Build an Arched Pergola - Canadian Woodworking Magazine

Home Improvement: Adding a pergola to your deck is a great way to expand its usage. If well thought out, it will provide shade when you need it, and create a cozy space in which to relax.

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Build an Arched Pergola



Photos by Matt Dunkin

One of the many factors to consider when approaching an outdoor living space like a deck or patio is how to create shelter from the sun during the hottest summer months, while still allowing light through during the rest of the year. This is even more imperative if the outdoor living space is attached to, or directly adjacent to your house; a permanent structure can block a significant amount of light during the winter months when direct sunlight is most welcome.

The answer is the pergola, a wooden framework of posts supporting a horizontal trellis over which leafy vines can grow to provide shade during the summer, but allow light through during the early spring, late fall, and throughout the winter months. Pergolas define a space, turning a deck, which might otherwise feel too exposed, into an outdoor room, combining our need for shelter with our desire for a vantage point. The pergola is the perfect sheltered place to sit and enjoy a meal on your deck or patio during those relaxed summer days when you yearn to be outside.

Unlike porches with a full roof, pergolas do not encounter a significant amount of snow load or stress from wind, and so are considered more ornamental than structural in terms of needing to comply with building codes. They need to adequately support their own weight and the weight of whatever is grown on them, but beyond this pergolas can provide an opportunity for creativity and a significant amount of flexibility and variation from a design perspective. By the very nature of their vertical posts and horizontal frame, the average pergola can look a bit boxy. On a couple of deck projects, I have attempted to escape that tendency by creating pergolas with a bit more visual interest, incorporating a gently curving arch into the framework of the horizontal trellis supports. I’ve built two now out of western red cedar and in this article I’ll outline my design and construction process as well as the technique I used to make them stand out.
 
The Posts
While they do not bear an intense amount of weight, pergolas have an implied weight about them and posts need to be sized accordingly. Because of the visual weight of the vines and horizontal members, to place them on anything less than a 6x6 as a post will make the pergola look top-heavy and flimsy. Both pergolas I’ll refer to in this article have used 6x6s that are integrated into the floor framing of the decks into which they are built; additionally, one forms part of a built-in bench while the other is incorporated into the deck’s railing. In each case, that intermediate support is important to laterally brace the post and make the pergola above solid. Because the post tops are exposed to rain and snow, I cut angles on them to shed most of it away.

The first pergola I built was to be narrow and cover a built-in bench, and so I used only two posts to support two parallel arches. The second pergola was to be built into the corner of a deck and would form a right-angled triangle, so incorporated three 6x6 posts tied into the deck railing. Hereafter I’ll refer to them as the “parallel” pergola and the “triangular” pergola to avoid confusion.


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Create a Space – A pergola sets the mood for your deck. The parallel curved arches add intrigue to this otherwise simple, yet inviting, pergola over a bench.
 
The Arches
When it comes to building most things out of wood, straight lines are simple and inexpensive to create, while incorporating curves can increase complexity, expense and waste. In approaching my arched pergola design, I drew inspiration from a stunning railing my father built for my elderly grandfather several years ago. In searching for a way to make more accessible the winding stone stairs down to the lake at our family cottage, he created a curved handrail by ripping fine strips of cedar, steaming them and then laminating them together in place to trees and strategically-placed cedar posts along the serpentine pathway. The resulting handrail has been both beautiful and functional and appreciated for many years now.

Adapting the technique of ripping, bending and laminating, I decided that the degree of curvature I would need for the pergola I envisioned was less dramatic and so I omitted the steaming step. I created a bending form 16 feet long from a full sheet of plywood, another partial sheet, and wood scraps. A block at the apex of the arch held it about 10" for the parallel arch and a slightly more dramatic 12" for the longer span on the triangular arch. Intermediate blocks helped to round out the shape of the arc.

Next, I ripped approximately 1" strips of 2x cedar after experimenting with the degree of bend I would require. I kept them in sequence after ripping them by numbering them to aid in re-assembly. After the width of the table saw blade was removed, a 2x8 provided a finished width of 5 -1/2" when laminated for the parallel arches, and the longer triangular arch was cut out of a 2x10 and finished down to near the width of a 2x8. In order to be able to glue them together, I needed a smooth surface and so I gang-planed both cut surfaces on a thickness planer to clean up the ripped surfaces. I laid the first strip down on the form and fastened each end to the form with one screw to bend the wood into place. With a small paint brush, I applied a generous coat of waterproof wood glue to the face of the strip on the form and then clamped the next strip in place at one end and worked along it fastening deck screws every 12 to 16" to connect the pieces mechanically while the glue dried and as insurance against the possibility of glue failure in the future. A second pair of hands made this go much more smoothly the second time I did it. The thickness of subsequent strips allowed me to connect multiple strips together with longer screws as I went. I made sure to keep screws away from the areas near the ends where I would need to cut a finished angle on the arches to avoid damage or injury from hitting a screw with my saw.

