Before you begin, produce an accurate scale drawing and get it approved by your building department. Keep in mind that the decking will overhang the joists by 1½ inches or so on all sides. You’ll also need to plan out the footings and beams. Because they extend 3 feet beyond the footings, massive beams made of three 2x10s are required. Large beams call for larger-than-average supports—4x6 posts and 12-inch-diameter footings. Expect to spend four to five hours building batterboards, figuring out the layout, stretching lines, and determining footing locations. To lay out the deck, you need to be able to measure and check for square, fasten screws, and pound stakes. It’s also a good idea to double-check your drawings for accuracy once they’ve been approved and to study the way the deck will be assembled. Then drive a batterboard into the ground near the house, more or less centered over the post location. Pound the stakes until the crosspiece is 6 inches or so above the ground and stable. For soft, wet soil, use long stakes; shorter stakes are fine for hard ground. Project tip: To mark the center of a posthole, hold a plumb bob (a chalk line will work in a pinch) with its string barely touching each layout line. Drive a stake or landscape spike into the ground to mark the spot. If the design calls for three or more posts supporting the same beam, the middle posts do not require intersecting string lines. Just measure along the line or use a spacing jig to mark the center of the footing.
