need to impliment a "Site is down for maintenence" redirect

I'm brand new to F5 and I'm looking for urgent advice on load balancing - potentially using Irules but am open to suggestions.

 

I need to impliment a "Site is down for maintenence" redirect that auto kicks in if all servers are in pool are unavailble.

 

 

Basically we have a simple setup - if members of the pool are active and servers are up then availble to accept traffic and serve the pages . But if Members are not inactive and down then, stop directing traffic to that Pool. and irule should redirect to other server where i have hosted maintenence page.