Making a pergola out of an existing structure

Hi, everyone! I'm a newbie here and need some advice on my newest garden project. I hope I put this in the appropriate subforum.

I have an odd structure surrounding our backyard patio. It's like a pergola, but without a roof, with sliding glass doors on the short ends to create some shelter from the strong winds we have here. My goal is to add some form of lattice on top of this structure and let grapevines climb over it.

My questions are these:

• How sturdy does this structure have to be to allow grapevine to climb it? (I have no clue how heavy a grapevine becomes... I hope to get some grapes at least, so that'll add to the weight.)
• What kind of material is appropriate to use as lattice? Some ideas are:
  - Sticks/wooden rods - too weak?
  - Wood (how thick does it have to be?) - too heavy if large boards are used?
  - Rebars (my colleague used this for his pergola) - too heavy? Will rust - not good for the plants?
  - Wires - risk putting pressure on the sides of the structure and collapsing it inwards if the grapevine becomes heavy?

Details: The structure is a bit rickety at some spots. The posts are concrete/cement put into the ground, and with wooden posts fastened in the concrete either through two metal platings or one really fat screw. The double-point posts are pretty sturdy but the single-point posts are less sturdy. The parts with the sliding doors are a LOT more sturdy than the free-standing part.

Adding pictures of the structure along with my drawn plan for the lattice.

Thanks in advance!

 

Welcome.
You'd be surprised how heavy a laden grapevine can be, so I'd make sure you err on the thicker side. 6" x 2" from the house to the edge of the structure, over the uprights with a 4" x 2" 'lattice' in between. It may sound like overkill but it's also no fun for the person who has to dig you out of a collapsed pergola!

 

I'm struggling to read the measurements, but on first sight I think you are going to seriously need to upgrade the sizes of all the existing timbers, especially the ring beam running across the tops of the supports.
Looks like some cross bracing would be a good idea as well, surprised its not blown over if you are in a windy position.
Your idea of the 54 degree roofing would help it to stay square but it needs some bracing laterally as well.

It almost looks like someone was going to put a roof on it and then decided the structure wasn't strong enough.

 

Thanks for the suggestions!

Some clarifications on the dimensions: The whole area is 615 by 340 or 325 cm (depending if you count to the brick wall or the wooden wall of the house). The existing posts are 7 by 7 cm. The top boards (where I'd need to fasten the lattice) is 4-5 cm by 7 cm, with the wider being the horizontal dimension. In my original plan the boards would be placed at each 40 cm along the sides, creating a square lattice with sides ~28.28 cm (not accounting for material thicknesses).

The central posts (marked with circled X's on my drawing) of the free standing part are 240 cm and 200 cm from their respective ends, meaning that there's quite some distance without support at times. At the widest part I'd put up some sort of climbing support, but exactly what I'm not sure of yet (that's the small problem).

@pete , With cross-bracing, do I understand you correctly that you mean vertical X's across the open areas at the free standing side? And as a detail that I didn't think would be relevant, there's a marquis/awning (brown stripes, visible in one picture) that rolls out and slides over the top of the structure, creating the missing "roof". So the idea was that you could choose when to have a roof and not. It's windy here because of the flatness of the area, but they built the streets perpendicular to the common wind direction, so it's usually quite calm. When it's really windy though, I think the streets may create wind tunnels. But the structure seems to have been standing for a number of years (if not tens), so that's a little promising at least

@flounder, Right now my plan was to not build the lattice all the way to the house so as to not create too much shade. Do you think I need to rethink that decision? I have a problem surrounding the house also because the windows are taller than the structure, meaning that I can't fasten the boards in a lot of places.

I'll gladly take any advice I can get on how to strengthen the structure! It would be awesome if this could work!

 

Maybe some corner bracing it doesn't have to be completely an X shape just a bit of triangulation in the top corners.
Just to sort out the "rickety" parts you mention.

 

Yep, triangles, in the corner bits...with four by twos. That'll keep the centre open and the option to add as your grapevine becomes rampant, er, I mean established

 

Update: I investigated a bit more today.

I think I will need to add an additional post (7x7 cm) along the longest free standing side (marked blue in the schematic). I also noticed I have a bolt that's missing at one spot (the red-marked post), which I think adds to the instability. Easy to replace at least.
The short ends (with the sliding doors) can hold my weight (~50 kg) if I hang from them. Good enough?

Starting out with adding a post and some corner bracing/X's seems like a plan! If those areas can hold my weight after that then maybe I should be ready to move on?

What are people's opinion on rebars/concrete steel bars? They are thin for their strength, which increases the odds that I can keep the marquis functioning, but rumor has it that they risk bending/slacking over time due to weight and aging. I asked a friend what thickness he thought would work and he answered 12 mm diameter.

 

I think rebar will sag, personally, the longest one looks like over 3 metres.
It will also rust, ok you can paint it but I think it will rust in time.