By which time Cullen has fastened the side of the tarp to be angled higher to both walls, and is tying corner number three into place on an exposed beam.

There's some residual stiffness from his (pointless, useless, unnecessary) ride to Skyhold (while injured). He rolls his shoulders, makes a face, turns his attention to the last corner. Maybe after that, he can call a halt to all tower improvement projects.

(But it's not that the Inquisitor had a point in ordering him to take time off.)