Cut a circle out of ½ inch thick wood with a jigsaw with a radius of 24 inches. Remove the center of the circle, leaving a width of 3 ½ inches. Saw the circle in half, to make two semicircle bands.
Make nine slats of 27-inch long wood to cover the roof of the arbor. Use 1 by 2 boards to make these planks.
Saw four posts of the 4 by 4 lumber to 78 ½ inches for the structure of the arbor.
Select the type of trellis you want the roses to grow on. Cut two pieces of this trellis to 50 inches by 22 inches.
Cut four thin strips of wood to support the trellis in the arbor out of 1 by 2 lumber. The strips should be 22 inches long. These will frame the trellis.
Fit the open top caps onto one end of each 4 by 4 post. Put some wood glue on to each end of the post before putting the caps on. Let the glue dry.
Glue the strip to the trellises along the 22-inch length sides of each trellis. The 1-inch side of the strip should be glued to the thin ends of the trellises.
Place one trellis against the side of one of the posts, right below the cap. The unframed edges should be placed against the post, with the framed sides facing up and down. Glue the trellis in place.
Attach another post to the other side of the trellis in the same way. Fit the second trellis between the other two posts as well. These are the walls of your arbor.
Use a nail gun to offset finishing nails through the edges of the trellis frames and into the posts.
Stand the frame pieces up so the trellises are parallel to each other and so you are able to walk between them. The caps should be facing upward. Have a helper hold them up in this position.
Fit one of the semicircles into the front caps on the frames. The semicircle should be facing you, with each end in a different piece of the frame. Put the other semicircle in the back caps.
Run glue inside the gaps between the cap and the bottom of the arches. Fill these gaps with strips of wood until there is no more empty space.
Place the slats along the top of the arches with a nail gun. The slats should be perpendicular to the semicircles and rest on both of them.
Nail the first slat into the peak of the arch, the middle point of the semicircles. Space the remaining slats evenly over the rest of the arch and nail them in place.
Insert the four posts into the ground at least 1 foot deep.