When the glue dried, I removed the initial anchor screws, scraped the glue squeeze-out off and sent the arch through the thickness planer again on each side to give a nice clean face. Placing it on the ground “arch-up,” I used a level to draw plumb marks on either end for the final vertical cuts on the mitre saw. I used a small router with a round-over bit to finish the edges and ends of the arch making it blend in with the other pieces of milled dimensional lumber. The result was a curved piece of wood that was relatively simple to make and yet was more appealing and imaginative than a straight horizontal piece of wood.


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Keep Things Simple – A flat platform to work on and some spacer blocks aid in producing a pleasing arc.

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Flatten the Faces – Once the curved arches have dried, Dunkin planes their faces smooth.
 
Trellis Cross Members
Traditionally, a pergola would have horizontal trellis members laid in two layers, perpendicular to one another; far enough apart to allow light to pass through and yet close enough to support the growth of vines. To create a simple but pleasing finish detail on the ends of each trellis cross-member, I cut it square and then with four successive mitre-saw cuts created a chamfer on the end.

For the parallel pergola, set on two posts, I inset a horizontal 4x6 piece of cedar 32" long into the top of the post and spanned them with two identical arches. Across these arches I used 2x2s set apart at 4" centers. Wanting to give the arch a bit more visual interest, I varied the length of the 2x2s to create another arc in the distance they protruded over the side of the arches beneath. This curved arrangement is cast in shadow during the brightest part of the day across the deck in an interesting pattern.

For the triangular pergola, only the hypotenuse of the triangle would have a curved arch to it, while the rest of the frame would be formed by straight horizontal 2x8s. Because those 2x8’s differ in length, the cross-members create a soaring asymmetrical pattern, especially since I again arranged the length of the cross-member overhang in each case to create a secondary arc. The spacing is much wider between the cross-members (10 inches) but they are of thicker stock given that their span is much longer – a 2x6 ripped in half and set on edge.


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Cross Members – A series of cross members provide some of the shade. If they are cut at different lengths they will add visual interest with curves or other patterns. Dunkin also took the time to chamfer the ends of the cross members, adding a small, but classy detail to the pergola.
 
Assembly
Because of its size, it was relatively simple to assemble the parallel pergola’s components in place from a step ladder. I toenailed screws through the arches into the 4x6 supports below, and was able to screw the cross-members from above with 3" deck screws. Given the size of the triangular pergola and the height of its deck off the ground, I had to get more creative. I created the 2x8 triangular frame, attached it together, and left it resting on the railing. I then experimented with the layout of the cross-members and cut them to finished length and chamfered their ends all without needing a ladder. I fastened the cross members to the frame by screwing down through the cross-members with 4" wood screws. Then, with everything ready, I enlisted help to raise the entire pergola up to near the top of the posts, level it and screw the frame to the posts with self-tapping lag screws whose head sunk flush with the face of the wood frame.


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Keep Things Low – Until most of the work is done keep the main part of the pergola at a comfortable working height. Once it’s ready, you can lift it in place and attach it to the structural framework.

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Fasteners – If you use exterior fasteners, you will be better off in the long run. Be sure to use a heavy enough product – lag screws in this case – as wind and other elements can punish a pergola.

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Greenery Creates Shade – There are many options when it comes to choosing a species to grow on your pergola. Check with your local greenhouse or nursery to determine what’s best for your area.
 
Finishing
It is with a certain amount of resignation that I build something out of wood and then place it outside for the sun and rain and temperature extremes to do with it what they will. Even the most carefully constructed outdoor project will begin to show signs of weathering in the first few months after being placed outside. In an attempt to protect the parallel pergola, I finished it with Benjamin Moore’s Arborcoat exterior stain system, which involves the application of one coat of stain followed by one coat of a protective clear coat. For me, it was most critical to have the laminated arch sealed from potential water damage so I stained the arches before putting them up. Subsequent advice from staff at my local supply store was that I would get better adhesion of stain and clear coat if I waited a year for the oils in the wood to dissipate. At a minimum, I made sure that the laminated arch of the triangular pergola was clear coated over the winter, and next year it can be given a quick sand down before beginning the process of finishing the entire pergola.
 
Made in the Shade
Now that the framework for each pergola is in place, it’s time to consider what you might grow on it to give the most shade from that glaring summer sun. While many creeping vines will suffice, I’ve had a couple recommended to me. Wild grape vines are native to our area, will grow nicely and have the benefit of providing fruit for jams or wines should you be so inclined. I’ve also heard that hops vines have amazing climbing ability and that their growth can be an impressive 8–20 inches per week under the right conditions. And if you are into brewing your own beer, you will have a seasonal supply of hops within reach, grown on the very pergola which will keep your deck cool this summer.


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Plan Ahead – A pergola that is well thought out and complements the design of the deck and surroundings is a great addition to a home. Consider overall design, as well as function, wood selection and finish to arrive at the best pergola for your space.

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MATT DUNKIN
Matt Dunkin


Matt is a green building contractor specializing in renovating existing homes in Peterborough, Ontario.  In his off time, he writes and spends time adventuring outside with his family.