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The Extend Roof Slab tool to extend a slab or roof slab to a plane defined by another slab, a roof slab, or a wall. The target itself does not have to intersect with the extended slab/roof slab, as long as it projects an intersection in the current user coordinate system (UCS).
To Extend a Roof Slab:
1. Select the target flat plane. It will be highlighted in green.
2. Select the first edge to lengthen
3. Select the second edge to lengthen
4. The result will look like this:
Another Example:
Notice that the roof slab has extended to where the wall would be if it were that tall.