Updated 2019: A closure plate and a register plate are two different things but do a similar job. A closure plate is used when you have a chimney liner and the closure plate seals the chimney for cosmetic reasons, to stop old soot falling on the stove and to stop heat disappearing up the chimney. If the closure plate developed a fault (e.g.hole) smoke could NOT enter the room.
A closure plate can be made of any non-combustible material. In the picture on this page the closure plate is made of 12mm concrete board which is much easier to cut than metal.
HardieBacker® 12mm cement backerboard (previously named HardieBacker®500) has been evaluated in accordance with the protocols and acceptance criteria of EN 12467 and was found to be compliant. HardieBacker® 12mm cement backerboard is fire rated class A1, meaning the product is non-combustible. HardieBacker® 12mm cement backerboard is CE approved and BBA certified since May 2004.
Available from Travis Perkins/B&Q etc.
A register plate is used when there is no chimney liner and acts as a barrier to the smoke and fumes in the chimney – it must make a sturdy seal between the room and the chimney and it MUST be made of galvanised or stainless steel (or other non-rusting metal) at least 2mm thick.
Why does it need to be made of metal? Well if a register plate failed then smoke could enter the room. This could happen if a brick fell from inside of the chimney.
A register plate would usually have access doors to allow the sweep to access any chamber above it to remove fallen soot.
It is rare nowadays that liners are not fitted (although stove flue pipes might be connected directly to pot lined chimneys) and therefore registern plates are less common on new installs. Modern wood burners are highly efficient and this means that LESS heat goes up the chimney. It is the heat that gives the draft (hot air rises and the hotter then the faster it rises). The more efficient the stove the less heat is wasted up the chimney and the more likely that a liner, rather than a cold void, will be what the stove requires to function correctly.
“Avoid the void” is a common Hetas expression heard on training courses.