Attic conversion - how to get the walls to meet a sloping roof

Hi there community,

A simple problem without a simple solution it seems. Please help.
I have sketchup v. 2021.

You have to give us a clue.
At the very least an image, better still an image and your model, and a description of what your are having difficulty with.

The simple solution is to raise and lower, or trim, walls to match the height of the roof at any point. Not difficult to do manually.

Not seeing your technique or what you are starting with, I’d say one way is to raise the walls up higher than the attic ceiling (underside of roof structure) and use “intersect faces” to cut the wall faces at the correct height and delete the excess. That way you don’t have to fuss with getting the top edges of the wall face right at the ceiling–it will meet up automatically. Just as the carpenter might do… set up (at least one) board where it will go and mark the cut to the slope.

In addition to @pbacot 's method of intersecting surfaces, you could also use solid tools if the roof and wall are solids. Made the wall overly tall as described. Copy the roof into the wall group, temporarily group the wall again and subtract the roof from the wall. The roof will disappear (thus you don’t use the original) and you delete the unwanted excess tall part. Explode the remaining part named “difference”