A pergola is an arbor or a passageway of columns supporting a roof of trelliswork on which climbing plants grow. Most pergolas are square or rectangular. If your landscaping design incorporates curves, though, you may want to build a pergola that looks more organic. A circular pergola is a truly interesting and organic piece of outdoor architecture. You may find it slightly more difficult to build than a square one, but the result is well worth the effort.
Download or buy a plan for a circular pergola, or draw your own plan. It should keep you on track as you build, and you may need it for your permit or variance.
Check with your local building authority to learn if you need to get a permit or variance before building your pergola. Be sure to have it before you start construction. Because you need to dig holes for the posts, you should also check with your area's "call before you dig" service so you don't encounter an underground water, gas or electrical line.
Determine where you want to build your pergola. It should be on as level a surface as possible. Look at the site from all sides, and look out from inside the proposed interior to make sure you've found the best possible spot. Remove the sod from the area and level the ground with a shovel if necessary, using a three-foot level to make sure it's exact.
Put a peg or stake in the center of your pergola's space. Decide how large you want the internal space to be. If you're building a 10-foot diameter pergola, you can use six support posts, but if you want it bigger you'll need eight. Tie a string to the center stake that is half the diameter you want, and use it to mark the inside diameter of the pergola.
Insert stakes at the 12, 2, 4, 6, 8 and 10 o'clock positions on the circle for a 10-foot pergola. Make sure that the lines from 12 to 6, 2 to 8 and 6 to 10 o'clock are all exactly the same length. Dig an 8-by-8-by-18" deep hole at each stake, with one straight edge along the circumference line and the rest outside it.
Level the bottom of each hole, and insert an 18-inch long prefabricated concrete footing with a bolt attachment. One person should hold the footing in place while you check to be sure that one edge is even with the circumference and the top is level with the ground. Fill any empty area around the footing with dirt and tamp it down securely.
Raise a 6"-by-6" support post onto one of the footings. One person should hold it in place while you check to see if it's level and plumb, then bolt it into place. Repeat with the remaining five footings. Check again to see that each post is the same height, and saw off any that are too tall, then smooth the top.
Use graph paper to draw the circle of top beams for the pergola, with the interior diameter of 10 inches. You need six curved pieces of flat wood to attach to the tops of the beams to create a circle, the circumference of your pergola. Extrapolate from the sketch the actual size and shape you need to cut.
Make a sturdy pattern for the curved beams. The pattern should be an arc that's one-sixth of the circumference, or about 5 1/4' long on the inside, and 4" wide. Trace it or cut around it with a table saw or circular saw to make six of them from the 2"-by-12" boards.
Place the beams in a circle on top of the support posts. Use a ladder to reach the top of the beams, and bolt each one securely to the post at each end. You may want to drill bolt holes in the board ends, then mark the spots on the post tops, and drill starter holed there before attempting to secure the bolts.
Take a 2"-by-4"-by-12' board and nail it to the tops of two opposite posts. This will be your cross beam to help hold the rafters in place. If you've decided which space will be your main entry to the pergola, place this board perpendicular to a straight line in from that space.
Cut 2"-by-2" boards for rafters. The longest will be just more than 10 feet, and they'll decrease from there. Refer to your graph paper circle or plan to determine how many rafters you want, and the lengths they'll need to be. Use a circular saw or jigsaw to notch each rafter where it crosses the center cross beam, and nail the rafter to the pergola on the arc beams and perpendicular to the center cross piece.
Add premade trellises and plantings to the side of the pergola opposite the entry if you want to create a shadier pergola. Consider adding a half-wall of brick or stone along that side if you don't want plants.
Finish the pergola by building a circular deck or a round stone or brick patio to the floor of the pergola. A deck or patio can extend through the space you want to use as an entry to create an invitation into the curved space.
Making your own plan allows you to create exactly what you want. It doesn't have to look like an architect designed and drew it. The plan only needs to be clear to you, and perhaps the city building department, so that you can use it as you build. When working with power tools and wood, use safety precautions including goggles and gloves. Because you'll be putting heavy tall posts in place and measuring long distances, this isn't a job for one person. Get a friend or two to help you build. Since you need to dig several holes (six for a 10-foot pergola, eight for a larger one) you may want to use an augur or power augur to save time and energy. You may need to drill a starter hole for the bolts holding up the support beam. Use a good quality wood for the curved top beams. A cheap wood may split when you cut an arc from it. Lay out the top pieces on a flat surface to make sure they form a complete and regular circle before attaching them to the top of the support posts. You may want to cut six arcs from newsprint or other large paper before cutting the wood, so that you know the pattern can make the circle, and can make any necessary adjustments before you start cutting. You may need to use a drill to make starter holes for the bolts in the tops of the beams. Each arc-shaped top piece will cover half of the top of a post at each end. Make sure that the bolt holes are far enough from the end of the arc that they don't split the wood. Make a template to cut a 1"-deep by 4"-wide notch in the center of each rafter so that they fit snugly over the center cross beam. How many rafters you make is up to you. More rafters will provide more shade, while fewer of them will keep the inside of the rafter sunnier. Base your decision on your planned use of the pergola. Your 10' in diameter pergola is the perfect size for a table and chairs or for a pair of comfy outdoor chaise lounges to provide a relaxing hideaway.