Oh Grasshopper,
A root wad you want to fish! Hmmm, several ways to attack this there are. The hard part is to get the fly in the zone when the fly is at the “end of the line”. Meaning if you cast right up to the root wad then the fly will sink and comeback towards you and thus out of the zone of the root wad. Using euro nymphing the same can happen. I would cast upstream of the root wad and closer to the shoreline of the root wad allowing the fly to sink before the root wad and hopefully when/if the fly “comes back towards you” it will drift under the root wad.
If using a dry fly I would measure my cast before release so that it lands in the exact spot I want it to. You do know that BIG fish hide in the root wads – that’s why they are BIG cause you can’t get to them. You will most likely get stuck which will spook the big trout out of the wad. BUT, if you do get a good cast close enough and the big trout comes out from the depths of the root wad you will be rewarded. The problem is the trout will head right back to the wad to get you tangled which you wanted to avoid in the first place. If it comes out to take your fly don’t give him any leeway to go back – bust off, tangle or big fish? Hopefully you will be rewarded. Same problem with undercuts. Tight lines!
