Decks demand precisely placed piers. That's where batter boards come in. Made of scrap 1x3s staked in the ground beyond the two outside corners of the deck, these temporary wickets hold mason's strings stretched above where the deck's rim joists will run. To make sure the strings are square, slide the ends that are tied to the batter boards from side to side until the two diagonals—shown as dotted lines in the drawing, left—measure the same. (Tack nails into the batter board where the strings line up correctly so the strings can be removed and restrung as needed.) The point where two lines cross marks the center of a corner pier—drop a plumb bob here so its string just brushes the intersecting lines and mark the ground with spray paint or powdered lime. The lines can be used similarly to place interior piers.