We have a grape arbor over the sidewalk that leads in a sweeping curve from the edge of the driveway to the pedestrian door into the barn.  It is 30 feet long and about five feet wide, and is probably a bit over 35 years old.

The upright posts are 6″ by 6″ and are still solid.  However, the whole upper structure was badly deteriorated.  The horizontal stringers were 2″ by 4″ boards that were nailed or screwed to the outside top of the posts.  The slats that lay across the top five foot span, perpendicular to the stringers, were 2″ by 2″ boards.  Many of the slats were missing and those that remained were fragile and failing under the weight of the grape vines.  So we decided it was time to replace everything but the posts. 

We made a few design improvements in the process. 
One, we upgraded from 2X4s to 2X6s for the stringers.
Two, we ran stringers from one side to the other at each post, creating box frames that will better hold everything together.
Three, we mounted the 2X6 stringers on top of the posts, rather than attaching them to the sides.  This gave us a much needed additional 6 inches of vertical clearance when walking under the arbor.
Four, not liking the fact that the last stringer had been attached to the barn’s facia, we added one more 6X6 post as support for that corner nearest the barn.
Five, we cut the slat ends at 15 degrees, just for some visual appeal, and added a 1″ by 8″ face board (also cut at 15 degrees) as a finishing touch.

For the construction effort:
First, and as carefully as we could, we pulled the grape vines (before they put out grapes) off the old arbor and gently arrayed them on the ground.

Next, we demoed all of the old slats and stringers.  Then we placed (after removing an unexpected huge boulder) a new 6″X6″ post.  See boulder video.

After that, we cut and affixed all the stringers.

Cut and attached all the slats with their 15 degree ends.

And added the face board.

Finally, and again as carefully as possible, we hoisted the grape vines up onto their new arbor.

And a bit later in the season, the vines started producing their first grapes on the new arbor.