Learning how to build an arbor made from wood that is arched is not as difficult as it looks. The trick is in knowing how to make the arch. Cut two, 2 x12 inch lumber pieces (pressure-treated pine is a good choice) in half to make four pieces. Lay out two of them to make a capital T, ensuring the bottom, vertical bar of the Tee is centered in the middle of the horizontal piece.
From the center of the top piece, measure 30 inches down to the bottom piece, driving in a nail to which you have attached a length of twine. Tie a pencil at the end of the twine so that the pencil measures 30 inches away from the nail when the line is held tight.
Holding the pencil with the twine taut, go from left to right of the top piece of wood, drawing an arched line. Reset the pencil three-and-a-half inches lower along the twine and draw another arch beneath the first.
On each bottom side corner of the top piece of lumber, draw a 45-degree angle (long side facing inward) and cut with a circular saw. Use a saber saw to cut out the rest of the arch. Repeat, using the other pieces.
Attach and Reinforce
The next part of learning how to build an arbor with an arch entails affixing and reinforcing these half arches to form a complete, stable arch. Using a scrap piece of lumber at least eight inches long, butt two of the half arches together (duct tape, if needed, to keep them from sliding around) and slip the scrap piece beneath them.
With a pencil, using the arch pieces as a guide, trace along them, marking on the scrap wood to four inches on either side of the center where they are butted together. Using a saber saw, cut out what is now a gusset to be used to fasten the two pieces together. Make another one for the back arch of the arbor. Use 2 ½-inch screws to fasten the gussets to the arches.
Cut two, 2 x4 pieces however long you want the arbor to be deep. This next step in how to build an arbor will be to connect the top arches. So, we will say, cut them 39 inches long (which will make the arbor 39 inches from front to back), and, using three-inch screws, attach to the insides of the arches at both ends. Use 42-inch long 1 x4s to make seven roof slats and attach with two-inch screws, beginning with the first one directly over the seam where the two half-arch pieces meet.
Building the Bottom Part and Finishing Up
Cut four, 72-inch 2 x4 inch pieces for posts in this next part of how to build an arbor. Then cut 1 x 2 lattice pieces 42 inches long, as many as you want for each side of your arbor. Between two 2x4s, center them horizontally eight inches apart and attach on the outside of the 2x4s with two-inch screws. Repeat with the other set of 2x4s. To reinforce, add vertical lattice pieces set 10 ½ inches apart and attached with 1 ¼-inch screws.
Attach the top arch piece to the bottom posts/lattice piece as the last part of how to build an arbor. Do this by butting one side of the arched top to one side of the posts/lattice and affixing with three-inch screws. For optional stability while setting in place, add 1×2s horizontally to the back and/or front of the arbor. You can also dig holes and set the posts with cement for further (permanent) stability.
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