Most mains-powered Z wave devices will extend the mesh. These are called “repeaters.” Battery powered devices do not repeat, as it would use up too much battery life.
See the following FAQ:
All of that said, I’m not quite sure why we revived a three-year-old thread today, as there are now a number of different options that were not available three years ago. Pretty much everything in this thread is out of date, regardless of the protocol being discussed. 
There is a how to article in the community – created wiki on automating an outbuilding which is up-to-date for anyone interested in the OP’s original question, which was how to control devices in a shed away from the main building.
http://thingsthataresmart.wiki/index.php?title=How_to_Automate_an_Outbuilding
With regard specifically to extending Wi-Fi networks, there are now multiple Mesh Wi-Fi systems to consider, including The newest one from Samsung which has a built-in SmartThings hub as well. 
