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Hanging A Cantilevered Pergola on a Wall

07/18/2020 12:12 PM

Hi friends,

I want to hang a wooden pergola above my patio but without any freestanding supporting posts. It would be around 30 feet long and 6 to 8 feet wide, and made of 2x6 dimensional cedar. It would be attached to a regular household brick wall with a number of wooden L-bracket supports.

As an electrical engineer, Ohm's or Coulomb's Laws are of little use to me for this project.

So, I'm looking for the equations and calculations to help me figure out the physical size and number of the supports that would be required given the weight of the cedar (C), the length (L) and width (W) of the structure.

More importantly, I also need to be able to calculate how many and what size lag bolts would be needed to hang this on the brick wall.

Thanks to all for your assistance and any suggestions,

Graebeard

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#1

Re: Hanging a cantilevered pergola on a wall

07/18/2020 1:02 PM

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#2
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Re: Hanging a cantilevered pergola on a wall

07/18/2020 1:28 PM

Show us your triangle of support...

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#4
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Re: Hanging a cantilevered pergola on a wall

07/18/2020 1:50 PM
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#5
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Re: Hanging a cantilevered pergola on a wall

07/18/2020 2:15 PM

If you plan on getting plan approval, building permit and inspection, you'll probably need an architect to design it...not cheap....a free-standing kit might be the cheapest way to go...

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#15
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Re: Hanging a cantilevered pergola on a wall

07/20/2020 1:23 PM

A small, structural engineering (consultant/engineering)-office, with a licensed structual engineer, who has substantial experience in wood/timber structures, and current local applicable conditions, Codes and Standards, would be a more effective engineer/consultant to hire...

(even though a Civil Engineer, typically, is legally allowed to do structural engineering on structures two-stories, or less, in height, a 30-foot cantilever is not just another typical wooden structure...)

(...and, 2x6 30 foot long cedar lumber may not be deep enough to span such a distance, under severe weather conditions... Captain Moosie would know more than I, but you should probably be considering something like a glu-lam truss, as well... ..because even a light load at the end of a 30' moment arm might be enough to bring down your supporting wall...)

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#3

Re: Hanging a cantilevered pergola on a wall

07/18/2020 1:44 PM

Drill some 1 inch or greater holes your brick wall and into the joist work of the ceiling on the other side of the brick wall. Have some pipe cut to about 16 ft and stick half a dozen of them through those holes.

Secure the pipe inside ceiling with blocking and proceed with your pergola.

I left out some details ensure that you know what you're doing

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#6

Re: Hanging a cantilevered pergola on a wall

07/18/2020 9:07 PM

Spacing of supports contingent on lumber length, but probably every 8ft...Lag bolts 6"x7/16 M-5 every 18"...subject to revision to comply with code....

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#7
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Re: Hanging a cantilevered pergola on a wall

07/18/2020 10:01 PM

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#8
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Re: Hanging a cantilevered pergola on a wall

07/19/2020 1:28 AM

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#9

Re: Hanging a cantilevered pergola on a wall

07/19/2020 4:51 AM

Brick is just a cladding and is not very strong when force is applied perpendicular to the wall. It is reasonably strong in compression but not in tension. If you look at awnings on brick buildings they are cantilevered to the wall and then long through bolts run through the building to anchor the braces to the building.

The brick is only held to the stud walls by the occasional bracket nailed to the stud and inserted in the cement between courses of brick. Unless you built the house you can only hope that the appropriate requirements were followed but the inspector can't be there all the time to make sure it is up to code. I think some of the other suggestions shown where the pergola is free standing and bolted to a cement foundation.

More than once I have seen an awning peel off a building taking part of the wall with it. Best supply a hard hat to any guests under it.

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#10
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Re: Hanging a cantilevered pergola on a wall

07/19/2020 7:00 AM

kiln dried cedar? It would be like a someone tapped you on the head with a piece of Styrofoam.

