What is a max beam length for pergola to grow vines on?

[ruthc]

Hello all, 

I am about to build a pergola to grow vines on for summer shading. The span will be 8 metres wide x 3 metres deep attached to the house. I need the majority of it to be timber, so the vines aren't burned/overheated. What outer beam material can span the longest width (preferably the full 8 metres), without needing any extra support posts in-between the main corner posts? Could the outer beam be another material (steel) if its able to single span the 8 metres? Or is it even possible to span that distance in a single beam? What is the maximum single span width I could realistically do that can also support the timber rafters every 600mm? 
It will be an open roof (no roofing - only grapevines attached), so do I need to worry about wind rating? The pergola will be go upwards (20 degrees) as it goes out, away from the house, in order to maximise winter sun. So it will have an upwards angle towards the harsh north winter winds. I live in country Victoria.
Lastly are the support posts best concreted directly into the ground, or better in stirrups?

[JacobZ]

Hi @ruthc,

Welcome to the Bunnings Workshop community, it is fantastic to have you with us.

Due to the scale of this project, it would be best that you speak with a professional builder and engineer for their assistance with this project. A structure of this size, regardless of whether there is a roofing material or not, should have engineering plans to ensure it is safe. By attaching it to your house, you are also introducing another element that needs to be considered within the planning stage.  You should also check what requirements your local council have in terms of building approval, as in most areas, a permanent structure of this size will require submission of building plans.

The only timber materials that are capable of spanning that distance are engineered timber products. Being outdoors, it would need to be H3-treated. No stores will stock these products, so you would need to special order them through your local store's Trade Desk.

Our supplier Tillings Timber makes SmartLam GL13s, which can be ordered with the correct H3 treatment. On the larger end, these beams can span 8 metres and above.

You would need to confirm suitability with an engineer, but typically, concreting posts into the ground would be more suitable than stirrups.

I am happy to assist further, but any final designs will need to be run by a professional.

Let me know what you think.

Jacob

[KirstyG]

Thanks for the advice JacobZ, we have a similar project to RuthC. 

We have an existing pitched arbour type structure built into our back patio, however it has deteriorated with age and needs to be replaced. We have reached out to tradies but none want to take on the work, so back to doing it ourselves...

Would you have any advice on where to start with suitable timber and any joinery techniques to make the structure functional but also neat? 

Existing beams are laminated timbers.

 

[JacobZ]

Hi @KirstyG,

Welcome to the Bunnings Workshop community, it is fantastic to have you with us.

Unless my eyes are deceiving me, you appear to have a ridge beam that is rising as it moves away from the house, where there is no post; rather, it is held in place by compression from the rafters.  

Unfortunately, I can see why you've had trouble finding tradies to take on the work, as there is nothing particularly typical about it. The physics involved in this design is far beyond what a typical tradie and certainly me, would be capable of designing and constructing. 

If you wanted to replace the structure, you would have to speak with an engineer for their assistance in selecting materials and designing a method of construction.

Allow me to tag @Nailbag, @Dave-1 and @R4addZ for their thoughts.

Jacob

[R4addZ]

Hi @ruthc & @KirstyG 

This is one I doubt anyone would like to give advice on as it is, as @JacobZ has pointed out, Something you would need a builder for and also probably a structural engineer. However you may be able to assist each other. I suspect that, already having the structure in place, @KirstyG  may well be able to get their local planning office to supply a copy of the submitted plans. This should have the names of the professionals involved in the original construction which would lead you to them for reconstruction. This would also give a wealth of information to @ruthc to point her in the right direction for her project.

[R4addZ]

Just a quick point for @ruthc 

You don't just need a beam capable of spanning that length, you also need to factor in the loading the vine will place on it plus a safety factor.

[Dave-1]

Good Evening @KirstyG 

Now that is a diiferent roofing structure, I like it but dont know how I would replicate it other then copying it piece for piece and then supporting the new as you replace the old. Hoping that there are plans submitted for it as I think it would be a difficult build to replicate. There is a lot of physics as @JacobZ has mentioned. And yeah I can understand why tradies may not want to touch it. Its not a total negative tho as you have an example of it being built  

A drafts person/engineer would be where I would start as ruling may very well have changed between the time it was built and now.

Dave

[Dave-1]

Good Evening @ruthc 

That 8m span is a pretty large span (when I say pretty large it is pretty much huge ) First up, do you have any photos to shopw us some more detail?

As @R4addZ has mentioned its just not the weight of the span itself that it will be carrying, the weight of a grape vine would be substantial, plus the battens and you fast get to unkown strains on the structure.

You could go for a steel beam (tho even 8m you will still need to have it drawn up professionally) and then face it with timber as an insulator between the metal and the sun plus vine. Pretty much a steel sandwich is what I am thinking.

Dave

[Nailbag]

Hi @ruthc 

I can't really get a bearing from that wide-angled photo of how it currently looks. having said that, an 8m spend in any situation is on the extreme end of the scale. This is something that a local domestic building engineer will need to nut out for you. But I'm suspecting the size of the beam and its weight along with future weight of the vine is going to be impacted by the structural integrity of there existing posts and cross-members to support anything. It's going to be a major project in any case to achieve.

Nailbag