I tried it initially with one plane but I couldn't figure how to tell the program to set the low point to the two long walls instead of starting low on the window wall and sloping up towards the rest of the room so I followed a similar chief tutorial that said to draw half the slope then copy it onto the other side, is there a way around that?
My new problem trying it that way by setting the pitch to zero is that it locks the bottom and top heights of my slope to be the same, is the option I'm meant to change on a different page?
Edit: I figured it out, I haven't solved the main issue but your comment made more sense while watching the video and I got it to work with one plane.