I've used the method I describe. My sds max bit is 18" x 1-5/8".

I've used types of angle iron for demanding applications.

With the welder and some planing a nice addition can be made. that 8' out is pushing it without some extra supports either through the wall or to the ground.

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#11

Re: Hanging A Cantilevered Pergola on a Wall

07/19/2020 9:23 AM

Before continuing with your pergola plan, you need to ascertain that your brick wall is not a brick veneer but rather a real brick construction!

In Real brick wall construction the wall is about 8 -10 inches in thickness.. while a brick veneer will be just an aesthetic looking brick only!

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#20
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Re: Hanging A Cantilevered Pergola on a Wall

07/20/2020 4:56 PM

GA! Agreed, but in addition, to support such a load, that real brick wall better have some significant reinforced concrete behind the brick!

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#22
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Re: Hanging A Cantilevered Pergola on a Wall

07/20/2020 5:32 PM

I concur with you.. as typically the local building department will require main header beams about 2X12X10 or whatever length, be bolted and anchored onto the brick wall.

This main header beam I think is where all the external loads will be borne and transferred onto.. Isolating the rest of the house, and before any structure can be built external to the existing wall..

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#12

Re: Hanging A Cantilevered Pergola on a Wall

07/19/2020 11:25 AM

Would you design a control circuit by staring with the parameter that it will draw 11 amps? Your objective is to create a pergola. You want it to look like it is made from cedar. Don't limit yourself by actually making it from cedar. Make 5 or 6 'C' frames from three lengths of 4" x ½" steel bar welded together. Drill some holes in the base leg of the C and slide through some rebar so that it keys into the concrete when you sink the base of the C below the patio base with the back of the C up against the house wall. Tie it back to the wall to hold it steady but not to derive any structural assistance from the wall. The top of the C cantilevers out the required distance1. Mill a 4"x ½" slot into the 2" face of your 6x2 cedar then slide it over the vertical section of the steel and glue in place to give the illusion that the cedar is taking all the load. Clad the top in a similar way but with a 5" x ½" slot. This allows you to slide a 1" x ½" timber in behind the steel to cap it off. If the milled timber is slid over the steel from the underside the capping will not be visible to anyone on the patio. By stopping the slot short of the end of the timber1 your reinforcement will not be visible and the cantilever will appear to be solid cedar. This solution has the advantage that the house is not compromised in any way and it may not even need buildings approval.

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#13

Re: Hanging A Cantilevered Pergola on a Wall

07/19/2020 4:30 PM

It could be supported from overhead....this would lower the roof though...and imo not as structurally sound...

https://www.shadetreecanopies.com/awnings/house-awnings/alpine-brackets.asp

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#14

Re: Hanging A Cantilevered Pergola on a Wall

07/20/2020 12:20 PM

what is meant by "hang"? is there a wall on both sides of this patio? i see everyone offering a cantilevered design, but that would have a support post, and want it done "without any".

30' is a looonnng way my man. and i if you plan on supporting it on one side i think is a real issue. I can think of several reasons why this cannot be done with lumber alone.

you may want to consider putting a fireplace or bar on the other end of this 30' run to support your pergola.

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#16

Re: Hanging A Cantilevered Pergola on a Wall

07/20/2020 1:33 PM

SE posted a lot of nice pictures. All have significant structure above ground but don't show the hidden part that's required to carry the load. ( I did not follow his links which may have that information)

Aesthetically your idea of cantilevering a pergola 6-8 feet out from your wall seems cool but the structure required to support that much cantilevered load has to be built in from the start or added in. Most houses are built TO code and not even a tiny bit more. Adding support structure can be very expensive. I once had a quote for taking out a supporting wall and putting in a hidden beam to open the floor plan up and it was cheaper to add an entire room.

I suggest having a qualified engineer look into your existing structure first. You may decide having an outer support beam and posts isn't so bad after all.

