Thoughts on shimming vs. trimming uneven rafters?

I usally pull a string line across the top of the common rafters to see exactly how far off things are. If it’s just an eighth or a quarter of an inch, I’ll take a Makita power plane to the high ones. It’s much faster than trying to re-cut anything once they are alredy nailed in. For the low spots, I keep a bundle of cedar shims or some scraps of Luan on hand to build them up. You realy want that sheathing to sit flat, otherwise your shingles will look wavy once the sun hits them. I’ve tried the 'hide it with drip edge' trick on a lean-to before, but it usually comes back to haunt you when the plywood starts bouncing under your feet while you're nailing off the sheets.

On a simple shed, I draw the line at structural integrity. As long as the bird's mouth has full bearing on the top plate, I don't sweat the tiny deviations. Just get it close enough that your fascia looks level to the eye from the driveway. I usually aim for within an eighth of an inch across an eight-foot span.

I actualy disagree with the idea of shimming individual rafters on a shed. Shims can compress or shift while you are trying to nail down your 7/16" OSB, and it’s a pain to keep them aligned. Even if you didn't plan for a sub-fascia, adding a 2x4 sub-fascia now is the smartest move. It forces those tails into a straight line and gives you a much better nailing surface for your finish boards. Instead of micro-managing with a power plane, I take a long 8-foot level or a dead-straight 2x4 and run it across the tails. If a rafter is sitting too low, I’ll use a pry bar to lift the tail slightly and then nail the sub-fascia into it to hold that height. The sub-fascia acts as a structural bridge that forces all those rafters to play nice togehter. It is much more solid than gluing Luan strips everywhere.

For the high spots that are relly egregious, I whould rather pull the nails at the bird's mouth with a cat's paw and recut the seat. If the bird's mouth is off, the weight isn't transferring to your top plate correctly anyway. Better to fix the structural connection than to just mask the symptoms.

I've dealt with this on my last shop build. Instead of messing with shims that fall out, I use GRK RSS Structural Screws to pull the fascia tight against the rafter tails. If a rafter is stubborn, those GRKs have the pulling power to flex it into position before you nail off. It makes the whole assembly way more rigid than standard nails. Plus, if you need to back it out to adjust, you can. Just grab a box of the 3-1/8 inch ones and you'll see why they're better than 16d nails for this.

While the previous suggestions regarding sub-fascia and planing are technically sound for remediation, I find that people often overlook the critical importance of ensuring the plumb cuts on your tails are perfectly vertical before the fascia is even introduced, as even a minor deviation in the tail angle will cause the fascia board to tilt and create an uneven plane for your drip edge. I typically take a sliding T-bevel to every single tail to verify the angle matches the pitch exactly, becuase if your bird's mouth cuts varied in depth, it is highly probable your HAP—height above plate—is inconsistent across the run, which necessitates checking the verticality of each rafter tail relative to the top plate's level plane.