Arbor between two buildings

I need to build an arbor between my garage and my house and I need to know how to line up the beams so they are perpendicular to both buildings. They are 15' apart and they are parallel with each other, but one is set back from the other, so I can't just measure from the end. All I am coming up with is the 3-4-5 rule, but I doubt I will be accurate enough with that. Any ideas?
Thanks, Steve
P.S. I hope this is the right forum for this question.
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On 9 May 2004 05:44:42 -0700, snipped-for-privacy@mypcpro.com (Steve) wrote:

3/4/5 will get you nice and square. if in doubt, do it from both ends.
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Double the numbers to 6/8/10 and it will be twice as accurate. :-) If you get one beam perpendicular to a building, you can measure from it to the next beam. At least that's how it seems to me.
(Steve) wrote:

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How about 15-20-25 ? Use a laser pointer or pull a string straight across the facing of the building set back to get a beginning reference mark on the opposite wall. You can tack up identical spacers projecting from the face to clear obstructions (down spouts etc.) and deduct the size of the spacers to get the corner reference. Measure the cross beams location from the setback corner and the reference line on the perspective walls. A level or plumb bob can elevate your reference line.
--
Chipper Wood

useours, yours won't work
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I saw an interesting item on TV where they were building houses on floating barge hulls to make boat houses. The hulls were in constant motion so levels were of no use. To keep everything level they had to use measurements and large oversize squares cut from full sheets of plywood. Find a sheet of plywood, check it for squareness (they usually are) and cut a large triangle 4' x 8' out of the sheet. Cut the center out to lighten the weight if you want and use it to check your squareness between the buildings.

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You can construct a beam that's exactly perpendicular to both buildings only if the two buildings are exactly parallel to each other.
Since "exact" is never attainable, you might want to ask yourself what accuracy you really need. Also, a 3-4-5 triangle scales perfectly to a 9-12-15 one. If you do that at each end of the smaller building [probably the garage] and hang string between them you'll get good baselines from which you can measure. A framing square should be used as a final check. Don't expect perfection, you probably don't need it.
Norm
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