I'm building a 4' wide walkway around the side of my house coming off of a deck. It's about 9' off the ground. While placing the joists, I've discovered the last 6 or so are shorter dimensional heights coupled with a slope on the beam. If it were only an 1/8th", I'd live with it but they're closer to 1/2" and a couple are 5/8ths. I can't find 2x12's rough sawn in my area or I would just use those. I've placed the taller joists to that end already and I'm still short. I'm considering placing a strip of treated wood or plastic wood on top of the beam and resting the joists on that or shimming each joist up individually. Is that a bad idea? Should I shim on the top of the joist instead? What's the limit on shims anyway?