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Once the outer edges of the expansion are marked off, dig post holes about 2 feet deep. Cut the posts a fewer inches longer than needed, they can be trimmed later.
Lay down some gravel (image 1) before placing the posts in the holes and adding concrete (image 2). After the concrete has cured, beams can be attached to the posts. These beams will support much of the deck’s weight so use at least 2x8 boards.
Use lag screws to attach 2x6 or 2x8 ledger boards to the area of the original deck frame where you plan to add on. Pre-drill and attach beams to either side of the posts using carriage bolts. Each post should be checked with a level to make sure it is plumb before you brace it on two sides of the post.
Next up are the joists. Check each one to see how it crowns and install it with the arch up. Attach these joists to the beams with joist hangers to give the structure extra strength. The joists should be placed 16 inches apart on center in the same direction as the existing decking boards.
Treat all the lumber with a wood preservative before you install the decking boards and railings. Lay the decking boards of the deck addition perpendicular to the decking boards of the existing deck. Pre-drill the ends of the decking boards to avoid splitting the wood when you drive in deck screws. Always wear a mask when cutting treated lumber.
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