This isn't the roof quest, but the sand quest, correct? So, Sirina mentions the sand in her home becoming a problem when you return from the erupted volcano. When you have the flying saucer, don't travel directly to the Elven forest. Instead, land on Hermit Island and return to Canacopra. This is one of several quests you can do there. Get the quest first from Sirina. If you then talk to Ritchie, the guy in the tent to the left of the inn, he'll tell you he's good at digging sand. You need to talk to him a couple of times to trigger this. When he tells you he's good at digging sand, the idea comes to your party that this may be the man for Sirina. Just keep clicking and let the events transpire.
If you have the quest and Ritchie doesn't tell you he's good at digging sand, it may be that you have to do a couple of other quests first, like the monkey and new slave quest. Just do whatever you can, but be sure to keep checking in with Ritchie before you leave the island in the flying ship.