Q: 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 footings?
—Amy Baumgart, Medford, OR
A: 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.