Question 920522
Knowing the Perimeter would allow You to find the length of one leg 
of a non right triangle knowing the length of the two other legs
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Re Reply:
Two measurements
10-20 feet away from a house..  laser measurement 1 to the top of the gable 
and laser measurement 2 to the opposite end of the gable.
If I am understanding correctly, 
You have a triangle formed by Your two measurements and the far rafter. 
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For these two measurements to work for You, 
You would need to know either the Pitch of the Roof 
0r the angle of elevation of each of the two measurements.