|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 3-4-5 rule from high-school geometry. It says that any triangle with sides that are 3, 4, and 5 feet long forms a 90-degree 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 10-foot mark on the tape lines up with the 6-foot 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.