The rule that two people were to be on the roof at any time was quickly broken when Barry decides to go on the roof himself ("we didn't build it so it's gotta be safe" is his rationale), leading to Keith also joining Darryl and Merle on the roof.
Merle - Merle begins by cutting ad-hoc without first measuring, and uses a circular saw and a chisel to cut the hole for the toolbox handle.