Selecting an open versus a solid roof is yet another important determination. In hot climates, an open roof will not provide enough shade to make the space feel comfortable. But in milder climates, the open design affords the opportunity to enjoy the trees and sky and creates an airier feeling.

Covering the solid cellular vinyl pergola in this outdoor seating area, this Boston restaurant’s owner chose tasteful manually retractable blue canopies.

On the other hand, a solid roof can enable the pergola to remain useable during warm rain showers. However, in the wintertime, the solid roof might collect too much snow, which is a weight load that most pergolas are not designed to handle. In addition, architects will take the surrounding architecture into consideration.

“The roof type is also based on the style of the home’s architecture and whether there is precedent to lead us in the direction of a gable, shed roof, or if a detached cover will be the best solution,” Terry says.

Attached Versus Detached

If the homeowner is interested in an attached pergola, for example, then an important aspect of the design is the issue of attachment. Typically, they are attached with a ledger underneath the roof overhang, but some older houses may have a very low roofline, in which case, the pergola would be too low.

“Sometimes the cover can be attached to the roof, but it is not the most aesthetically pleasing solution and involves flashing and reroofing the area,” Terry says.

Another option is an open-gable attached patio cover, which involves building a new gable on the roof. While this is an added expense, an advantage is that this type of structure does not put any extra weight on the roof because it is built with an independent framework that supports itself.

Another possibility for low rooflines is “finding a location where a detached open roof cover could be placed on the property, still meet building code setbacks, and be functional and attractive from a layout point of view,” Terry says.

Designers should also consider which door will grant access to the covered patio and if any of the house/building’s windows will impact the placement of support beams.

An attached pergola might also positively or negatively influence the home’s interior. For instance, south-facing windows may produce excess solar heat gain that can be mitigated by the pergola, resulting in a more comfortable environment. On the other hand, if that side of the house/building has a low roofline or does not receive much sun, then the attachment of a pergola could make that room feel dark.

On spacing the support beams, Mark Willard, R.L.A., director of planning, Daft McCune Walker, Towson, Maryland, relates that for a 7- to 8-foot-high pergola, an 8-foot on-center column spacing works well for architectural proportion. Additionally, an 8-foot x 8-foot plan area works well to contain a variety of furniture placement options. “If you spread the columns out too far, the structure does not feel structurally sound (visually) and begins to feel top heavy," he says.

Similarly, when mapping out the massing and proportion, the pergola should not dwarf the home in terms of member size, height, or overall size, nor should it appear too small.

Offering a rule of thumb, Mark Clement, a contractor and carpenter who runs MyFixItUpLife.com, suggests a ratio of 1:1.618. Consequently, if the overall pergola height is 96 inches, then the length would be approximately 155 inches.

He further advises that the pergola measures no less than 80 inches from the ground to the girder bottom, suggesting that structures built too short look like mesas, and if too tall, appear like towers. If the design includes a ceiling fan, then the pergola must be tall enough to allow at least 88 inches under the blades.

Further offering a sense of size and proportion, Terry’s recommendation for a one-story, 2,500-square-foot house is a 14-foot-deep x 14- to 16-foot-long pergola. “If the home’s rafters are 2 x 4, then I would use that sized member as a reference point. In general, the top layer of an attached patio cover, known as the lath or lattice, could be between 2 x 2 and 2 x 4 or 4 x 4, depending on the overall massing of the home.”

As the spacing of the lattice determines the amount of sun that penetrates the cover, Terry likes to embed the lattice into the second level of rafters to create a more custom, built-in look.

Installation Tips

The girders are typically installed in each side of the main post with the rafters and optional purlins will be installed on top. Consequently, the two girders will support the weight of these members.

As builder Matt Weber explains in the article “How to be a Pergola” on the well-read Extreme How-To blog, “usually the rafters are slightly smaller than the girders and are installed vertically, bridging over the girders at 90 degrees and overhanging each side with the decorative rafter tails.”

Size and design style will determine rafter spacing, but commonly this is done 16–20 inches apart. For a professional, uniform look, it is important to keep the distance consistent between all the rafters. Similarly, the rafters should be spaced evenly between the front and rear rafters, and each rafter should be perfectly square and vertical to the girder.

As a decorative enhancement more commonly used on larger projects, purlins are often installed vertically over the rafters at 90 degrees.

The pergolas structural design must take into account lateral resistance, uplift, and snow load to withstand the forces of nature and meet building codes.

Uplift capacity is of particular concern, so having a clear, established load path is critical. When setting these up, it is important to have a well-anchored foundation that both anchors the whole structure and bears the load of the top piece. It is best to use steel that travels through the entire post and can be bolted into the beam it supports.

Joists should be anchored with joist brackets, and lathing can be attached to the joists with stainless steel screws.

For an attached pergola, it is recommended to use a ledger which is anchored into the framing of the structure it is attached to.

Beyond the basic nuts and bolts of the structure itself, pergolas can incorporate a number of additional features, such as lighting, greenery, a ceiling fan, and/or a dynamic canopy.

Some manufacturers offer rain-resistant fabric, and the canopies can be motor or manually operated, and come in a number of mesh fabrics, densities, and colors.