Use a roof, that way they can follow the structure.

Infrastructure is a bit of PITA with Revit.

Do you use Civil3D also with your projects? If so there is a nice link (bi-directional) that you can do with Revit for corridors and profiles that change dimensions using subassembly editor.

Otherwise a good method is to add some adaptive points to your deck component.
(Look up "revit adaptive components tutorial" in Google and you will find a ton of tutorial videos)

If not you are left with extrusion by path, which is also fine, but if you are changing the dimensions of your profile frequently and depending on what is changing in your profile dimensions, this might just mean a ton of modelling in-situ pieces.

As usual, with revit there are a few ways of doing the same job, you really need to evaluate what the objective is of the changes and which workflow will best suit the project you are on.

If you are using civil3D then I suggest you check out the subassemblies tool, and some tutorials on corridors as this will do the job you are asking. If not - extrusion by path, and if thats not suitable, "model in place" in revit for each piece. A parametric family might also do what you need but without seeing what your project needs its impossible to say which workflow is the one you need.