A pergola is an exterior structure designed to provide a degree of shelter and shade in landscaping. It may also be used as an arbor, supporting the growth habits of vining plants such as grapes or certain species of rose. During the Renaissance, pergolas formed "green tunnels." Willows or grape vines planted to either side of a path were trained on columns of timber to meet and arch over the walkway, supported then by cross-timbers to provide a shady respite or shelter from a rain shower.
In contemporary times, a pergola may still be employed in a landscape over a sidewalk or garden path. The structure may be constructed from pressure treated wood to avoid rot, from resin or from metal. Walkway pergolas are usually rectangular in their overall shape and, like the original green tunnels, extend across a length of ground that may otherwise be too sunny for comfort or too open to be visually attractive. A walkway pergola may act as a connective element between the house and the garden or within a large garden.
Like walkways, free standing pergolas are independent of other structures. A free standing pergola, though, is usually smaller and acts as a defining element for a sitting area within the landscape. The columns may form a square or rectangle, with the cross beams creating the overhead. This allows the area to be shaded but not completely void of sunlight. As with walkways, vining plants may be trained to climb the columns and across the beams to create shade and add greenery to the site.
A pergola may also be attached to the house, extending from the roof line to support columns. Crossbeams or lattice work are added to create the roof of the overhead for the pergola. Extensions are similar to covered porches in that they define a specific outdoor area, lending to it a sense of extended living space from the interior of the home. The difference is that a porch roof is a part of the structural roof of the home. A pergola overhead is not shingled nor is it integrated into the support of the home.
An attached pergola may also be styled as a free standing pergola and attached to the home without the benefit of the roofline. An example of this may be on a two story home. The pergola is securely fastened to the exterior wall of the house at a position that approximates in height with the interior ceiling on the first floor.