prehung exterior door too tall for rough opening

I am getting ready to replace an exterior door and I discovered that the fiberglass door we selected (therma-tru) is too tall for the rough opening. (Opening = 81.75" but appears to be slanted by 0.25" so I really only have 81.5" to work with after it's leveled; door is 82" and they recommend an 82.5" opening.)
So I have identified the following possible solutions:
1. The door dealers told me they could cut the door down at a cost of $67, though it sounded like taking an inch off was getting to a limit where the standard storm door wouldn't fit.
2. I read in this group that somebody cuts the header back to make room for therma-tru doors which tend to be taller than everybody else. Is that a reasonable thing to do? (Wall is not structural.)
3. I could reframe the rough opening which seems like a bit of work.
4. I could lower the subfloor. As it happens, the subfloor is rotted out and needs to be replaced anyway, so I could replace it by something thinner right where the door is to sink the door into the floor. (Floor is vinyl tile on top of 1/4" plywood on top of 3/4" subfloor.) I could support this with blocking underneath.
Anything on this list reasonable?
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snipped-for-privacy@cam.cornell.edu wrote:

That sound questionable. Since they will not be trimming a storm door, it is probable that it will not fit.

If the wall is not structural then I see no problem with this. I vote for trimming the header.

This will work, obviously. What you will end up with will be the same dimensions as trimming the header so it should not be necessary.

I don't like this idea. To lower the subfloor in that one area would create problems. The floor in that are would forever be lower than the rest of the floor. That doesn't sound good. Trim the header.
Lawrence
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Trim the header with a router and finish the corners with a chisel.
0.25" off the bottom might not screw up the threshold considering exterior doors come with that built into the frame. Could eliminate the slant with the router as well.
Power planer might be able to shave enough off the door frame to squeeze it in.
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PipeDown wrote:

How would you go about trimming the header with a router? I mean, I could cut away material to make a sort of bowl like shape but then I'd have to do the edge somehow. I suppose I could affix guide rails to the sides of the header for the router. But I have to say this sounds like more work to remove possibly an inch of material than something like the sawzall suggested elsewhere in this thread.
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Buy a door that's the right size. It's called a custom door. I've gotten several. You have to order it and pay in advance and wait 4-6 weeks to get it.
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snipped-for-privacy@gmail.com wrote:

It's hard to see what the advantage of this is. I imagine it would double or triple the price of this door replacement effort rather than just adding $70. The door manufacturer says that cutting an inch off their door is not a problem and that they would still honor their warranty. So why would I want to pay the big bucks for custom doors (storm door too, presumably)?
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I'd rework the header. That should not be a big problem. That would leave everything as it should have been in the first place. You didn't say what kind of siding you are dealing with. That might be the fly in the ointment. Let us know about that. The other options are making a bad situation worse. TB Cornell B.Arch 66
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snipped-for-privacy@bellsouth.net wrote:

When you say "rework" the header do you mean trim it or do you mean something else?
I have asbestos (concrete) siding. I'm not sure how easy this stuff is to cut.
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Get out the sawzall and cut a 1/2" off that header. Then go and get coffee for the crew!
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I think lowering the floor before the door is reasonable, and in fact might be attractive if the tile flushes with the door threshold. Cutting the header can be done, if you have a sawzall. Sometimes you get lucky and there is a filler board or a flat two by or something. Cutting the door is probably the most work.
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