Good Heights for Pergolas
Grapevine Pergola
One of the most common types of pergolas is the grapevine pergola. Constructed over a patio or path, these pergolas are typically 7 feet or higher in height. Grapes will hang down between the overhead trellis cross members, allowing them to ripen. Additionally, grapevines tend to grown in all directions, including down, so space must be given to allow an adult to stand and not brush against vines and leaves. The pergola should be built at a height that will enable the homeowner to reach hanging grape bunches without the need for a step-stool or ladder.
Pathway Pergola
A pathway pergola, or traditional pergola, is one that is constructed to give shade and decoration to a pathway or walkway in your garden. These types of pergolas can typically be built in an archway-style design, with a height higher than the grapevine pergola. Since there is no fruit to pick, a higher height will allow for a more comfortable and less claustrophobic feel for someone walking down the path. Also, if your pathway pergola contains rosebushes, care will want to be given to allow plenty of room to prevent brushing against thorns. A height of 8 or more feet is typical.
Deckside Pergola
Building a pergola alongside a deck, near seating or a barbecue, is a great way to provide shade for your outdoor seating or cooking area. Care must be taken to construct the pergola with sufficient height to not only clear the grill top when it is lifted, but to prevent the pergola from being burned by flame. A pergola constructed near a grill or cooking surface should be at least 6 feet above the heat source. Additionally, any vines or plants growing on the pergola should be pruned back away from the heat source to prevent fire.
House-side Pergola
A pergola constructed along the outer wall of a house can be a great way to cut home air conditioning bills by providing shade. Construction height of such a pergola depends on your region, and what height will maximize shade time for your wall. Pergola height should be higher than the tops of your first floor windows, but low enough to allow your vine or plant to grow on the top of the pergola and not immediately be clogging your gutter, or spreading onto your roof.
Source...