Hmm... I think you may be confused about what AirPlay is.
It will only handle sending music from a device you're using on the backyard deck, through your home network, to an AirPlay compatible receiving device located in your basement.
AirPlay won't help you in getting zone 2 audio from your basement to the speakers on your deck.
If you were intending for the AirPlay compatible receiving device to a new AV receiver with built-in support for AirPlay there are a couple of possible limitations of going this route. One is that I don't think any such AV receiver has wireless networking built-in. You'd either need to connect the AV receiver to your home router through a wired Ethernet cable or use a wireless bridge to connect it. The other is that many AV receiver with built-in AirPlay support only support it in the main zone, not zone 2.
Otherwise it's like jdsmoothie said. AirPlay uses your home network. If you can get good connectivity with your iPhone or iPad on your backyard deck then you shouldn't have a problem with AirPlay.