How do I pick the right size J-channel for my vinyl siding?

Match the J-channel pocket depth to the full profile depth of your siding panel, not the material thickness. A lap siding with a half-inch projection needs a half-inch or five-eighths J-channel so the panel can slide in and expand without buckling. Trimming a short piece around an exterior box is an easy DIY task with tin snips, but full exterior siding work usually requires building permits and a licensed contractor.