Math For Checking For 90 Degrees In A Corner - Carpentry - DIY Chatroom Home Improvement Forum

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11-27-2006, 01:12 PM   #1
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## math for checking for 90 degrees in a corner

Can someone tell me the math you use to check for square of a corner, the math makes a triangle with a 90 degree angle which would be the corner.

11-27-2006, 01:37 PM   #2
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3-4-5

Measure out 3 feet (or 3 anything...inches, feet...but the longer the better) on one side and mark it. Measure out 4 feet(or units) on the other side and mark. If the corner is 90 degrees, the measurement between marks will be exactly 5 feet (units).

If it's not 5 feet and you are looking for the exact angle, it gets a little more complicated (trig), but if that happens come back and we'll step you thru it.

 11-27-2006, 01:38 PM #3 Member     Join Date: Sep 2006 Location: Canada Posts: 1,733 Rewards Points: 1,016 Do you mean the 3, 4, 5 rule? You measure 3 feet across one wall, and make a mark, then measure 4 feet across the adjacent wall, and make a mark, and the measurement between those points is 5 feet. Is this what you are looking for?

 11-27-2006, 07:16 PM #4 Newbie   Join Date: Nov 2006 Posts: 10 Rewards Points: 10 Yes thats what he means.... the rule is (AxA) + (BxB) = (CxC)... Which in the end with the 3 4 5 rule... It would equal out to... 9 + 16 = 25... Do you follow??? Last edited by joasis; 11-27-2006 at 07:59 PM.
 11-27-2006, 07:58 PM #5 General Contractor     Join Date: May 2006 Posts: 876 Rewards Points: 500 The basic carpenter's method is the 3-4-5 rule, or multiples of it...makes things simple, We use it for checking forms when seting up next to buildings if we don't shoot with a transit.....some guys build a large square for this...lots of old home improvement books show it.
11-27-2006, 08:17 PM   #6
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Quote:
 Originally Posted by yummy mummy Do you mean the 3, 4, 5 rule? You measure 3 feet across one wall, and make a mark, then measure 4 feet across the adjacent wall, and make a mark, and the measurement between those points is 5 feet. Is this what you are looking for?

You're getting good .....
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11-27-2006, 08:48 PM   #7
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## atlantic

I'm real good!

 11-27-2006, 09:37 PM #8 renovations   Join Date: Aug 2005 Location: Canada Posts: 430 Rewards Points: 250 There's a bunch of them, 5:12:13 works well for long skinny buildings.
11-30-2006, 08:50 PM   #9
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## Alternatively . . .

Pythagoras is smiling today. Maybe this is belaboring the obvious, but I don't know of anything squarer than the corner of a sheet of plywood.

Correct me if I'm wrong, you pros out there, but isn't that square enough for all building projects?

 12-01-2006, 08:40 AM #10 Member   Join Date: Mar 2006 Posts: 2,083 Rewards Points: 1,000 How about a carpenters square?
 12-01-2006, 11:40 AM #11 General Contractor     Join Date: May 2006 Posts: 876 Rewards Points: 500 Most of the DIY'ers would be surprised at how out of square most construction projects can end up...especially by us experienced guys. I was taught by a craftsman, and it is a point of pride for me that we build true and square, but I can't count the number of homes and commercial buildings that are not just a little, but way out......one of the nationally known home builders...the one that builds a house for you on land you own with nothing down, to 90% completion, and let you finish the rest to "save" money, is the worst overall...I have seen rooms...10 x 12, out 4 inches, no joke, and roofs that are a foot out, not to mention floors and doors out of level and plumb....but what would you expect from a crew that flies in and literally slaps it up in a week with #3 lumber, and wants to move on to the next one. A mobile home that has been moved a few times is more square.
 12-01-2006, 02:21 PM #12 Member     Join Date: Sep 2006 Location: Canada Posts: 1,733 Rewards Points: 1,016 I had my home built 3 yrs ago, and now that I know how to check to see if the rooms are square, there isn't even one room that is perfectly square. So I believe what you say about even the so called "pros" can't be perfect. So, I am really happy that the basement that I am renovating, my first use of the 3,4,5 rule, and I am out by 1/4 inch. I say that is pretty good. Mind you it took me 2 months to frame 2 walls.
 12-01-2006, 03:15 PM #13 General Contractor     Join Date: May 2006 Posts: 876 Rewards Points: 500 Just 2 months? sounds like you are a pro...that is the way my crews do it if I am not cracking the whip.
 12-01-2006, 04:10 PM #14 Member     Join Date: Sep 2006 Location: Canada Posts: 1,733 Rewards Points: 1,016 If you want to continue making money, you better crack that whip.
12-01-2006, 04:16 PM   #15
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## joasis

When I was building my house, and I asked the contractor why was he taking so long and why all of a sudden there would be 2 weeks where no one was on the job site.

His reply was that he cannot make a living just on my house alone.

This was my first time building.
If I had to do it again, (and overall it was a good experience), I would research all aspects of building from footings to fixtures, and if I had the time, I would contract it out myself, and I would be on the subs butt 24/7.
And I bet I would end up with a much better built house.

Joasis, as a general contractor yourself, would do you think about this?

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