Therm-A-Plate is installed by lining the letter box aperture with pre-cut sections of the appropriate width, typically 45mm for FD30 fire doors and 54mm for FD60 fire doors.
For small letter plate apertures up to 50mm high, 4mm thick intumescent sections are used.
For deeper sections up to 120mm high 6mm thick intumescent sections are used.
For the largest 120mm x 300mm apertures the intumescent sections are additionally fixed with 4no. 30mm x 1.5mm steel pins at 50mm from the aperture ends and 10mm from the door faces.