Options for adding lower gearing for bike with Shimano 105 drivetrain I have a 2015 Salsa Vaya 2 with a Shimano 105 drivetrain. It has a Shimano 105 Triple front derailleur with a 30/42/52t crankset. In the back it has an 11-30T 10-speed cassette (installed for me by the LBS instead of the stock 11-28T one). I was told at the time that I can't get a lower-geared cassette on this bike. 

Now I'd like to get into some light touring, and want to lower the gearing for climbing with load. What are my options? I see some 105 cassettes with larger big cogs (e.g. 11-34 11-speed). Can I just drop one in? Do I need a different rear derailleur, a new shifter, or any other parts to ensure things work?
There are a couple of parameters you need to pay attention to:


The rear derailleur's wrap capacity (how much chain it can wrap up)
The biggest gear the rear derailleur can fit under.


With your current setup, your derailleur needs to wrap 41 teeth of chain. It looks like you've got this derailleur, which can wrap 37 teeth and fit under a 32-tooth gear. So you're already beyond capacity in terms of wrap, but if you don't cross-chain, it's probably not a big deal. You could probably get away with a 34-tooth bottom gear by adjusting the B screw on the derailleur (the one pointing straight back, under the hanger) to push the derailleur down.

The current generation of 105 medium-cage derailleurs is designed to fit under a 34-tooth bottom gear and handle 39 teeth of wrap.