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#17

Re: Hanging A Cantilevered Pergola on a Wall

07/20/2020 2:26 PM

Hi Graebeard,

Just my initial thoughts.

You really need to have a pretty accurate design of your structure so you can workout the weight. Without that figure it's a bit "how long is a piece of string" and it might give you a whole different perspective on the job.

Best regards and stay safe,

John

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#23
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Re: Hanging A Cantilevered Pergola on a Wall

07/20/2020 6:40 PM

In answer to your querie "How long is a piece of string", well I have found acouple of startling string facts, no reference to quantum mechanics who don't do normal services but returning to the question the length of a piece of string. Firstly it is only long enough to fit between its ends, never longer or shorter and that it is twice half its length.

Hope this clarifies the age old question on the length of string question.

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#18

Re: Hanging A Cantilevered Pergola on a Wall

07/20/2020 3:33 PM

So flesh it out, give us a sketch of what you want....

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#19

Re: Hanging A Cantilevered Pergola on a Wall

07/20/2020 3:33 PM

Many, like me, are quite concerned with the strength of your brick wall. Local codes will specify the pounds per square foot load that the floor/roof has to withstand. This leads to a rather large overturning moment that the wall has to resist. I don't think it will unless you tie to some substantial structure behind the wall. Note that those illustrations showing the pergola supported from the ground have a rather substantial vertical post. If you can't support with vertical posts at the 4 corners and intermediate on both sides, I think you are asking for trouble.

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#21

Re: Hanging A Cantilevered Pergola on a Wall.... Ice, snow, wind, resonance.

07/20/2020 5:05 PM

Ice, Snow, Wind, and Resonance

8'x30' is a lot of surface area even if your pergola is skeletal and not sheathed. Everyone else has fully warned you about brick being vulnerable to tensile forces but they have been skipping the potentially aggravating circumstances. If this structure is to live in Toronto, then weather should be in your considerations for sure. An ice glaze can add a whole lot of static load even to a skeleton. Wind can get it jumping and that cantilever can be a big spring to build up oscillations in a substantial moment structure. The ice load can hold on and the low spring stiffness(wood and/or steel cantilever) can combine with that additional ice mass to create a low frequency, high queue, oscillator which can accumulate stored wind energy like the Tacoma Narrows Bridge.

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#24
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Re: Hanging A Cantilevered Pergola on a Wall.... Ice, snow, wind, resonance.

07/21/2020 10:26 AM

Sorry for a late submission. This is my first chance to respond.

I fully concur with thewildotter. The support system you are desiring adds a lot of potential loads (wind, snow load, and live loads - while under construction) to a system that was not designed with the intent to support these loads. There may be residual ability in the structure to do so. But given the the size of the pergola you described, I am very doubtful that the existins structures you want to support this pergola have the residual ability to absorb these new loads.

The walls you want to use to support this strucutre will most likely need reinforcement, bracing, etc. This is not something that can be provided in a forum like this. The additioinal bracing requires a review of the existing structure and what loads the existing structures are already supporting. You really are much better off creating a free standing structure like many of those pictured in this forum. I would think there is probably a way to blend these new supports into the existing walls so they look like they are part of the original structure.

There are lighter weight options that may cause the loads to be less than the residual support strength in the walls. But this is not a recommendation to do that. If, after reviewing the walls and their residual support ability, you may be able to to dsign a a pergola that does not overstress these walls. Peryhaps something like canvas with aluminum framing? But, without checking and analyzing the existing structures in greater detaail, I would still strongly recommend you construct independent support structures.

Best of luck in figuring this out.

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#25

Re: Hanging A Cantilevered Pergola on a Wall

07/22/2020 2:24 AM

Wow, lots of support from this forum, and it's greatly appreciated. Much food for thought although I feel I wasn't as clear as I could have been. Basically what I envisioned was a 30 ft plate secured to the wall, with 6 ft long boards every foot or so cantilevered at 90deg to the plate. There would need to be L bracket supports every several feet as well. Since it is an open construction with no sheathing, snow or ice loads should be minimized.

