I need advice on how to fit the new fire door casing 6" / 150mm around the stair case.

How much of the door frame can be cut at the bottom around the second step on the stair case without jeopardising the fire rating of the door set? if you look at the pictures you will see how the original door casing was wrapped around the stair tread and stair string, on the hinge side it's going over the stair string and on the latch side the door casing is cut around the stair string.

Can the new fire door casing be fit like the original door casing was fitted?

Or Can the new fire door frame be cut around the stair tread so not to damage the second step by leaving 70mm of the fire door frame at the bottom where the stair string and door frame meet? that way it will avoid the second step on staircase on both sides.

or does the whole door casing need to be continuous and go all the way down to the first step so would have to cut into the second stair tread and riser? like some bm trada installers have suggested.

thank you for your help.

In order to comply with the fire door manufacturer's evidence of fire resistance performance (and therefore BS 8214: 2026) the door framing sectional dimensions* must 'continuously' be at least that specified by the installation instructions or certification data sheet for the door leaf. Therefore, the door frame should not be interrupted at any point.

*commonly 32mm thick by 70mm wide as a minimum, but check the door leaf installation data.

Are you sure that this needs to be a fire door?

Building regs (part K) requires a safe, level, landing at the top and bottom of a flight of stairs and no door swing can encroach closer that 400mm to the front of any step.

The current situation creates an enormous tripping hazard