Ok so you need to know there is many build for soraka. The "basic" soraka with chalice and armor is an option but not the best. I start to expect a zeroraka to be possible who would build roa and would refuse to use W at all. but it's just a theory for now.
However there is the 10+ Soraka that I start to understand now... You might want to dig into it. That Soraka build turn around casting heal more then 10 time. More like 20-30x in a single teamfight. Which can potentialy heal for about 750% of her health (10k hp per teamfight). That Soraka will rely on hp5, spellvamp and lifesteal to recover what she use for her heal (not only the basic starcall). Going over the 10th w have some weird effect. The major one is that any health items now turn into plain negative effect. At the 20th heal for exemple, A ruby would have cost you 150hp from your base hp pool. Try a build that contain not a single HP item, including sight stone. I don't feel I reached the cap yet at all...