Why won't "attach top/base" attach both side sides of gable roof?

[svalen]

This happens just about every time I try to "attach top/base"  

Why does this happen and how can I get rid of this other side of the wall?

[barthbradley]

Split the wall in half, Attach each side, and then use Join Geometry to hide the edge between the Walls. 

[constantin.stroescu]

• Try to slightly move up your roof and the problem will be solved. Or create a wall on 2 levels, so to avoid the triangle profile.
• another way is to use Edit Profile for the wall and while in Edit Profile , lock the upper lines to Roof Bottom