Here's How: Design and Build a Pergola Over a Walkway
Dear James: I have a walkway in my yard that could use some shade and sprucing up. People have told me to build a pergola. How do I do this, and what is the difference between a trellis, arbor and pergola? -- Larry M.
Dear Larry: A pergola is an attractive means of providing some shade over an outdoor walkway, but it is more often used to spruce it up a bit. Any of the three items you mentioned -- trellises, arbors and pergolas -- will provide shade over only a small area. To shade a long stretch, use trees or an awning.
Trellises and arbors have more design features in common than with a pergola. A trellis is typically a vertical latticework that is attached to its own strong framing or mounted near and supported by the house wall. It is ideal for growing climbing vines and for shading a section of house wall or patio because the foliage is dense from the ground upward.
An arbor is basically a trellis with two sides and an arched top. The latticework typically covers the entire sides and the top. Some use a more open lattice pattern over the arched top. These are almost always located over a walkway or a birdbath in a garden area.
A pergola is a much stronger structure than the previous two. It uses heavy vertical posts and horizontal purlins and joists for the top canopy. It works well with climbing plants with heavier stems or a trunk because there is no supporting latticework on the sides. The amount of shade it provides depends upon the spacing and size of the top joists and the type of foliage growing over them.
A good location for a pergola is at either the beginning or the end of a walkway. It can create an attractive, inviting entrance to the walkway or to the patio or garden at the end of the walkway. No matter where you locate it, the construction methods will be identical.
A square pergola always looks nice. Its size will be dictated by the width of the walkway you want to cover. The heavy posts on the vertical corners should be located as close to the sides of the walkway as possible. This gives the entrance a more inviting feeling.
Pressure-treated lumber, especially for the posts, will be most durable. Although 4-by-4 posts will be adequately strong, using 6-by-6 posts will be more attractive. These are very heavy, so you will definitely need a helper to handle and assemble them.
The posts can be set in a concrete base, but it is better to mount them in steel post bases on concrete piers. The top of the piers should be an inch or two above the ground surface. The post bases are placed in the concrete just before it sets up.
Even though pressure-treated lumber holds up underground, potential rotting problems will be eliminated if it rests on concrete piers. In cold climates, make sure the bottom of the piers are below the frost line and taper out at the bottom.
Use horizontal 6-by-6-inch beams in the same direction as the walkway on top of the posts. Place narrower purlins on top of and perpendicular to the beams. The final horizontal joists will be placed perpendicular to the purlins.
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