rafter
榱 refers specifically to a rafter, a structural beam supporting a roof.
榱 combines 木 (tree/wood) with 衰, which likely contributes the sound. The character is used for a wooden rafter.
The wood radical 木 on the left suggests a wooden object, and the right side 衰 looks like a support structure. Picture a wooden beam (木) that supports the roof, like a rafter.
For スイ, imagine a rafter that is so smooth it looks like it could be used as a water slide: smooth slide -> スイ, and the wooden rafter becomes a swift water chute.