Lattice-can't select area inside a closed shapes

We use SU4S for CAD class. Trying to draw 4’x8’ sheet of lattice. Drew perimeter and 2" border. Drew first angled wood slat using Line going from corner to corner, then copying that line 2" to right. I can’t select either large triangle to delete them. Select tool picks the entire inside area of original rectangle, not the area inside shapes that are inside the original rectangle. I Don’t know why I can’t select or delete either large triangle. Am I approaching this all wrong? Any ideas? Thank you. Lattice Mr. R.skp (138.3 KB)

How did you draw the 2" border? I tried replicating the process using the offset tool to get the border. Then I drew the diagonal, moved a copy of it 2" to the right and cleaned up extra edge fragments. The faces were split appropriately and the large triangles could be deleted. So, your workflow seems like it should have worked. I can’t explain why you had an issue. Sorry!

I did use offset. And I tried it all again and now it works. I don’t know why not work before. It works now. thank you.