the side door on our brick-faced garage (about 60 years old). I measured the interior framing, and it seemed like a 30x80 door would fit. We ripped out the old wooden door, and tried to slide in the new one BUT the brick around the exterior of the door opening was protruding, so it wouldn't work (I didn't measure from the outside). I trimmed the brick all around enough to handle the width of the unit (using a chisel), but now we see that the opening is too short (on one side - I guess the floor isn't level). The bricks above the door sit on a metal L-bracket, and it's too low too accomodate the door unit (by about 3/8 "). I thought I could pull out the metal piece along with the mortar along the bottom of the bricks, then replace it with a flat piece of metal that would be thinner, until I realized it was an L. Can't seem to cut it with a reciprocating saw.

So...what do I do? Can I shorten the door? I'm thinking from the bottom. Take it off the hinges and cut the door with a circular saw. Also, I'll have to take off the metal sill and trim the side pieces (sorry I don't know the technical terms for this stuff). I've already messed up the door some, so I can't return it, and I don't want to order a custom door (it's only a side-door on my garage, like I said.).

HELP! For now, the door is leaning up against the opening. If I squint my eyes, I can pretend that's it's installed, and it looks very pretty. I hate to give up and call in someone who knows what they're doing (probably because I don't want to be laughed at).

Any help would be appreciated. obviously, I don't know what i'm doing.

 

The first thing you should do is remove the brickmould, if your door has come with preinstalled brickmould. Once the brickmould is removed, you will probably find that the door will fit into the opening just fine. Center it in the opening and get the door squared up by using shims at all 4 corners. Nail or screw through the jamb to secure the door to the framing. Ensure the door is plumb, and that when it closes, it contacts the weatherstipping evenly all the way around. If the top corner of your door sticks out, that's a sure sign that it's out of plumb. You may have to tap one corner of the door in, and tap the opposite corner out to get the door plumbed up. Once you are satisfied with how the door operates, then you can put the brickmould on. Trim it to fit if need be, but it's a lot easier to put the brickmould on afterwards in a brick opening that's too small.

 

If I tell you nothing else it's this. Leave that brick header alone (that's the L shaped thing). If you cut that out you will have bricks falling out on you. Sounds like your not too far off though. If one side will fit and one will not then I would try to copy the angle of the ground assuming that's where your slant is. Then cut the door down a bit. I am assuming we are talking about a wood door here. Make sure you leave the ends of the frame longer then the door. Ditto on the brickmold if that's the case.