How to Build a Perfect Backyard Pergola

Mark out the perimeter

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You’ll need to mark out a perfectly square layout for the posts. Cut the sod away and mark the perimeter of the pergola with stakes and string. To start, position two strings exactly perpendicular to each other using the 3-4-5 triangle method (in this case, your measurements will be 9, 12 and 15 ft.). If you’re not familiar with this trick, search online for “345 triangle.” Once you get two lines squared, the other two will be easy. But double-check your layout with diagonal measurements before you mark the posthole locations (shown in step 2). Check the layout by taking diagonal measurements; equal measurements means the layout is square.

Position the postholes

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To keep the post groupings positioned precisely, I cut plates from treated plywood and fastened them to the posts (shown in step 3). That way, you can position, plumb and brace each assembly of three posts as if they were one post (steps 4 and 5). Bracing the assemblies takes several trips up and down the ladder. Align the post bottom plates with the strings and then outline them with spray paint. 

Build the post assemblies

First, prep the lumber: Every stack of treated lumber contains some beautiful wood and some ugly stuff. Take the time to pick through the pile and select good material—your project will look much better. When you get the lumber home, you’ll be eager to start right away. But I strongly recommend that you let the lumber dry for a few days. Stack it with spacers so air can reach all sides of each board. Then stain it before building. Staining this pergola after assembly would be a slow, messy job. I applied two coats of Behr Semi-Transparent Waterproofing Wood Stain (No. 3533). Once your lumber is prepped, screw plywood plates to the posts, as shown. Joining the posts this way saves you the hassle of positioning and plumbing them individually.

Set the posts

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Place each post assembly in its hole. You’ll need at least one helper for this job. Be careful so the sides of the hole don’t cave in.

Brace the posts

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Start by screwing some horizontal 1×3 braces onto the tops of the posts. Make sure the post assemblies are spaced the same on the top as they are on the bottom. With the spacing established, you can brace the groups diagonally to the ground with stakes. Keep at it until it’s close to perfect. No need to buy extra lumber for bracing; just use the 1x3s that you’ll later use for roof slats.

Fill the postholes

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Use a tub or wheelbarrow to mix the concrete, then toss it into the hole. The mix isn’t critical because you don’t need to trowel a finish onto it. Just make sure you get it packed into the holes evenly around each grouping. I used about four bags per hole, but get a few extra bags just in case. Let the concrete harden at least one day before removing the bracing.

Install the headers

The next day you can install the headers. Overlap the corners as shown in Figure A. I removed my braces at the top, one at a time, as I leveled and installed each outer side header and then finished with the front and back. Be sure to take the thickness of your slab into consideration as you measure the distance to the bottom of the headers. Leave at least 80 in. between the slab and the header. Taller people may want to nudge it up a couple inches.

Once the outer headers (B and C) are in place, cut the posts flush with the top of the headers, I used a framing square to mark them and a circular saw to cut as deep as I could. I then used a handsaw and when my arms felt like they were ready to fall off, I used a 10-in. blade in my reciprocating saw. Next, cut and install the inner 2×10 headers at the front and back and then the rafter supports (E) cut from 2x8s. Rip the 2x8s to 6-1/2 in. at a 7-degree angle. The slightly wider 6-1/2-in. edge should go toward the inner side of the pergola. The rafters will rest on this support and extend to the outer side headers (C).

Set the ridge beam

Measure the distance at the center of the pergola from front to back at the top of the headers. Cut the 2×10 ridge beam (F) to this length and drive screws at an angle into the headers. Measure from the top edge of the ridge to the inner edge of the side header (C) on each side, starting at the midpoint of the ridge and the header. Cut the rafters to fit. Ideally it should be 7 degrees, but if your ridge is cupped slightly you may need to adjust the cut. Because my ridge was slightly cupped, I had 6-degree cuts on one side of the ridge and 8-degree cuts on the other side.

Install the rafters

Cut the rafters to fit flush with the top of the ridge beam and 3/16 in. above the outer headers. Screw them into place.

Mark the gable rafters

The gable rafters (H) are the same as the common rafters except they sit atop the front and rear headers, so they need to be scribed to fit. Tack a 2×4 to the end of the ridge beam and then align it with the header and in the same plane as the other rafters. Scribe each end of the 2×4 to create a pattern for the other gable rafters. Fasten these to the ridge beam by toe-screwing at an angle or by screwing through the opposite side of the ridge at a slight angle into the rafter (this method give a cleaner installation and less chance of a protruding screw). Finally, screw through the side headers into the rafter ends, making sure your spacing is even.

Install the slats

I cut 2-1/2-in.-wide roof slats from 1×6 material because it was better quality than the 1x3s available at the lumberyard. Start installing the slats parallel to the ridge and work your way down each side. Overhang the front and back of each course about 5 in., then you can string a line and trim them once they’re all installed. Use a 2-1/2-in. spacer as you screw each row to the rafters. Check your progress every fifth course to make sure you’re staying perpendicular to the rafters and that you’ll finish with a full-width slat at the end.

Once all the roof slats are fastened and trimmed, cut the ridge covers (K) and nail them over the exposed end grain at each end of the ridge beam. I mitered the ridge cover tops to fit tightly under the roof slats.

Add plants for privacy

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For privacy and greenery, I built a trellis on one side of the pergola. If you want the feel of an outdoor room, you could add trellises on two or three sides.

My trellis is simply 1x4s and a 2×4 joined with 1-5/8-in. screws. I built it on the pergola floor, then stood it up and screwed it to the posts. I sized the planter boxes to hold 6-in. plastic pots and made them 25 in. long, but you can make them whatever length will work.

Finishing up

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If you stained the lumber before assembly, all you have to do now is coat any unstained ends of parts. Be sure to stain the top ends of each post to reduce water absorption and cracking. For extra insurance, I coated the post tops with stain followed by exterior paint. If you plan to pour a concrete floor as we did, go to familyhandyman.com and search for “concrete” to find several articles about working with concrete.

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Figure A & B

Figure C & D

Figure E & F