Fantasy books about city on giant root bridges

I think I had bought and read this back in the 80s; it may have been a duology. It follows the adventures of an (iirc) female character who lives on a root bridge based city strung between the walls of a very deep canyon. In the first part she's working to help build a new root bridge, which involves measuring up roots on opposite sides of the canyon, then cutting them at the correct heights that the ends can be dragged together to meet in the middle and be grafted to each other, after which houses can be built along them. Someone attempts sabotaging her work with a fire, though I believe she succeeded in cutting it correctly. I mostly remember her going on about "measure twice, cut once" while marking where her cut needed to be made, and her racing to make the cut while trying not to be overcome by toxic or hallucinogenic smoke from the sabotage.

I think there was also hang gliding using some of the giant leaves that sometimes floated down into the canyon? Though that may have been the way they disposed of corpses and not as a means of transport.

And the second book (or possibly second half of a single book) was about an expedition down to the very distant floor of the canyon to find out what was making the roots sicken, which was giant acidic slugs.