Building Overhead Structures, Pergolas and Arbors
An overhead structure, pergola, or arbor is frequently used to make the outdoors more inviting, to extend livable space, to create a shady retreat, and serve as a major landscaping element.
Amount, Specifications and Supplies:
Most overhead structures are built from wood. It is easy to cut, shape, and fabricate; it comes in a wide range of sizes and offers a variety of species, grades, and textures. Most importantly, wood has a natural appearance that looks good outdoors. Go to Varieties of Wood for Outdoor Structures for information on wood types.
Depending on the design of your overhead structure and the site conditions, specific construction detail will vary. All overhead structures should be designed to ensure maximum safety.
It is recommended to use at least 12-foot lengths of 4 x 4 or 6 x 6 inch cedar, redwood or pressure treated posts for the main structure. Untreated wood will not hold up when exposed to constant moisture and weather and should not be used. This height is recommended to allow for enough headroom after the timbers are positioned in the ground. For the remaining structure 2 x 4's or 2 x 6's are most commonly used. For larger structures 2 x 10's or 2 x 12's may be necessary. These can be nailed, screwed, or bolted directly to the posts.
Tools and Equipment:
- Power or hand auger (12" diameter)
- Manual post hole digger
- Tamping bar
- Twine or construction string
- Framing square
- Carpenter's level (2 foot minimum, 4 foot preferred)
- Line level
- Hand saw (crosscut)
- Retractable tape measure
- Power circular saw
- Power drill
- Miter saw
- Jig saw for decorative ends
When choosing a site for your overhead structure, you must consider its relationship to the house and yard. It should be positioned so that it is easily viewed and accessed from the house, but not so close that it is unnoticed when looking out the window. It should be a pleasing accent to the existing landscape but not so dramatic that it is the only thing you see. Because this becomes an outdoor room, consider the natural canopy created by existing trees and try to fit the overhead structure into this scheme. It is also important to know the sun's path. An overhead structure can serve as wonderful protection from the afternoon sun. The overhead structure should be softened and blended into the landscape with plants for the best effect. For more information on sustainable landscape design see the Design section of SULIS at www.sustland.umn.edu/design.
Step-By-Step Process: Building a Garden Arbor
- Choose the site where you would like to build the Arbor. The Arbor should be designed with site considerations in mind. In Minnesota before digging call Gopher One to mark all underground wires at 651-454-0002.
- Carefully measure and mark the areas where the posts are to be positioned. If your design calls for corners that form exact 90-degree angles, use the 3-4-5 rule to square the corners. You will need another person to assist and two tape measures. Find the exact area you would like a corner of the overhead structure to be located and drive a stake into the ground. At a 90-degree angle, mark a spot exactly 3 feet away from the stake. The second person should hold the end of one tape measure at this spot and place the second tape measure on the stake. Next, pull out the tapes until the 4-foot mark on the tape attached to the stake intersects the 5-foot mark on the other tape. Drive the corner stake into the ground at this point. Repeat the process starting at the new corner stake. This triangular method works in any multiples of 3-4-5, (for example: 6-8-10 or 9-12-15). For maximum accuracy, use the largest ratio possible.
To check your rectangle, measure the distance from one corner (A) to the opposite corner (C). Then measure the distance from corner B to corner D. The distance from A to C should equal the distance from B to D. If the distances are not equal adjust (move) the stakes as necessary.
- Use either a 12-inch power auger or a manual posthole digger to dig postholes a minimum of 54-60 inches into the ground. It is important that the posts are positioned below the frost line to ensure the structure will not heave with freezing and thawing. Each municipality sets standards for these depths. Check with the municipality in your area for the minimum requirements.
Lay 4 to 6 inches of pea gravel, class 5 fill, crushed rock, or a concrete footing pad as a base at the bottom of the holes. Use the tamping bar to compact the base. For information on other footing styles see Deck and Outdoor Structure Footings.
- In our arbor example the posts should be 12-foot lengths of 4 x 4 inch cedar, redwood, or pressure-treated timbers. Remember, pressure-treated lumber is the strongest of these woods. Purchase posts that are straight and square to prevent cracking and warping. Twelve-foot posts will allow for a 7-foot ceiling. Taller structures may require 8, 10, or 12-foot ceilings. Longer posts are more expensive but allow for a taller ceiling. Place the posts square in the hole by using a carpenters level on adjacent sides of the post. Backfill with crushed rock or class 5 fill 12 inches to temporarily set the posts. Support lumber can be temporarily attached to the posts to hold them in position.
The crushed rock or class 5 fill is used to assist in drainage and keep any water from settling next to the post. Use the tamping bar to compact the fill. Next, run string around each of the outside edges of the posts to help position the posts in a perfectly square or rectangular format. The string should just touch the outside edge of each post while being held taut in each direction.
Remember to use the 4-foot level to insure that the posts plumb. Backfill and compact the fill in 3-4 inch increments. Continue this process until the holes are completely backfilled and compacted, and the posts are plumb.
