Why can't I build a pipe here in Prison Architect?

Why can't I build a pipe here in Prison Architect? - Low angle view of metal scaffolding standing near repairing building against cloudy sky

I'm trying to connect plumbing up for a toilet in a cell, but the game won't let me build a small or large pipe here. Is there a reason why?

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Pictures about "Why can't I build a pipe here in Prison Architect?"

Why can't I build a pipe here in Prison Architect? - Urban multistory building at construction site

Why can't I build a pipe here in Prison Architect? - House Floor Plan

Why can't I build a pipe here in Prison Architect? - Hallway With Window

Liz Phair - Why Can't I? (Official Video)

Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.

Images: Josh Sorenson, Mathias Reding, Pixabay, Jimmy Chan