Trim is not what you want - it has a completely different function in 15 than earlier versions.
Crop in 15 is the same as the "old" Trim.
If you right-click a wall (e.g.), you'll see amongst the pop-up context menu commands the attached: Crop to Single-plane Roof
If you are using the new multiplane roof, then read about the new Trim commands in the help file and watch the youtube videos... Best done in 3D with the status bar displayed, to show you the steps to perform - you'll select the trimming body (roof) and then be asked to move your mouse over the part to be kept - which will be highlighted as feedback until you click and perform the operation - allows trimming the top or bottom.
Cheers,
Karl
One of the forum moderators
AC 28 USA and earlier • macOS Sonoma 14.7.1, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB