How do you square a porch using triangulation?

There are lots of articles on the web about 3 - 4 - 5 triangles.  Each, of course, explains that the angle between the 3 side and the 4 side is a perfect right angle.  Few give positive illustrations on how to use this principle.  I like those that include pictures.  Here is one:

http://www.mathopenref.com/triangle345.html

They use a rug for illustration, not a porch, but the idea is there.  I like the idea that they show, graphically, that the numbers can be anything as long as the proportions remain the same.  In big stuff you may want to use 6 - 8 - 10 or even 12 - 16 - 20.

Mathematically this and other ways work but amazingly you can never seem find a room or porch with square corners. LOL

You can measure across corner to corner like an X shape. Left to right, right to left. Both measurements should yield the same.
The 3-4-5 method can be used as inches or feet. You start at a prefered corner, measure down 3 (")(') put a mark. Then making an ell shape measure down the other joist 4 (")(') and make a mark. The measurement betwwen the to marks should come to exactly 5 (")('). This means the structure is square.
If you take a framing square and put it on two or more corners, this will tell you the same. You just have to move the framing until it conforms to the square.
Does this make sense?

You have been given excellent info on squaring up your project. Remember you will want to do this on both of the walls coming off the original structure. Find your first wall then after squaring up that corner, move down the length of the main building structure to the width of the porch. Find your corner there using the 3-4-5 method. Now measure out the distance your porch will extend from the main structure. Square that corner and move over to the opposite one. Now use Ron 45's suggestion of measuring all corners "Diagonally". This measurement should be exactly the same on any diagonal corner to corner measurement.
You can't go wrong if you use these suggestions from the members.

The 3-4-5 or 6-8-10 is called the "Pthagorean Theorem". I recall this from geometry back in school. It basically says that Side A squared + Side B squared = Side C squared (3x3 "9" + 4=4 "16 = 5x5 "25". And I always thought the stuff they taught in school was useless, lol. And definitely run your tape on the diagonal. It is always best to check it both ways.

Posted By Oldman on 09-07-2010 07:13 AM
You have been given excellent info on squaring up your project. Remember you will want to do this on both of the walls coming off the original structure. Find your first wall then after squaring up that corner, move down the length of the main building structure to the width of the porch. Find your corner there using the 3-4-5 method. Now measure out the distance your porch will extend from the main structure. Square that corner and move over to the opposite one. Now use Ron 45's suggestion of measuring all corners "Diagonally". This measurement should be exactly the same on any diagonal corner to corner measurement.
You can't go wrong if you use these suggestions from the members.

Great reply Barry!

the 3/4/5 method is the absolute best you can use.. based on a square sided triangle... 3ft mark on one edge, 4ft on other edge, 5ft on the top edge... shifting the porch until you acquire 5 ft between the marks..... The really kool thing about the 3/4/5 is multply uses of it meaning you can increase it by the same number and it still works
examples........3/4/5 times 2.....6/8/10..... 3/4/5 times 3..... 9/12/15 and so on up the line