- Measure each of the posts individually and saw them off at the height that corresponds to the finished elevation. Do this by tying a construction string at the desired height of the first post and by hanging a line level on the string between each pair of posts. Next, using a level or straight-edge, draw a cutting line across each of the upright posts. Snapping a chalk line also works well. Cut off the tops using a circular saw. If your saw does not allow you to cut the core of the posts, use a handsaw to finish. ***Note: If decorative post tops are desired they will be easier to create before the posts are installed.
- In our arbor example the roof has a roof support beams on two of the four sides (if rectangular, they are usually positioned on the longer sides). The support beams for our arbor are constructed of 2 x 6 or 2 x 8 inch lumber and attached on each side of the support posts. The beams are connected with the appropriate length carriage bolts to fasten the support beams to the posts (drill recessed holes for head of bolt and nut). The support beams can be finished on the ends to give more character to the structure. This should be done prior to mounting them on the support posts.
An example of our arbor's design and construction.
An example of a different style of arbor design and construction.
- Once the four support beams are in place, attach the surfacing lumber with 2 x 6's or 2 x 8's. For a more decorative appearance, and to add strength, use a saw to notch the surfacing lumber so it is recessed into the support beams. Securing the surfacing lumber depends on its dimensions. Lumber can be toe-nailed or screwed to the support beams. ***Note: It is recommended that you measure the distance to be covered, and then calculate the spacing distance and the number of surfacing boards you will be attaching. The spacing depends on the amount of sun or shade desired, the span between posts and the size/strength of the surfacing lumber. Use a spacer board cut to the correct length to accomplish this.
An example of our arbor's surfacing lumber design.
An example of a different style of surfacing lumber.
- Additional post sections can now be added between the support beams and surfacing boards for additional strength and aesthetic appeal.
- If additional support is needed cross bracing can be used between the corner posts and the end surfacing boards.
A small to medium garden arbor.
If you desire to have a climbing vine drape over the top of the outdoor structure, the following list of vines work well and are hardy in Zone 4:
- Perennial vines:
- Honeysuckle (Lonicera japonica)
Kiwi Vine (Actinidia kolomikta)
Clematis (Clematis species)
- Annual vines:
- Scarlet Runner Bean (Phaseolus coccineus) Black -- Eyed Susan Vine (Thunbergia alata)
Additional vines can be found in the Plant Selection Program in the Plant Selection section of SULIS.
If the outdoor structure was not built over a pre-existing patio or alternative flooring structure, you will most likely want to create a flooring structure to finish off this project. For Paver and Brick Installation go to www.sustland.umn.edu/implement/paver.htm in the Implementation section of SULIS.
More Outdoor Structure Details:
Outdoor Classroom pergola, Department of Horticultural Science -- Trial and Display Garden, St. Paul, Minnesota.
4 x 4 inch cedar posts 2 x 6 inch cedar support beams 2 x 6 inch cedar surfacing lumber
Pergola interior includes:
patio, deck, seating, planters,
trellis, and hanging baskets
Outside view, North side
Outside view, West side
Sustainable Garden pergola, Department of Horticultural Science -- Trial and Display Garden, St. Paul, Minnesota.
4 x 4 inch cedar posts
2 x 8 inch cedar support beams
2 x 6 inch cedar surfacing lumber
Agriculture Theme pergola, University of Minnesota Southern Research and Outreach Center, Waseca, Minnesota.
16 - 6 x 6 inch cedar posts
2 x 8 inch cedar support beams
2 x 8 inch cedar surfacing lumber
1 x 2 inch decorative lattice
Blanchette, Bob. 1997. Professor of Plant Pathology, U of Minnesota. Personal interview.
Doyle, Bob, Ed. 1997. Sunset Deck Plans. Sunset Books, Inc. Menlo Park, CA. pp. 14-45.
Gardenstructure.com. 1999. "Garden Structures and Plans." http://www.gardenstructure.com
Garlinghouse -- Build it Yourself Project Plan. 1999. "Swing and Arched Arbor." http://www.treatedpine.com.au
Lantto, Ken. 1997. Lantto Construction Company. Personal interview.
Lifetime Books. Complete Book of Outdoor Projects. pp. 297.
Nelson, Jr., Wm. R. 1990. Landscaping Your Home. Revised edition. U of Illinois at Urbana-Champaign College of Agriculture Cooperative Extension Service. pp. 74
Pedersen, Brad. 1997. Professor of Horticulture. U of Minnesota. Personal interview.
Sauter, David. n.p. "Wood Overhead Structures." Landscape Construction.
Stevens, David. 1999. Garden Features and Ornaments. London. pp. 14-15.
Vandervort, Don. 1999. Sunset Patio Roofs & Gazebos. Sunset Books, Menlo Park, California.
Von Trapp, Sara Jane. March, 2000. "Building a Pergola." Martha Stewart Living. pp178-184.
Woodsmith Store -- Project Plans. 1999. "Garden Arbor Plan." http://www.woodsmithstore.com/garaplan.html
This implementation report was developed by Joanie Somerville, Stephanie Jutila, and Michael Sonnek, students, University of Minnesota Department of Horticultural Science.