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Before you begin, check local building codes, and obtain a building permit if necessary. Call your local utility company. They can help you mark your yard for any underground utility lines. Determine what type of addition you want to add. Create a simple image of the deck, and map out the railings, benches and posts. Decide whether you want the extension to receive partial or full sun, and identify any drainage issues that might arise. Tear down any old posts, stairs and railing that must be removed from the existing deck. If the old posts are set in concrete, cut them flush with the frame of the deck. Remove any shrubbery or trees that might be in the way.
For a two-level deck, attach 2" by 6" ledger boards around the edges of the existing deck to create a molding effect, and attach 2" by 8" joist hangers below the ledger boards. Temporarily nail the ledger boards and joist hangers in place. Then drill pilot holes, and permanently attach the boards with 3/8" lag screws. Space the lag screws approximately 3' apart.
Use stakes and a string line to help keep the deck square and level. Attach a piece of masonry string to the house or the deck at a height that will provide a good reference line. Use a line level to level the string. Set up a reference line for the sides and back of the deck, then check the corners for square.
One easy way to check whether a corner is square is to use the "3-4-5 rule." Measure a point 3' from the corner on one string and 4' from the corner on the other string. A diagonal line stretched between the points will be exactly 5' long when the corner is square.
Use a posthole digger or a gas-powered auger to dig 2' postholes for the deck. Fill the bottom of the holes with a 2" layer of gravel, set the 4" by 4" posts in position, and secure them in place with concrete. You may need to attach supports to the posts to keep them plumb while the concrete sets.
After the concrete cures, create a beam system to support the deck. Use 2" by 8" boards bolted to either side of the 4" by 4" posts. Drill holes completely through the boards and the posts, and secure them in place with 3/8" by 7" carriage bolts.
Install joist supports on the joist hangers around the deck. Space the supports so that the centers are 16" apart. Seat the joists in the bottoms of the supports, and secure them with galvanized nails or deck screws.
Cut 4" by 4" pressure-treated lumber to length for rail posts. Make a 1 1/2" by 7 1/2" lap cut in the lower ends of the posts so they'll be flush with the deck when installed from the inside of the frame. Drill pilot holes in the posts, and attach them with lag screws. Use a level to keep the posts plumb as you attach them to the deck.
Begin installing the deck floor with 5/4" decking boards, arranging the planks in any design you like. Keep in mind, however, that intricate designs will take longer and produce more waste. Attach the planks with #8 2 1/2" galvanized deck screws. Drill pilot holes in the ends of the planks to prevent splits.
When positioning your planks, try to keep them bark side up. You can identify the bark side by looking at the curved growth rings on the cut ends of the planks: the outside of the curve is the bark side.
When building with pressure-treated wood, butt the ends of the boards tightly together.
Attach 2" by 6" rail frames to the posts. Drill pilot holes, and attach the rails with galvanized deck screws. Attach one rail flush with the top of the post and another rail 4" above the floor.
Attach 2" by 2" balusters to the outside of the rails. Start in the center of each rail section and work toward the ends. Use a block of wood as a spacer to help you keep the balusters evenly placed.
If you plan to install a bench, leave a gap in the rail. The standard height for a bench seat is 18" from the floor, and the back should have a 7-degree slope.
Create the bench using 2" by 6" pressure-treated lumber. If your bench will be 8' long, you'll need four support frames: two on the end and two in the middle.
Drill pilot holes in the frames, and attach them to the deck floor and rails by toenailing them with 2-1/2" galvanized deck screws.
Use 5/4" decking boards to create the seat slats and back slats. Try to use boards that have as few knots as possible. Don't enclose the bottom of the bench. That way, people's feet can fit comfortably underneath.
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