Answer: When you think of trellis, you need to think of it in combination with the plant it is intended to support. Trellises in many forms has been used for centuries, and the basic idea of providing a support suited to the plant in question has not changed.
Roses will send their canes wending their way through a trellis to some extent, but may also need to be tied to it. Roses also tend to bloom best when their canes are stretched horizontally, so this is a consideration for the trellis design. Ease of access for pruning is also a consideration, as is the trellis' ability to hold the weight of a full grown rose especially when wet and the weather is windy. Clematis is a much more flexible twiner, and also has little tendrils to wrap around the support. For these reasons, a rose needs a support with fairly wide openings while a clematis needs a finer mesh to grasp hold of. These plants are often seen growing together with the trellis basically supporting the rose and the rose supporting the clematis.
Traditional designs include the rose tower, the obelisk, and the flat panel either free standing or hung from a wall as well as rose arbors of nearly any dimension. Wood or metal are usually used, although plastic is coming into vogue as well due to reduced maintenance needs.
Less tradtional interpretations can be made to suit almost any style and taste as long as the mechanics of support and structure are taken into account. If the structure you wish to use does not provide enough opportunities for the plants to twine through it, add some wire mesh to help them.
The secret is to begin early. Be sure the support is firmly anchored and strong enough for the mature weight of the plants, then plant the plants rather than adding the support after the plant is in place. Then check on the plant regularly so that you can gradually direct the new growth in the direction it needs to go. Pruning should also be accomplished according to the prescribed guidelines for the plants in question -- the techniques depend on the type of rose and type of clematis you are growing. Be sure the varieties you grow together are compatible in this respect, also.
Both roses and clematis can be successfully grown up into trees and larger shrubs, can be allowed to scramble on the ground, and can be trained onto nearly any structure from a picket fence to the side of a barn (Hang a sturdy mesh such as a cattle panel painted the color as the building and it will "disappear") to a child's swingset to an old tree stump.
For more about growing roses with clematis, see the article "<a href="http://www.garden.org/articles/scripts/articles.taf?id=737">Roses and Clematis</a>."
Good luck with your project!
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