Je pense que la solution la plus simple est de calculer la hauteur de l'intersection :
\documentclass[border = 8 mm]{standalone}
\usepackage{tikz}
\usetikzlibrary{positioning}
\usetikzlibrary{3d}
\usetikzlibrary{calc}
\begin{document}
\begin{tikzpicture}
% ground
\begin{scope}[canvas is xz plane at y=0]
\draw (0, 0) rectangle (15, -10);
\end{scope}
% ceiling
\begin{scope}[canvas is xz plane at y=10]
\draw (0, 0) rectangle (15, -10);
\end{scope}
% vertical edges
\begin{scope}[canvas is xy plane at z=0]
\draw[blue, dashed] (0, 0) -- (0, 10) ;
\draw[blue, dashed] (15, 0) -- (15, 10) ;
\end{scope}
\begin{scope}[canvas is xy plane at z=-10]
\draw[blue, dashed] (0, 0) -- (0, 10) ;
\draw[blue, dashed] (15, 0) -- (15, 10) ;
\end{scope}
% blue diagonal
\begin{scope}[canvas is xz plane at y=10]
\draw[blue] (0, 0) -- (15, -10);
\end{scope}
\draw[red] (0,10,0) -- ($(15,10,-10)+(0,{-1*sqrt(15*15+10*10)*tan(10)},0)$);
\end{tikzpicture}
\end{document}
![alt text][1]
[1]: https://texnique.fr/osqa/upfiles/document_LVRrIar.png
