Build a garden gateway pergola

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Every permanent outdoor project begins with the challenge of creating a plumb, square, accurate structure on top of the inevitable irregularities of the ground. This project is one for which one construction strategy proves useful again and again: set the posts in or on the soil first, then cut them to length later. In the case of this arbour, concrete isn’t necessary around the posts because all four work together to support each other as they sit within holes in the ground.

That said, try as hard as you like, but you’ll never dig post holes consistent enough to allow posts to be cut to final length before placing them; the tops won’t be level. The trick is to set overlength posts into holes (I began with 12' posts for this project), plumb them with temporary 2x4 braces, then fill in soil around them, compacting the earth around the posts as you go. I use the end of a sledgehammer handle for this process, as it fits nicely between the post and the hole’s edge.

Trimming four 6x6 post tops to about 8' tall after they’re in the ground isn’t easy, but it does create perfectly level tops, which you definitely need for everything else to work. You can’t just cut the tops square either; that would look terrible. Instead, I made four cuts 18° from square to form a pyramid shape. Six-by-six posts are too thick for a standard circular saw to complete these cuts, even when tackled from each of the four sides. You’ll need to finish these cuts with a handsaw, then refine the pyramid facets with a belt sander spinning an 80-grit abrasive. Yes, it’s worth all the trouble.

The roof boards straddle the crosspieces with interlocking dados. Custom cut these dados to account for any variation between the spacing of both crosspieces.

Getting cross

With the main posts in place, now it’s time to work on the main 6x6 crosspieces. Cut them to length on the ground, prepare pyramid details on their ends, then enlist some help for the next step. You’ll need to prop up the crosspieces with scrap lumber so they’re level, then mark the location of the lap joints on the posts and the crosspieces themselves. Take the crosspieces down, then cut these laps with multiple kerfs using a circular saw every 1/4", knocking out the waste with a mallet and chisel. You’ll need to do this work with the posts in place, of course, and that’s not easy. Put the crosspieces themselves on sawhorses for the job.

Before you do any of this work, let me tell you about a limitation of handheld circular saws and how this is actually an advantage in this case. The depth of cut of most 7 1/4" circular saws is about 2 1/4", but to create half-lap joints that allow 5 1/2"-wide 6x6s to fully overlap, you need a 2 3/4" depth of cut. You could make the half-lap cuts deeper with a handsaw, but this process is a huge pain and actually counterproductive. Since most pressure-treated timbers have rounded edges, a full half-lap actually looks bad because the rounded-edge profile creates gaps when it meets the face of neighbouring wood. A better option is to go with half-lap joints that don’t mesh fully because the saw cuts weren’t quite deep enough. The offset joints look much better, especially if you orient the overhang of the crosspieces on the outside faces of the posts, as I did.

With your half-laps cut, hoist each crosspiece up, fit the joints together, then bore 1/2"-diameter holes to accept the hot-dipped galvanized carriage bolts that hold everything together.

Use the grid diagram (see plan) to draw and cut the shapes on the ends of the roof boards. You can make these any shape you like, although I did spend a long time refining this design. The plan’s details show how I drove 4"-long HeadLok screws into predrilled holes through the fragile ends of the profiles to stop the wood from cracking and falling off over time. Fasten the roof boards with 8"-long HeadLok scews driven down through predrilled holes in the top edge of each roof board.

The offset lap joint connecting post and crosspiece is easy to cut and looks better than a full-depth lap. A 1/2"-diameter x 7"-long carriage bolt holds the joint together.

Lattice Work

The lattice assembly on each side of the arbour comes next. To simplify the joinery for fastening the 4x4 lattice top and bottom parts, I used a trick. The plans (side view) show how I fastened 2x2 strips to the inside edges of the 6x6 posts with screws first, then cut 1 1/2" x 1 1/2" dados in the ends of the 4x4s before fastening these to the 2x2s with 1/4"-diameter carriage bolts. No joinery is needed on the ends of the 4x4s to mesh with the sides of the 6x6s.

Cut 1 1/2" x 1 1/2" dados only in the 4x4 lattice tops for the lattice strips, then fasten all four lattice top and bottom pieces temporarily using deck screws in places where carriage bolts will be used later. Place one lattice strip into the first dado in one of the lattice tops, plumb this strip with a level, then mark the location of the corresponding dado you need to cut in the bottom strip. Repeat until all dados are marked, then remove the bottom 4x4s and mill the dados.

You could have cut all the dados in all the 4x4s earlier, at the same time, but making them after marking automatically compensates for any slightly out-of-plumb conditions. The lattice strips are visually prominent, so accuracy matters. I cut all the dados using multiple cuts with a dado blade in my tablesaw.

With all four lattice top and bottom members installed again, secure the vertical lattice strips using deck screws for speedier positioning. Install the short, horizontal lattice strips next, spacing them evenly and using a level to determine the final position of all the parts. Once all the lattice parts are up, it’s time to replace the deck screws with carriage bolts. Clamp a deck-screw joint, remove the screw, bore a 1⁄4"-diameter hole for the carriage bolt, then install the bolt and remove the clamp. Repeat this process until all the deck screws have been replaced.

The lattice bottom bolts to the lattice end strips, which are screwed to the posts.

Finishing Touches

Large chamfers on the edges of posts and crosspieces look great. Even though making these details is a dirty, dusty job when working on vertical posts, the results are worth it. When all was done, I also improved the colour of my arbour with a one-time finish called Eco Wood Treatment. It’s a powder that you mix with water and spray or brush on. It reacts with the wood chemically over a few days to create an even, slightly weathered grey-brown colour that makes my arbour look like it’s been around for a while.

Bulider's tip

If a project calls for long, thin pieces, such as 2x2s or 1x1s, it’s always better to saw your own. You can cut these pieces from 2x8 or 2x10 lumber. You’ll get clearer, warp-free wood that way. Wider boards are always cut from the largest, highest-grade logs.