Pergola 18' Span Length?

And to answer your question Stampie, I'm going to put a pergola over the back patio.  This thread made it clear my first "back of the napkin" idea won't work, so I'm looking for a good solution that won't break the bank.  I was figuring around $8K for the simple canvas sail frame, including hiring somebody to help put it up, but it looks like reality might intervene with that goal.

In reply to jwagner (Forum Supporter) :

If you are ok with a post in the middle, all your span problems are solved. Add a beam down the middle. That would mean the beam spans would be 12', and the rafter spans would be 9'. Those are easy spans (and 2x6 would be good for 9')

Still have the "scale question"... but there is nothing wrong with a canvas sail "roof", even if it doesn't look like a "pergola". 

SV reX said:

In reply to jwagner (Forum Supporter) :

If you are ok with a post in the middle, all your span problems are solved. Add a beam down the middle. That would mean the beam spans would be 12', and the rafter spans would be 9'. Those are easy spans (and 2x6 would be good for 9')

Still have the "scale question"... but there is nothing wrong with a canvas sail "roof", even if it doesn't look like a "pergola". 

I think we're on the same page.   Just measured where a post in the middle of things would go and I think it would be OK.  Now I'm trying to visualize how the wood frame/canvas sail pergola would look on the back of the house compared to the traditional lattice pergola pictured above.

In reply to jwagner (Forum Supporter) :

It would look good!

I would free-stand it from the house, and make it taller than that lower roof on the right. Otherwise, it's gonna look too squatty.

I was in Sam's today. Here's a cheap option:

You could install 2 of them and still be under Challenge budget.

If you just have to use long length beams you can also use LVL beams that have been aluminum wrapped with weep holes in the bottom of the wrap to make them weather proof.

I'm not qualified to size beams but here is a 24'x18' clear span structure consisting of 6x6 posts with double 14" LVL beams and 2x6 rafters for discussion only. Not shown is the gazillion 4x4 diagonals needed to hold everything together and up. This will require some substantial concrete piers. 

With diagonals.

VolvoHeretic said:

I'm not qualified to size beams but here is a 24'x18' clear span structure consisting of 6x6 posts with double 14" LVL beams and 2x6 rafters for discussion only. Not shown is the gazillion 4x4 diagonals needed to hold everything together and up. This will require some substantial concrete piers. 

Awesome, thanks!

Here's an idea...

This is a combination of VH's LVL idea and your earlier pipe idea...

How would it look if you installed LVL's like VH's drawing, then installed 9' long pipes (perhaps PVC) instead of the 2x6's.  Alternate them- set one high, and the next one low.  Then weave your canvas over the pipes.

The pipes would only be 9' long, so they wouldn't have to be very large diameter. 

In reply to SV reX :

You could also slide a light gauge steel sleeve inside the pvc to stiffen them.

It'll depend on your skills and tools, but my first thought after reading "18 foot span" was "laminated arch".  Which looks to be DIY-able for not terrible money. It'll definitely be time consuming.  

But after looking at the picture of the space, I'm not sure it would look right as the arch might block some of the second floor windows. 

Below are pvc screw down end caps for 1-1/4" pvc piping and 1" x 20' steel EMT conduit to reinforce the pvc pipe. The end caps come 10 per bag. There are 14 cross bars needed if you also use them at the two ends of the pergola.

Schedule 40 pvc 1-1/4" inside diameter: 1.360"

1" steel EMT conduit outside diameter: 1.163"

With PVC purlins.

Thanks VH - researching engineered beams and pipes/conduit now.  Don't think PVC is the answer.  Would need to be painted and I'm not sure that's the look I want.   But I was already thinking about conduit, and maybe sliding a conduit into another might be enough strength to avoid serious sag.  Shouldn't rust, cheap, and would look OK without coating.

Might have found a good solution:  chain link fence top rails.  They come in 21' lengths, various gauges, and if I'm willing to order a bazillion of them from China, I can get powder coated in 9 to 17 gauge.  Probably not...

With these, I think I could skip the center support and cut these to about 19' for the purlins (learned a new word today)  Time for a discussion with my builder man.

In reply to jwagner (Forum Supporter) :

What would they attach to at the ends?  Still gonna do the perimeter beams?

You can buy fence top rails in black.

I'm pretty sure they will sag if they span 19'.

10ft is typically max spacing for chain link fence posts, for whatever that's worth. Not sure how much of that has to do with wind/lateral loading and how much has to do with supporting the top rail to prevent sag.

In reply to STM317 :

Right. Although the canvas will make everything perform completely differently.

In reply to jwagner (Forum Supporter) :

This isn't that important, but I just thought we could communicate a little better if we are in the same page...

A "purlin" typically goes side to side across the rafters. A "rafter" typically follows the pitch of the roof.

Most pergolas don't have a pitch, but the roof framing members are usually still called "rafters".  
 

Not a big deal.  I know what you mean when you say "purlin".

STM317 said:

10ft is typically max spacing for chain link fence posts, for whatever that's worth. Not sure how much of that has to do with wind/lateral loading and how much has to do with supporting the top rail to prevent sag.

I'm guessing that the 21' top rails have support posts in the middle in a chain link fence, but I was hoping that by going to a thicker gauge rail we could minimize the sag.  Standard rails at the home supply stores seem to be 17 or 18 gauge.  Master Halco has 21' lengths in much heavier gauges.  For example:    1-7/8” O.D. – PE    2.72 lb/ft  .145" nom. wall thickness (9 ga)   Don't know about the cost of the heavy duty stuff. yet.

Need to figure out how much that would sag across a 18' span.  Might still need the center rail support.

SV reX said:

In reply to jwagner (Forum Supporter) :

What would they attach to at the ends?  Still gonna do the perimeter beams?

You can buy fence top rails in black.

I'm pretty sure they will sag if they span 19'.

Was thinking we'd have paired 2x10s for the perimeter span, and it looks like we'll need to split the 18' with a center support.  As for the end attachment, perhaps drilling holes into the inside beam to insert the "purlins" (rafters).

Edit - We probably don't need the 18' end beams for anything other than appearance.  Maybe some kind of trim piece would be just fine.

You are still gonna need the full perimeter- all 4 sides. Without it you won't have anything to attach bracing to, and it will blow over. 
 

Unless your posts are set really deep (like 3 or 4 feet) in concrete. 
 

I know it's just a decorative structure, but you are in WI. You still have wind loads, snow loads, and frost heave to deal with. 
 

 Don't underestimate how much wind that canvas will catch.

In reply to SV reX :

Was figuring on attaching one side to the house.  Wind yes, but I'll take down the canvas in the fall.  Unless we have a late May snow - which has happened - don't have to worry about that.

In reply to jwagner (Forum Supporter) :

Attaching to the house is good. But the other side is 18' away. Your are gonna have to have some bracing, etc to keep it standing. 

Absolutely nothing to do with the structure questions, but I saw these while visiting a friend of a friend - Shades clip on with small carabineers and can be slid along the cables to provide shade where you want it.  Storm coming?  Retract against the house.  In winter, un-clip them and store away.