[Sanna]

Hi all,

I'm trying to build a cantilever pergola in my backyard but not sure if this will be structurally sound. I'll be removing the front two columns to create more of an open space concept. Does anyone have any advice to how to achieve this and if it's possible?

Thanks advance!

 

[GottaFixIt]

Are you saying that you want to support it with just 2 posts in the back?

 

[Sanna]

yes, only two posts. Any suggestions?

 

[oh'mike]

I believe that the best solution will be a welded steel L shaped column--with the leg of the L on top to support the roof structure.

The frame must be stout and well anchored --even an open roof like a pergola will exert a lot of wind stress and uplift.

 

[Sanna]

Location: Ontario, Canada

Thanks!

 

[mako1]

There are no dimensions on your drawings that I can see.HOw can anyone even guess what the load will be on a span with no dimensions?What type and size of joists and so on?

 

[Bondo]

Ayuh,.... If ya only want 2 posts, move 'em to the center to balance the load,....

 

[oh'mike]

Yep---having the posts at the back will require long posts ,well anchored--

 

[GottaFixIt]

There is just a tremendous amount of force working against this design.

Not only will the weight try like hell to warp the posts (bending/compression), but you also have to deal with a lot of stress on the attachment hardware (shear), twisting from wind (torque), and even the post settings working their way loose in the ground (this design is going to move around a lot from wind, etc).

Some large outriggers (supports angled up from the post to the roof) would help with 1 & 2, but it's still going to be swaying in the breeze.

Here's a couple pics I found online:

Notice the size of the post & the outriggers, but also that there are more than 2 posts at the back. This will help with the twisting motion. The designs are wide while the beams are short, unlike your initial design, which has more of a square shape.

I'm sure it's possible for an engineer to spec some serious supports to make your design work, but the simplest answer is to change the design.

 

[red92s]

The weak link in a cantilever pergola is the post/ground connection. The roof framing is light enough that commonly available dimensional lumber and hardware would likely still perfectly adequate if properly arranged. But, the forces working to try and "tip" the posts become tremendous. There are a couple ways to really counteract that force, but the simplest is to embed the posts a LONG way into the ground . . . like 5 or 6 of embedment if the post is 8' tall. There are other ways, like tension cables on the backsides, or burying "feet" that entend out underground under the roof section.