Installing the new walls underneath the roof rafters in the attic can add support to the roof framing system.

You can also add some additional strength by blocking in between the roof rafters and on top of the wall.

The blocks will keep the rafters from twisting over time. As you can see there are two different ways to add support to the roof rafters.

I prefer using a framed wall, like the picture above and then blocking for additional support.

As you can see the system for framing a new wall underneath the roof rafters in the picture below will save you some money, by eliminating your top horizontal 2 x 4.

Both ways will work.