Thanks for all the good suggestions and points to consider.

Graebeard

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#26
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Re: Hanging A Cantilevered Pergola on a Wall

07/22/2020 3:02 AM

2 x 6 x 6' <12 lbs...

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#27
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Re: Hanging A Cantilevered Pergola on a Wall. Nice Trellis

07/22/2020 8:17 AM

Snow Sheathing Might Someday Grow There

Another "open construction with no sheathing,"

As built it might not have sheathing which can catch snow but years in the future...

Can you keep nature and/or eager horticulturalists from using it as a trellis ?

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#28
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Re: Hanging A Cantilevered Pergola on a Wall

07/22/2020 10:46 AM

Your 30 ft plate is the two 2x12x?? one being on top of the other, making it a 4x12x?? main header, which extends throughout the length of the project, which would be the required "main header" that is anchored and bolted every 4-5 feet onto the outside wall as described in post#11..

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#29
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Re: Hanging A Cantilevered Pergola on a Wall

07/22/2020 12:10 PM

A drawing is worth 1000 words!

Here's what I understand from your post:

How do you propose to attach the 6 ft long boards to the plate? There will have to be at least a strap joining the outside ends of the boards to keep them from flapping around in the wind.

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#30
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Re: Hanging A Cantilevered Pergola on a Wall

07/22/2020 1:27 PM

Those brackets are too short, they need to be over half the length of the board, preferably 2/3...The plate can only be used to tie the boards to the wall, it offers no support...the boards must be supported from above or below, but the purpose is to carry the weight, in order to do that the support must meet the board over half way out from the wall...

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#32
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Re: Hanging A Cantilevered Pergola on a Wall

07/22/2020 4:39 PM

I simply made a drawing of that which I understood Graebeard to have described, hopefully to find out if that understanding was correct.

I should have said so, and also should have said that I considered the concept dangerous!

I have some cantilevered beams that stick out about 6 feet outside my home's walls, but they are 4"x8" beams that also extend 10 or 12 feet inside the walls.

We haven't been told how far the wall extends above the proposed assembly, but agreeing with your ideas, one good way to provide support would be to have a set of cables connecting from around 2/3 of the way out the boards to a second 30 foot plate at least 3 feet above the boards. That upper plate is the one that would have to be supported firmly by bolts going through the brick and the inner concrete.

If there is no inner reinforced concrete inside the brick, then I say the assembly must have some other form of support not involving the wall, or it's a no-go.

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#33
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Re: Hanging A Cantilevered Pergola on a Wall

07/22/2020 9:23 PM

Yes I understand that, I was speaking more in general rather than admonishing your contribution...that is what he proposed although the measurements of the supporting braces were not specified...that was my understanding of the op's visualization of the proposed structure as well...Why he chose to add supporting brackets only every 4 or 5 boards makes no sense, without the boards being tied together that would require a brace on every board...I think Graebeard is having us on a bit....

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#31
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Re: Hanging A Cantilevered Pergola on a Wall. Cantilever is Not Levitation

07/22/2020 3:13 PM

Hidden Mechanism in Practical Cantilevers

Check out this video on cantilevers. Pay close attention to the discussion of a "back span". Often a cantilevered board penetrates the wall, runs farther on the hidden side of the wall and has stuff on top of it on the other side of the wall. It may appear to be levitation but it is not. Usually it is just a see-saw with half or more of the beam hidden and something sitting on top of the far side. In your case the torque on your plate from the moment of all those six foot boards will be HUGE even with just the inherent weight of the boards. As a demonstration. Take a single six foot board. Grab one end of it with one hand. Do not wrap any fingers under the bottom of the board. Now, with only that one hand, lift the other end of the board.

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