How to Plan and Post a Perfect Pergola
Pergolas can be the perfect backyard oasis, but they're also fun to build. They're a perfect blend of big honkin' timbers, some pretty neat carpentry and a little landscaping thrown in for good measure. The simple combination of posts and open rafters is ideal for highlighting entrances, masking storage areas, providing privacy and especially creating some shade. I built this one as a focal point for an outdoor dining area—and to reclaim an unused patch of the backyard.
By Mark Clement
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April 23, 2009 12:00 AM Text Size: A . A . A
Materials and Design
It's just four posts and some boards, but a pergola must stand plumb, level and square through all kinds of weather. I chose attractive, naturally rot-resistant western red cedar for this job. I've built plenty of pergolas with pressure-treated lumber too--if you go that route, just make sure the lumber is rated for ground contact. Vinyl and composites, which often come in ready-to-assemble kits, also work well. For this project, I built a frame on the ground to mark my four posts' locations before digging. I moved the frame out of the way, dug the postholes and brought the frame back. Then I backfilled the 6 x 6 posts and fastened them to the frame.

This method works for pergolas in open lawn space. If you are building over an existing stone patio, don't anchor the structure directly to the patio. Instead, remove some stones, dig footings and weave the patio parts around the posts. To anchor to a concrete slab or footing, use post anchors like Simpson Strong-Tie's ABA66--these raise the post off the ground, letting moisture escape. When affixing to the side of the house, fasten the timbers securely to framing and flash the connection. One thing to watch out for: Tying the pergola to a deck can be dangerous. Get a permit to ensure your plans are safe. Finally, local laws require setbacks from a property line to the edge of a structure. Check building codes; at least 4 feet is typical.

As for the dimensions of the building, I never build a pergola that measures less than 80 inches from ground to girder bottom--the same as an interior doorway. Structures built too short look like mesas; too tall and they look like towers. Since the space often defines the pergola dimensions, there's no rule of thumb on length and width.

Codes don't specify spans for pergolas because they don't support weight. But span too far and the girders will "smile" as the center bends to the earth. Here, I used two 2 x 8s spanned 10 feet from post to post. To get a bulletproof connection, I bolted through the post with double hot-dipped galvanized through-bolts. For spans greater than 14 feet I'd add angle brackets. I'd also beef the girder up to a 2 x 10 or 2 x 12. Aesthetically, cantilevers less than 12 inches long tend to look chopped off (I used 18 inches). I like a pergola to appear to expand as it rises, so I bolt each girder to the outside of the post. (Note: This is a no-no on a deck girder.) Here, I spanned rafters 16 inches on center and mounted 2 x 4 stays on top.
Construction
I budgeted two days to level and prep the site, a day to dig and set the posts, and another to mount the girders and rafters. Last, I planned a day of yard repair. That schedule might vary for other builders, but there are a few universal bits of advice. First, renting an excavator saves time and effort, and also ensures the holes are straight shafts with flat bottoms. I use a 4-foot level to make sure the posts are plumb in both directions, then I anchor them to the frame. I learned this trick the hard way--posts want to move as they dry out, and the angle brackets hold them fast until I can get the rafters and stays in place.

I detail the pergola parts on the ground before hoisting them. This eases installation and vastly increases accuracy. For example, countersink 1 1/2-inch holes in the girders' faces to recess the bolt heads. Cut the rafter tails on the ground, line the rafters up, and use a square to mark each rafter bottom where it will meet the girder. Mark the girder tops for rafter locations as well. Then match the marks on each piece during installation--no measuring from the ladder required. As a result, everything will go up plumb, level, straight and square.
The Steps

1 A Toro Dingo's 16-inch auger makes short work of straight, 40-inch-deep holes for footings, or whatever depth will reach below frost line in your climate. For drainage, pour and pack 4 inches of gravel into the holes before installing the posts.

2 To lay out posthole locations, build a 2 x 8 frame. Its inside corners mark the four posts. Dig a small trench, level the frame, then remove it to dig the holes. Next, replace the frame, set the posts and fasten them to the sides of the frame so they'll stay plumb.

3 After fastening the posts to the frame, install 2 x 4 angle braces from the frame to the posts. These will keep the posts from twisting or warping before the rafters go up and lock the posts in place for backfilling. You can remove the braces later.

4 Posthole depth varies. It's best to install the posts long and trim them to final height, measuring from the frame upward. Finish the circular-saw cut with a handsaw, then install a cleat 6 inches down (inset) to hold the girders while fastening.

5 Temporarily set the girders with deck screws above the cleats, then install the through-bolts in the girders and posts. Allow the girder to rise above the post an inch or so in case you decide to run wire for lights or power.

6 Cut the rafter tails on the ground, then mark their 18-inch cantilevers against the posts. After wedging one end up top, walk the other end up the ladder. Toe-screw the rafters to the girders using a 3-inch deck screw.

7 A 2 x 4 rafter stay atop the rafters every 24 inches adds the right detail while holding the rafters in place. Install stays over the girders first, then over the rafters. After two coats of tinted oil-based stain, add a layer of sod, then a table and chairs.

Read more: DIY Pergola Plans – How to Plan and Post a Pergola - Popular Mechanics