Sub frames around fire door sets

[Guest Elliot]

When building a sub frame around a fire door I know the guidance states solid wood should be used.   Is it acceptable to use ply wood?  It is spanning the width of the wall and is positioned side on so you can see it’s layers.  
my concern is whether the layers would break down too quickly in a fire and would be point of failure.

This is around fd30 and fd60 door sets. 

[Neil ashdown]

Contact the fire door leaf / fire door-set manufacturer for advice.  They will have evidence of fire resistance performance for the door and be able to advise the requirements for subframes.