rafter
桷 refers specifically to a rafter, a structural beam used in traditional roof framing.
桷 combines 木 (tree/wood) with 角 (horn/angle), likely indicating a wooden structural piece with an angled shape, such as a rafter.
A wooden (木) beam cut at an angle (角) to support the roof: that's a rafter.
For カク, imagine a carpenter cutting a wooden rafter at a precise angle, making a sharp 'kak' sound with each saw stroke: kak -> カク.
Toringo crabapple (Malus sieboldii)