I would suggest
not
splitting walls unless absolutely necessary, since it just gives you more things to adjust if you change things in your model.
Solid Element Operations can be the best solution in this case (Edit menu).
Extend the wall above the dormer ridge, select it as the target, the three roofs that it touches as the operators, subtract with upwards extrusion and use own attributes.
If the exterior wall is not the same construction as the dormer side walls, as it appears from the thickness of your model's exterior wall, then this will give the result you probably want in all views.
If the exterior wall and dormer side walls are the same construction, for example frame, then the wall-splitting technique can end up being best, since you'll want the front and side dormer walls to clean up to one another, rather than display as a but-joint in plan.
Prior to 8.x, the wall-splitting technique was the only option.
IMHO,
Karl
PS In 9.0, you can split walls using the pet palette, rather than the split wall command. Mouse-down on a wall edge (mercedes) and you'll see options for inserting new nodes (= split).
PPS If you do use trim-to-roof, than as a new user you should also know how it works. If you have no roofs selected, than you are trimming to any and all roof that touch your selected wall(s). If you have one or more roofs selected along with one or more walls, then the walls are trimmed only to the selected roof(s).
AC 28 USA and earlier • macOS Sequoia 15.4, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB
Rubia Torres
