You square it off the best you can. Then you take
diagonal measurements if it is square or rectangular -
the measurements must be equal. Once you get it
square, attach temporary diagonal deck boards at the
corners you are not working on to keep in square.
This is how I do it anyway. I am not a deckbuilder,
just a DIY type guy that knows a little geometry.
Mac-It is very important that the deck base frame
be square to whatever you are building it against
plus to be square to itself so that when you install
the floor joists, dceckboards, deck rail, etc.
everything fits easily and uniformly. If the deck
frame is not square you will have all kinds of
additional headaches installing the components. The
normal allowance for a project such as a deck to
be "out of square" is approx. 1/8"-1/4" to itself in
a distance of say 20 feet. Nothing ends up exactly
perfect and a small incrementsuch as that will not
substantially affect the building of the deck. But
always strive to be as close as possible to be
truly "square". As far as how you check for square
on a deck which may have a substantial length and
width of say 14 feet out from your house and 30 feet
long, you must use geometry and general math
principles. If the deck is either a square or
rectangle you can measure from one corner to the
opposite diagonal corner and then do the same on the
other side. The measurements should be within 1/8"
of each other if the deck is square. Also to make
sure the deck is "square" to the house you can sight
the deck visually and also use the "pythagorean
theorem" of 3-4-5. When you measure the frame of the
deck at the corners one side should be 3 feet one
side should be 4 feet and the hypotenuse or the base
of that triangle should be 5 feet. Once you have the
deck frame square you do need to secure the deck
frame before working further so that you dont
accidentally knock the deck out of square and forget
to resquare it. By the time you get most of the
floor joists in the deck becomes too heavy to move.
So install some bracing on the deck frame and use
care so that the deck stays square while you build.
The 3-4-5 method works great...You can double it to 6-
8-10 and so on...Brace it well and just remove a brace
when you need to...Most likely as you approach a brace
during the decking process...
You can use a string to check for the straightness of
the outter box joists and when you apply the braces,
make sure you sight down the string one more time
after you nail the braces...
HOW TO PRE-CUT A DECK...
Let say your deck is 10' x 12'. Make a drawing of the
deck with the calulated measurements and number of
pieces needed of each measurement and just start
I can't tell you how many decks I started on time,
because of this method...
Rain sleet or snow, you can still pre-cut any deck
anytime... Your garage, the customers garage, just
about anywhere and when the weather breaks, your ready
Even if your pre-cut deck doesn't set-up perfectly
square, as you bring the deck boards across the deck,
you can always measure the remaining area the deck
boards need to be installed-over by taking three
Left-edge, middle and right-edge...
If all 3 measurements are the same, your golden...But
if they're different, start making slight adjustments
to each new course...
If you only an 1/8 of an inch off I wouldn't check it
again for several courses...
The difference in measurement from one one end of 12'
piece of decking to the other could vary at least that
much, plus some boards wil be about 5 1/2 inches wide
and some others 5 5/8 to 5 7/8 wide...So once you
begin to understand the issues that creates, being
dead-on square really isn't a big issue...
I've been building houses for decades and I don't
think I've ever started off with a dead-on square
foundation...Not once, so pre-cutting all you frame
ahead of time, doesn't hurt anything, despite what my
fellow handymen might have to say about my deck making
skills after reading my response to your question...
If your middle measurement is more or less than the
side measurements, just open or close the ends a
little on your next couple of course and recheck it
All you deck boards shoul be installed as tight
together as you can get them...
No more spacing deck boards like years ago..
They shrink to much now to be leaving a space between
them so if and when you make an adjustment, just make
it a minor one...That's why you start taking
measurement when you have several feet left to deck...
( Lets say at least 4 or 5 ft left to deck )
Only make the adj. on one end of the deck board at a
You can also take the remaining measurement and divide
it by 5.5 and see if you'll end up with a full deck
board or a sliver of a board and start making
The sooner you start to take the measurements the
sooner you'll take control of your decks final
12 feet or 144 inches divided by 5.5 equals
26.181818...So based on the average width of your
stock, you should be able to start-off with a full
board and end with a full board somewhere between 25
or 26 boards with the proper measuring and
adjustments...Even if your deck is not square...No
human eye or fellow contractor will ever
notice...Trust me on this...Good Luck...E.T.