Is a 15 foot tall pergola not feasible?

Question

Looking for some structural wisdom :)

I have a deck that sits 6 feet off the ground, and I want to build a pergola whose "roof" (beams/slats) are 9 feet above the deck. So, from ground (grass) to roof beams, would be 15 feet (6+9).

I don't want to build the pergola ON the deck, because I don't know if the deck can sustain that extra load.

My plan is to dig 4 new footers (one for each post) NEXT to the deck (two on each side).

Being that the deck is raised 6 feet up and the ceiling is 9 feet above that, these 6x6 posts would be 15 feet above ground level and 5 feet in the ground (6x6x20 posts).

Would the roof being so far above the ground cause any top-heavy weight/lean issues?

Answer 1

First big question....Are you planning the floor in the pergola at the level of the deck floor?

Next...

1. For posts 15 feet vertical height I do not believe that 5 feet depth in the ground is enough. You should vie for a depth of 6 or 7 feet deep.
2. For a structure like this you want the bottoms of the posts anchored well below the frost depth.
3. For the type of height that you are talking about it will be absolutely necessary to have some type of angle bracing that will help to keep the structure from racking and parallelogramming.
4. If you have a floor at deck height this can offer some alternatives where you can place some of the angle bracing below.
5. It the pergola is meant to provide some shade at the grass level from a top that is 15 plus feet off the ground it will be far less effective than if the shade top were more like 8 to 9 feet overhead.

To add bracing under the elevated pergola floor:

Answer 2

Preface: I'm not an engineer; just a working carpenter, so any of this should be validated by plan approval by your 'authority having jurisdiction' (AHJ). They may ask for engineering stamps.

Another note, there are many perfectly correct ways to build things. I'm only going to talk about what makes the most sense to me personally. That shouldn't imply that other solutions are worse -- some might actually be better.

This is a sub-answer, related specifically to splitting the posts of your pergola into 2 sections -- the length below the deck, and everything from the surface of the deck up. There's a few reasons to do this... if you're going to put a post below grade, it should be pressure treated (PT). Personally, I wouldn't be happy about my pergola posts above the deck being PT. Even if the PT wasn't an issue, actually standing 16' or 22'-6x6 posts would be difficult without a bunch of manpower or a crane. (Or a tractor, as noted above.)

So, let's build a support for the pergola that just happens to live below the deck. You can sink your posts in deep holes or simply make deep holes full of concrete with a bracket on top of the concrete. If this was my job, I'd use brackets on top of a tube of concrete (less lumber to buy and carry, better resistance to rot; though a bit more concrete to pour and if you choose a bracket that is actually set in the wet concrete, you need to take care to get it perfectly placed). Posts will go up to your existing deck and connect with sturdy through bolts. You'll also want to add extra joists and rim boards to thoroughly box in the top of the 4 post structure we're building. At grade, you'll also want to box the posts to prevent any movement. Further (as noted by @MichaelKaras), you'll want cross-bracing to prevent racking or twisting. Here's a terrible pic:

Those under-deck posts will be where you want the pergola posts to connect and carry straight down. A direct connection is far better than pushing the under-deck post to the underside of the existing deck. (Don't know what your deckboards are, but if they're wood, they aren't well suited to being sandwiched between low and high posts for a variety of reasons.) Various brackets and alternate solutions exist to connect posts... if you try to make it so that they will never move (up or sideways), then you're on the right track. They also should be almost as strong as if the two posts were one piece of wood.

But since they aren't ever going to be as strong as one piece of wood, you need to engineer some bracing at the top of the pergola that'll keep the structure from twisting and racking. I would lean toward a pair of 2x8 or 2x10s on each side, with a knee brace (8 in all) in each direction. Here's one I made earlier:

(This was a while ago, but I think it was about 13' square, 11' high, 2' knee braces, 2x10 lumber at the top. Lag screws to connect the knee braces to the posts; through bolts to connect the 2x lumber. If you really shoved it, it had a tiny bit of flex, but it was pretty solid. If you stare hard, you'll see that the knee braces are let into the post by a tiny bit for a better connection.)

If you need any points clarified, just ask.