Norm's Tricks Of The Trade
David: Question: We'd like to build a deck off the slider door in our bedroom. How do we figure out where to dig the deck's footing? Amy Baumgart, Medford, Oregon
Norm's Reply: The trick I use is the simple 345 rule from highschool geometry. It says that any triangle with sides that are 3, 4, and 5 feet long forms a 90degree angle between the 3 and 4 foot sides. The same is true for multiples of those lengths: 6, 8, and 10 feet, or 9, 12, and 15 feet. Longer lengths ensure pinpoint accuracy.
To use your deck as an example, rest one end of a board on a joist hanger attached to the ledger. Mark the top edge of the board 6 feet from the end resting on the hanger. Next mark the ledger 8 feet from the board. Drive a nail into that mark, hook a tape measure on the nail, and extend it to the mark on the joist. Now move the board and the tape from side to side until the 10foot mark on the tape lines up with the 6foot mark on the board. The board will then be perfectly square to the ledger. You can drop a plumb bob anywhere along the board to pinpoint the spot to dig your footings.
