Keith is right. There in Sydney you need to water more often with this media but not more than once or twice a week. The sponge rock absorbs enough moisture toward the inside of the pot to keep it hydrated. The roots look normal. These are older roots and when watered they turn a darker brown color. Remember that the root is actually inside the velamen which is the spongy water absorbing layer which holds the moisture for the root to take up. I still have a few orchids that are growing in charcol and sponge rock only along with some in lava cinder only. They grow well and are growing new leads. Rachel, when it starts turning cooler say in May or June, water less often. One way, and I feel a good way to tell how hydrated a plant is, is to water it thoroughly and pick it up. Remember how heavy it feels. Now let it go without water for a week. Don't worry. It will be just fine. At the end of the week, lift the plant again. It should be fairly light weight by now. Now water it again. Really drench it. Lift it up again and feel the difference. That is how to tell if it needs to be watered. You'll notice a big difference in weight. It is nearly summer there so feed it weakly, weekly. 1/4 strength orchid food every time you water. Every 4th watering, water it with plain water with no food. Give it lots of air. Where you have it hanging sounds ideal. One other thing. Locate your nearby orchid club. I know there are at least two in the Sydney area. You can learn so much from them. Good luck.
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It all depends on the genera and your environment. 
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