Although I initially planned to attach my trellises to my building, I instead ended up attaching my trellises to 4' or 5' rebar sunk into the ground a foot to 18" rather than to the building. This has worked well for me.

A couple of considerations:
- Is there a rain shadow created by your eaves or a drip line to avoid? If so, the plants should be outside of the rain shadow or drip line. (This is why mine are attached to rebar, so they can be outside the drip line.)
- Are the cross pieces small enough so that the leaf stems of the clematis can wrap around the trellis? Several folks have posted that wooden or PVC lattice type trellises are difficult for clematis to climb. I aim for about 1/2" diameter or less cross pieces.

Good information re: drip line. Thanks.

I think what you are saying is that under the eaves created by the roof the plant won't get sufficient water from rain?

The trellis we are taking down is of that wooden variety that you mentioned but the new one is metal (fake wrought iron?) so the leaf stems should wrap around without any trouble.

"I think what you are saying is that under the eaves created by the roof the plant won't get sufficient water from rain?"

Yes. Any plants that grow between my drip line and the house don't mind being dry, and don't include clematis. Clematis like even moisture, not either soggy or really dry soil. Also, if there is rain dripping off the edge of the roof, even from one story, the force of the water dripping discourages plant growth, though this wouldn't be an issue with gutters.