Problems making the deck framing square

by Mike
(Halifax, MA)

Hello to all! Having some problems making the deck framing square.

Just completed the frame on my deck which is 8 x 24. Haven't attached it to the beam yet and will once it is square. In measuring diagonally both measurements are 303-1/2" but when measuring from left to right the front and rear measurements are off by 2-1/4". The rear measurement from left to right is 286-1/2" and the front is 288-11/16". My brain is telling me that this is not possible. Anyone have an idea what I'm doing wrong?

Editor's Comments

Geometry does not lie. Your deck frame is not square. If it were truly square your diagonal or hypotenuse would be 303-9/16" (25'-3 9/16") and simultaneously your lengths, front and back would be 288" (24'). However since the back and front lengths are off by 2-1/4" you know for sure the corners are no longer 90 degrees.

Think of it this way. You can still have your diagonals equal in length and yet not be square. This would result in a quadrilateral but not a rectangle. Measure the length of the two shorter sides and and then compare them with your two longer lengths. You probably have four unique measurements.

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Dec 05, 2015
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Squaring your deck
by: S.D. General Contracting

Now that you have used the 3-4-5 method, measure your deck from one corner to the corner at the furthest point of your deck and them measure the opposite corners and you should have the same measurement on both if your deck is in an "L" shape.

Make the main portion without the "L" square so that then once you have the same measurements you can then go on and square the "L" portion using the same method.

hope this helped!!!!

Nov 11, 2015
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Square Deck
by: Mike

Yes, I think I found my mistake. The two shorter sides are equal but I don't think I measured 90 degrees off the house correctly. Used the 3,4,5 rule but not quite right. If I make the front rim joist the same as the ledger board (286 1/2 inches) and then check the frame again for square I should be ok. Does that sound right to you?

Editor's Comments

That is the right place to start. If all your pieces or sticks are cut to the right lengths then it becomes very hard to mess up the framing of the deck. With each joist cut correctly it simply has to come together perfectly, or very close to perfect.

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