So a lot of zerg I see / know have trouble with protoss forcefields, especially when they are trying to be aggressive early game. A strategy that can be used to bypass the pesky ramp forcefield tactic is burrow + tunneling claws on roaches. After researching burrow at your lair and tunneling claws at your roach warren your roaches can burrow underground and slide under the forcefields! I like to use this to burrow up the ramp and into their mineral line, then pop up and bam.
Obviously if your opponent has cannons down or an observer out this isn't nearly as powerful but you can still use it. Burrow your roaches and quickly get them up the ramp past the forcefields then unburrow them and engage. Hopefully they will not take too much damage in the process; this works great against protoss who feel safe behind the fields and tech up quick instead of building a bigger army.