Casing for door out of square

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A door I'm trying to put a casing around isn't square. How best to work out and cut the mitre angles for my top and side pieces? A large protractor, or is there a device that fits inside interior angles (such as inside the door frame) to measure them?
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I have a device, not sure what it is called, it has a wooden handle with a thin metal blade that folds out like a pocket knife that is handy for finding angles, it is a pretty common tool, not hard to find. You may encounter another problem, once you find the angles, the cuts may not have the same face length. Unless it is horribly out of square, it may be better to just cut the miters 45 and at least have part of it right.
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Eric in North TX wrote:

Is that the one that has a slot down the middle of the blade? I bought one of those in a tool-buying frenzy about ten years ago, and didn't really know what it was for.

The trick there should be to take whatever the angle is and halve it, I think. For example, if the angle is 92 degrees, cut each piece with an outside angle of 46 degrees. Then the joined edges should match.
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That's the one.
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I've always called it a "bevel."

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wrote:

Bevel tool.
-- Oren
I have not failed. I've just found 10,000 ways that won't work. Thomas A. Edison
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I think your tool is called a "T-Bevel". Not sure how it got its name, but I've used one before and it helps for those angles that are only slightly out of a square 90 degrees. Cut one piece at 45, tack it up and use the bevel to determine the cut for the other side. I generally do the top piece at 45 on both ends to the correct length. Then cut the 'legs', leaving them a little longer than they need to be. They will have enough 'flex' in them to be able to check the mitered cut, and re-cut a time or two if necessary. Once I'm good with the miter, I cut the bottom of the leg square to the correct length.
If the jam is out more than a few degrees you are better off using a protractor and making each cut half the total angle. That way they are the same length on each side and the molding will line up better at the cut. A bevel is still good for doing that.

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You can use the 345 rule to check square and then use those measurements to find the angle.
The 345 rule is, pick a corner and measure 3 inches one way, and 4 inches the other way, and the ends of those two lines will be 5 inches IF that corner is square. Of course you use larger numbers for larger corners (ie multiply by 5 will mean 15, 20 and 25 inches if square).
Now if you don't get 5 inches you need to figure out the angle using the Cosine Law. Much simpler just to use this online calculator.
http://www.analyzemath.com/Geometry_calculators/cosine_law_calculator.html
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Noozer wrote:

Oh, trig. Of course. :-) That certainly avoids having a special tool. Thanks.
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LOL!
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or x**2 + y**2 = r**2. No Cos needed. All calcs have x**2 and SQRT fns pretty much.

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Eric in North TX wrote:

The correct name is "bevel gauge". I call it an "angle guesser"
You fit it too the corner, tighten the clamp. Then all you have to do (hah!) is bisect that angle. How to do that? I could at one time on a planet long ago and far away explain it using geometry. Real world is "Take a guess then make repeated cuts" with your your saw changing the angle slightly each time until you get it right. Use scrap wood for the tests.
Harry K
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On Sat, 30 Dec 2006 12:41:38 -0500, Harlan Messinger

Is the top jamb level?... use a two foot level under the header jamb.
Speaking for myself. Take a four foot level to the top frame jamb and mark each side jamb where the reveal is set (horizontal), adjust reasonably, but keep level. Cut the normal 45 degree angles. and nail the top casing.
Determine the reveal line at the top mark and mark a (vertical). Measure and cut the side casing. Same 45 cut.
Doing the verticals from top down, adjust the casing a little to set the reveal as best you can.... nail it... as you move down I take an extra blade off at the bottom and not force the casing. If I can hold it up, match the cuts and shoot a nail in it. I work my way down, adjusting the reveal as necessary.
-- Oren
I have not failed. I've just found 10,000 ways that won't work. Thomas A. Edison
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Since you have the casing off why not just shim the door back to square. Seems faster than messing around with odd angles and still ending up with a crooked door.

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On Sat, 30 Dec 2006 19:12:11 -0500, "calhoun"

That would require a level ... <grin>.
-- Oren
I have not failed. I've just found 10,000 ways that won't work. Thomas A. Edison
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calhoun wrote:

Just replacing the old moldings was a big enough challenge for me! Pulling out the jamb and starting again seems risky.
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On Sat, 30 Dec 2006 19:51:24 -0500, Harlan Messinger

He did say shims and I mentioned a level a few times. Why would you consider taking out the jamb to shim it?
-- Oren
I have not failed. I've just found 10,000 ways that won't work. Thomas A. Edison
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Oren wrote:

Obviously I'm missing something. From what I see, the jambs are set level and plumb (supposedly) by shimming them--and then they're nailed into place. Once they're nailed, how can I shift them without taking the nails out?
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wrote:

Depends how many nails, and where they are placed. Sometimes you can fine-tune reality with some hardwood blocks and a BFH. Stanley wonder bars are great, too, if you have a solid header and jack studs to pry against. Just how much out of square are we talking about?
If this is an interior door, I'd be inclined to rip the sucker out, and reinstall properly. God invented Sawzalls and stubby steel-nail-cutting blades for a reason. A couple hours work, tops. Even if you get lucky on the trim angles, it will annoy you every time you look at it or use it. Exterior doors are a lot harder, especially if they are trapped in brick or siding.
What I would be more concerned about, and nobody in this thread has addressed, is why is the door out of square? Has it always been this way, or is this a recent problem? Bad install job, or settled foundation? Does it move around depending on time of year and weather conditions? How old is house? I'd only be inclined to fake a non-square door on an older house with no practical way to fix the underlying problem. If this is a modern (mid-20th century or newer) house, and the floors and walls were level, square, and stable, I'd definitely rip out, and reinstall properly, or replace if the door was junk anyway.
aem sends...
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snipped-for-privacy@att.net wrote:

I'm glad you asked, since I've measured the angles now. Figuring the easiest way to measure a diagonal is with a rigid rule, I used a yard stick with one corner on the header at 12 inches in, and the other corner against the jamb. I found that the deviations are small in angle terms: 90.62 degrees and 89.72 degrees.*
Why did I think the door was out of square? Because I had already cased one door, and the pieces fit beautifully, but when I tried this door, having mitered my three pieces of molding to 45 degrees, there were gaps at the molding joints that I couldn't get rid of however I shifted the pieces around the door. I figure (a) I was really lucky with the first door and (b) 0.62 degrees is really quite noticeable. Also, small though this deviation sounds as an angle, [80 * sin(.72)] shows that it makes a difference of about an inch horizontally over an 80-inch drop to the floor.
I think I need to change my question. Now it's: what's the best way to handle a tiny gap like this? Should I try to set my miter saw to cut at 45.3 degrees and 44.8 degrees, respectively? Or should I stick to the 45 degree detent and fill in the gap with something? But a bit of trig tells me that in an 80 inch drop to the floor, this is a difference of around an inch.
* THEN I found a leveling protractor in my tool chest, after all this--and confirmed that any deviation of the jamb or header from level or plumb is a fraction of a degree.

My experience is minimal, and I suspect that if I did this myself I'd wind up worse off than I am now!

I'm replacing existing moldings, just to replace plan, flat moldings with contoured ones. Come to think of it, the door doesn't fit too well either.

Built in 1974. Possibly just settled, though heaven knows a number of things were evidently done poorly here.
I'd only be inclined to fake a non-square door on an older house with

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