Michael Woodall

Arbors, trellises and pergolas used to have distinct meanings. An arbor, which meant tree, was used to create shelter. It would be created by bending trees and saplings together and covering them with vines.

Pergolas, which originated in Greece, were flat-topped outdoor rooms. In the 1800s, they became more known for shading walkways. Trellises, on the other hand, provided flat, openwork surfaces for plants and vines to crawl on.

Today, these terms are mostly used interchangeably. An archway marking the entrance to a garden could be called an arbor or a trellis. A beautiful outdoor room, or shaded breezeway with openwork roof, is now known as an arbor or a pergola. They generally include openwork of some kind, adding a character and texture all their own -- from the quaint wood latticework archways to more grand outdoor rooms made of stone.

Whatever you call them, these structures are a versatile backyard staple. Placed over a hot tub, they create an airy yet cozy shelter. Next to a pool, they offer a restful spot to enjoy the sun. They also provide shelter for outdoor kitchens, dining rooms or entertainment areas. They even can serve as a purely ornamental element for framing a view.

Use the following four tips when designing and building arbors, trellises and pergolas.

  1. Use human scale.

There are no hard-and-fast rules for sizing arbors and pergolas. But when it comes to height, professionals recommend staying between 8 and 10 feet. To make them taller would be to make them monumental, and make humans feel out of scale. If you’re dealing with some type of pitched roof, the plate height – or lowest point around the edge -- should also be set at 8 to 10 feet.

If you’re building in a hot climate and expect the roof to become solid with plants, make it higher. This gives room for the heat to rise without stifling the inhabitants.

  1. Calculate shade in advance.

Some arbors and pergolas aren’t meant to provide shade. If this is the case, many landscape designers will typically place rafters at 16 inches apart on center, going one direction. On the other hand, if you want to provide shelter from the sun, you’ll need larger pieces placed closer together. For example, Clement Butt runs slats perpendicular to the rafters. “They’re little tiny strips – maybe 2-by-1 inch -- and those could go every foot,” says the president of Classic Garden Design, LLC in Weston, Conn. “That gives you quite a bit of shade while still allowing an open canopy.”

Keep in mind, shade moves around with the sun. Except for noon, when the sun is directly overhead, part of the area will not be shaded.

If this is not acceptable, track the sun’s path. Consider adding some type of shade on the south and west sides. Or make the structure larger to leave the ideal amount of shade.

  1. Provide adequate support for plants.

Some species require more support, while others need a lot of surface to help them attach. This is important when designing an arbor or pergola that will have trailing plants or vines. Grapes and wisteria, for instance, are notoriously heavy and aggressive. Both have sturdy trunks, so the base should be equally solid.  For these varieties, professionals recommend using structural elements measuring at least 2-by-2 inches. If you’re going to grow wisteria on an arbor against a house, keep it away from the gutters and roofing. “It’ll crawl up the roof and under the shingles within one growing season,” says Rick Knight, co-owner of Seattle-based Urban Refinements.

Ivy, ficus and trumpet vines are self-supporting, meaning they’ll attach to just about anything with their runners or claws. Other vines need a little help. Clematis, for example, needs more surface to attach and grow on. This can be provided with a latticework or suspended wires on the structure.

Consider what kind of shade you want and for how long. In regions with colder autumns and winters, deciduous vines might make the most sense. They drop their leaves when the weather cools, so more light can come through.

Many landscape designers like to combine species that bloom at different times of the year. This provides a new show year-round.

  1. Build for the long haul.

Even the most delicate arbors need proper support. On his wooden structures, Butt begins with 6-by-6 inch posts on the lightest ones, and moves up to 8-by-8 inches when they get larger. He also recommends red cedar rather than white. “It has greater durability,” Butt says. If you want to save a little money, try pressure-treated posts with red cedar latticework above. These woods look different, so you’ll need to cover the pieces with an opaque paint. Butt recommends an oil-based primer and a latex water-based stain rather than a primer and latex paint. “You’ll have much less chipping and cracking,” he says.

If you’re hiring a carpenter to install the arbor, look for somebody who will dowel the wood together. “Nails tend to pull and rust, and the whole arbor will fall apart,” Butt says. “The proper way is to have tongue-in-groove or have them doweled on the top and then glued together almost like a piece of furniture.” Staples are also a no-no.

Regardless of the size, the arbor or pergola should sit on some type of foundation. Many people try to install the smaller archway types directly on the ground. “This exposes it to insects and rot,” Butt says. “Ideally you want to concrete them in or put them in some type of pad.”

A concrete pad will protect the arbor and keep it level at all times. For lighter arbors, you may be able to use compacted crushed stone. It doesn’t have to go down deep.

In most cases, a foot should be fine for simple archways. Smaller structures can just rest on the footings. Larger ones should be connected to the pier with an anchor bolt.