fitting a metal sheet behind a gas cooker?

My Mum wants a gas cooker fitted and has been told that because the vents on her new cooker are at the back and the fact it is being fitted against a wooden Tongue and groove wall she will need to have a metal sheet fitted to the wall.
Does anyone know what specifications this sheeting would need to have (size, type of metal, thickness etc) and where we should buy it?

Janine
October 2010