Hello.

This is a building-related question, but none of the existing forums fit the profile. Since pergolas are all timber, I am posting it here.
This is my first attempt at building a pergola. I saw many designs online. It is almost finished, but I have a few specific questions that I couldn't answer. Please, have a look at a photo below.

(for some reason can't have it shown here - either in dropbox or onedrive) I attached the picture to email...

1) Do I need a couple more battens on top?
2) Currently, the rafters are not screwed to the beams. There is at least 3.5" of wood before a screw will go to the beam. I thought maybe there is another way to attach them.  And, since the whole battens+rafters structure is rather heavy, do I have to attach the rafters at all? With the added wines it will be even heavier...
3) If I put 8' x 4' plexiglass sheets on top of the cross-members to make a roof against rain, would the grape wines be able to grow up from the sides and duck under the plexiglass? I presume that they could grow under, if I will be manually guiding them. There will just 2" of space between plexiglass sheets and the tops of the rafters.

Thank you.

Hi Slava, I'll take a shot but I'm hoping others chime in as well.

1.  If you're going to add a plexiglas roof, I'd add two more battens, one at each end of the rafters.  If they are about 2 feet apart that should be ok. If you aren't going to put a roof on it, I don't think you need the battens at all.  Will it be flat plexiglas or wavy?

2.  I think you should attach the rafters to the beams.  I would "toenail" them together with smaller screws.

3.  I don't know how grape vines act so I'm not sure if they'd want to grow under the plexiglas.  They may prefer to grow on top of it.

You don't say where you live.  Do you get much snow?  If you do, be sure that the plexiglas and rafters can handle the weight.  It's hard to tell from the perspective of the picture but what is the distance between the posts?  I wonder if the beams that the rafters sit on are strong enough as well (for snow).

Thank you, Mike!

I am in Virginia - should be zone 7b. This pergola is about 8' x 11' and the posts are 4"x4". You made a good point about snow load - I never thought about it...

The plexiglass sheets that I have are flat. It would be nice to have a roof above against the rains and snows, but my primary goal is to have a plenty of grapes to eat. When the grape vines will grow above the battens, it will be impossible to add the plexiglass, so the decision has to be made early. If the snow load is of concern, I could take the roof off for winter and return the plexiglass sheets in the spring. I don't know if that additional maintenance would worth it, though...  Another unverified guess is that under plexiglass the grapes will be sweater because of warmer air and the glass will prevent birds from eating the grapes.

The photo is the pergola's side view before rafters were added.

Ok, 11' long isn't as bad as I feared.  If you were more like 16' long I'd definitely worry about those beams holding up the weight of snow.  I'm not sure how beefy you need to be for snow in your area.  Maybe check with some other people in your area to see what works.  I'm not sure flat plexiglas can handle snow weight without a lot of support.  It may bend between the battens if they're only every 18" or so.

Can you attach the plexiglas underneath the structure for the summer?  Just a thought...

Something to consider with the vines under the plexiglass is stagnant air and localized high humidity buildup as the leaves transpire from photosynthesis. This can potentially create a nice environment for plant diseases. Those leaves and vines want to go skyward and will grow up against and touch the plexiglass, especially if there are only 2 inches of room. I would advise against it. You definitely want plenty of air flow around your grape vines. As far as birds go, I've observed them being very clever and will find a way to food if there's not much else around to eat. Maybe some bird netting over the grape vines will help perhaps?

I am a Carpenter by trade,  I agree with Mike on attaching the rafters to the beam. One thing about Arbors, or Pergolas is that the sun beats on them. The rafters need to be fastened to the beams and to the runners on top to reduce twisting and warping.  I would also recommend adding another runner nearer to the tails, they will warp to the side drastically without something holding them to form. The only other thing I would have done differently is notch the beams into the posts. Looks good!

One last thing to consider: How deep are your posts in the ground? I don't see any sway bracing, and the structure is going to get various side loads over time, especially as vines grow up and give something for wind to get hold of, and possibly lean unevenly against it. Unless your posts are extremely deep, I would recommend diagonal braces in both directions at the top of each post. They wouldn't have to be heavy to be effective in this case, and there are lots of things you can do to make them decorative.

Hi could you post what you decided to do? I have the same issue in that I'd like to design a pergola for grape vines with glass cover for rain protection for my backyard but don't know the best method. thanks!