rafter, porch
椽 centers on a structural beam that supports a roof: a rafter. The porch sense is a natural extension, as porches are defined by their roof-supporting rafters.
椽 combines 木 (wood/tree) with 彖, a component that likely contributes the sound and may relate to the idea of a beam or projecting structure. The exact historical development of 彖 is uncertain.
The left side 木 is wood, and the right side 彖 looks like a pig 豕 under a roof-like top. Imagine a wooden beam (rafter) shaped like a pig's back supporting the porch roof.
For テン, picture ten rafters lined up across the porch: ten -> テン, and each one is a wooden beam.