To make it possible to keep the new gutter straight and fine-tune its slope, we installed a series of 3/4-inch plywood “ribs” 20 inches on-center, scribing them to the bowed wall as needed to keep them in line, and adjusting them up or down in a few spots to help keep the bottom flat.
We cut them out using a router, providing a 73° slope for the sides and a notched front to receive the fascia board.
On one side of each rib, we fastened 2-by support blocking to the wall with a couple of 6-inch-long TimberLok screws. After the ribs were secured, we installed the rest of the Zip roof sheathing along the eaves, then inserted the sloped 3/4-inch plywood sides of our rough trough, cutting the top of the inner panel to the roof’s pitch.
Then came the fun part: fitting the sloped plywood bottom. First, we snapped lines on the plywood sides representing the 1/8-inch-per-foot slope we wanted, which worked out to about a 2 1/2-inch drop from the middle out to the ends. Next, we custom-fit the plywood bottom, which, because of the angled sides of the trough, became narrower as it dropped in height. This process took some time, but was made easier by having straight, parallel sides to work with.
From inside the trough, the slope looked more pronounced than the 2 1/2-inch drop we needed, but this was an optical illusion created in large part by our compensating for the out-of-level wall. We fastened the trough bottom with screws countersunk into the angled sidewalls.
We back-prime and install the fascia. The fascia was pressure-treated, solid select pine from New Zealand.
With the trough bottom in place, we installed a 3 1/2-inch plywood cap that would help to align and support the crown molding.
The cap and molding wrapped around both ends of the trough, which were sloped inward at the same angle (73°) as the sides.
The last step in prepping the trough was to back-prime and install the crown. We used Lifespan trim, a pressure-treated, solid select pine from New Zealand. I ended up milling the new crown, using knives from a previous remodel, to match the existing rake molding at the return.