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Blog entry by Kevin_S posted 12-23-2015 11:31 PM 4887 reads 3 times favorited 2 comments Add to Favorites Watch

I recently completed this pergola shown here in its mostly completed state.

I’m happy with the way it turned out, but having gone through the experience, there are some things I would do differently. The legs are 4×4 treated fir wrapped in re-sawn cedar. I should have just used solid cedar posts and come up with a different way of achieving a similar look. It would have been much easier and probably not much more expensive.
These pictures show the details.

Each 4×4 is wrapped in eight pieces of cedar; four thick (0.75”) boards around the lower portion and four thin (0.50”) boards around the upper portion. I milled down 2×6 lumber and re-sawed it on my table saw into one thick and one thin board each. I cleaned up the boards with minimal sanding and/or planing so I wouldn’t lose too much thickness.

The problem with this design is that rain caused many of the boards to cup badly (perhaps thicker boards, like full 2x stock, would have reduced this) which opened up large gaps along the edges. I had used my air nailer to attach the boards and speed things up, but those nails don’t have a lot of holding power and despite being galvanized they started to stain the wood almost immediately. I ended up redoing much of it with exterior grade screws and construction adhesive. That helped, but there are still some gaps.

Where the legs transition from wide to narrow I added some trim.

Again, in retrospect this is not the greatest design. This is classic cross-grain construction. When the boards swell due to rain and/or humidity those miters on the trim open up some large gaps. I’m not worried about stresses or structural problems though, because I used just one screw in the middle of each trim piece. The boards can swell and move without being restrained by the trim.

The main cross beams are in pairs mounted on both sides of the 4×4 posts. They are 2×8, notched where they meet and given a decorative cut on the overhanging ends.

For those ends I made a template out of scrap ¼” plywood. I clamped it down and traced the outline.

Then removed the template and used a jig saw to cut close to the line.

I scored the bottom of the 2×8 with a utility knife to reduce tear out of the end grain by the router bit.

I then reattached the template and used my router to get to the final shape. The bit I’m using has bearings on top and bottom and was just barely long enough.

For the notched joints I started by marking them out.

I cut the depth with my jig saw.

Then did a shallow bottom cut with my multi master on both sides of the board.

A quick whack with the hammer knocked out the waste.

Cross beams go both long-wise and short wise.

The short-wise beam in the middle is solid, or in other words it goes post to post. The web of the long-wise beam is two boards that butt up to the short beam. However, the top and bottom chord on the long-wise beam are full length 2×4s glued and screwed to the web. This all made for a very strong assembly. (I’m an engineer in case you couldn’t tell by now.)

The small cross beams are 2×4s notched to catch a support along the long center beam. You can’t tell from the picture, but the support is angled a bit to help shed water (more on why that is important later).

The overhanging ends of the small beams were given a decorative shape to match the larger beams.

Each post has two corner braces. They are more for decoration because the structure is more than rigid enough without them.

Each post needs a cap, but I didn’t like the stock ones available at the hardware store, so I made my own. I started by milling some of my 2x scrap and gluing it into a couple blanks that were thick enough.

I cut those long-wise on the table saw to form a peak. Then cut them into cap sized blanks and mounted them to a backer board with screws.

That let me run them through the table saw again to form the four sided peak of each cap.

Then it was time for some decorative stuff. I cut a small flat at the base/edge of the peaks and used a cove bit on the router table to make a round-bottomed notch.

I was able to do all four sides of each cap by removing them from the backer board, rotating them 90 degrees and reattaching them. Then running the same operations.

A roundover bit helped ease the edges.

Then I hollowed out the base of each cap with a spiral bit on the router table and a set of guides. I adjusted the guides between cuts to gradually work my way in from outside to the center. The image on the left shows a cap after a couple “laps”.

When I suggested I could build a pergola for the backyard my wife really liked the idea. She said she wanted a roof though, to provide protection from rain showers. I liked that idea, but I like the open, airy feel of a pergola. I didn’t want to build a gazebo or something that looked like a carport. So how to provide rain protection while maintaining an open feel?

My solution was to attach transparent polycarbonate panels (typically used for greenhouses) to the underside of the overhead beams.

This had a huge impact on the design and is why I put a beam down the center. I needed to provide a peak so the polycarbonate panels could slope from the center to the outer edge. This is why I alluded earlier to the importance of shedding water off that center beam and supports.

I could have just let the water run off the ends of the polycarbonate panels, but I wanted to control it better. That meant I needed some type of gutters “inside” the structure to catch and channel the water.

Regular gutters would like, well, regular gutters. Yuck. I investigated alternatives and found those fancy half-round gutters used on nicer homes. Good looking but not available in a DIY form that I could find. They are made on site by contractors that specialize in gutters. I only needed two straight sections of less than thirty feet total. I suspected I would end up paying a lot for two simple gutters.

I decided to make my own out of four inch PVC pipe. I glued together the length I needed which consisted of two sections of pipe, a coupler, and end caps. Then I cut that pipe in half lengthwise to form two half-round gutters. I used J hooks to support the gutters just like you would if it were being used as drain pipe. To disguise the fact that it’s PVC, I painted everything with black spray paint which has a hammered metal finish. They look like black metal pipe. I cut a hole at one end but rather than make downspouts I’m going to use rain chain once I find something I like.

The pergola was designed and modeled in Catia. Then I printed out pdf drawings to use in the shop. Here are some examples.

If you’ve read this far and have any questions about the design or how I did something I’m happy to answer them.


2 comments so far

View htl's profile


4872 posts in 1763 days

#1 posted 12-24-2015 03:04 AM

That’s one nice project and one great how to!!!

-- An Index Of My Model making Blogs

View Bobsboxes's profile


1393 posts in 3268 days

#2 posted 12-24-2015 04:20 AM

Very cool design, wrapping the treated posts is a great idea. I also built one, and love how it keeps the hot summer sun from heating up the kitchen side of house. My wife’s Mother came over and sat under it in the shade, she asked me if I knew anything about construction, where’s the roof?

-- Bob in Montana. Kindness is the Language the blind can see and deaf can hear. - Mark Twain

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