How to Square a Deck
Squaring the Deck Frame
Introduction: Have you ever
visited a neighbor to have a beer on his proud DIY
made deck and wondered why it seemed to be out of
square? Well if you have you are probably not the
first person to notice such mistakes in DIY
square the deck box frame is a common oversight in
DIY homeowners when deck framing. Unfortunately
this oversight will make the entire project look
Squaring the deck frame is a crucial step that must
be taken when building a house or building a simple
Some carpenters prefer to use the method based on
the Pythagorean Theorem, the 3-4-5 triangle while
others use the cross over method. Using either
method will insure your deck frame is square.
The examples below will help clear up any questions
you may have on how to square a deck frame.
The 3-4-5 method:
Start by taking a measuring tape and hooking
the tape on the ledger board. After you hook your
tape, pull a measurement away from the house
alongside the outside box and make a mark with your
pencil at the 3 foot point.
Take the measuring tape and hook the edge of the
ledger board and pull the tape to the right and
make a mark at the 4 foot point on the ledger
After you have made your marks at the 3 and 4 foot
point, hold your tape at the 3 foot mark or drive a
nail at the mark. Pull your tape across from the 3
foot mark and connect with the 4 foot mark and if
your deck is square it should measure 5 feet
across. If it isn’t, move the box frame to make the
5 foot measurement correct.
When the deck is square, nail a piece of scrap or
strapping on the deck in such a way so the deck
stays in place while you attach the decking boards.
The cross over method:
Once the perimeter boards (band boards) are in
place you can square the deck using 2 tape
This will only work if the deck has
symmetrical geometry, square or rectangular.
For a square or a rectangle measure
diagonally from corner to corner on both sides of
These measurements should be equal.
If the deck frame is an octagon, select two
distances from opposite sides of the polygon. As
each joist is added they should add strength and
rigidity and should in effect square your deck if
they are cut to the appropriate lengths.
Obviously the more square the frame the
Slight tolerances can be adjusted before the joists
are installed to prevent exaggeration.
Compare the opposite lengths of the
perimeter of squares and rectangle forms. These
lengths should be the same.
Using a square and a 4 foot level will also
help verify that your deck frame is level.
You can also use the 3-4-5 rule to verify
any right angles.
That is how to square a deck. Squaring the deck is
a necessary step in deck building to insure that
the completed deck looks like it was built by a
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