Offsetting curved egdes

Hi all, I’ve created some curved steps on a garden design and would like to add an extra line to each of the risers so I can create an overhang on the tread or make it look as though the top edge of the step is in a different material. I also can’t figure out how to slope the top of a curved wall so one side is higher than the other?
Thanks in advance

First group the wall, so it is protected from other geometry.
Double-click the wall group to enter it for editing.
Activate the MoveTool.
Hover the MoveTool’s cursor over one of the top end edges of the wall, until the edge is highlighted, and click somewhere along the highlighted edge.
Tap the Up Arrow key to constrain the tool to the blue axis.
Drag this end edge up or down, and click when you have the edge at the desired height.
SketchUp will triangulated and soften the top surface to make it look sloped.

So no actual extension is need to do this.

Again, it is the MoveTool you need. This time using the CTRL key copy modifier.

Hover the MoveTool’s cursor over the top edge of a riser, and tap the CTRL key once. You’ll see a plus symbol appear next to the MoveTool cursor. Click the top riser edge, and move downward (you can tap the UP Arrow key once to constrain to the blue axis if you wish,) …
Once you have begun moving in the proper direction, you can click to complete the copy, or just enter a distance or a point (which will be displayed as you type in the Measurements box.)

  • Distance: The number you type can be positive or negative, such as 20' or -35mm. If you specify a unit of measurement, that unit overrides your template’s default units. For example, you can type 3' 6" even if you are using metric system as your default.

  • Coordinates: The Measurements box accepts global or relative coordinates. To enter global coordinates, use square brackets, such as [3’, 4’, 5’]. To set relative coordinates, use angle brackets, such as <3’, 4’, 5’>.

Ref: SketchUp Online User Guide