Neil ashdown

When inspecting the gaps between the edges of the fire door leaf and the door frame: 1) Check the gap is between 2mm minimum and 4mm maximum, generally (consult the door leaf data sheet for certainty about the requirements). 2) Ensure that the smoke seal brush or blade (where required) is in light contact with the surface that it sweeps or compresses against, in the door fully-closed position. 3) Ensure that the door leaf does no stand proud of the door frame (and not proud of the other leaf of double doors). The door frame rebate does not provide significant protection aga

Obviously floor conditions can be outside the control of the fire door installer. Where the 3mm gap cannot be achieved options are fitting a threshold plate to the floor or fitting a drop-down seal to the door leaf bottom edge. Where fitting of those is not possible then the person or organization that has control of the building and responsibility for fire safety should make a risk based decision about whether the excessive gap is acceptable. If deemed not acceptable then remedial action to the floor condition will be necessary. The following is from ASDMA on BS 8214:

This type of self-closing device is designed to be easier to open. Contact the manufacturer to find out which device is best suited to your needs and that of the flat entrance door as these must be self-closing fire doors with restricted spread of smoke.

Ditto

The guidance you need as follows: BS 8214: 2016 https://shop.bsigroup.com/ProductDetail/?pid=000000000030332501 IFSA https://www.ifsa.org.uk/documents Unless the brush seals are offset, there shouldn't be room for two brush seals as the gap between the edge of the door leaf and the frame should generally be 4mm max and 2mm min. Same for the gap at the meeting edges of double leaf doors. Be careful to ensure that the brush seals do not prevent the doors from self-closing completely. In the door closed position, the brush (or blade) should only lightly contact the surface

If the seal is making a noise when the door opens, it is most likely from the brush or fin that provides restricted smoke spread protection. This type of seal needs to make light contact with the opposing surface in the door closed position so the noise is most likely due to friction. If the noise is a problem you could change the seals (all of them) for fire only seals (below) used in conjunction with the type of smoke/acoustic seal (bottom).

Refer to BS 8214:2016 Timber-based fire door assemblies - Code of practice for details about how to seal the gap.

Is it a new door? Do you have the name of the manufacturer (is there a label on the top edge of the door)?

Possibly repairable depending on extent of damage and of course, so long as the core of the door is not damaged.

Follow guidance BS 8214:2016 Timber-based fire door assemblies - Code of practice https://shop.bsigroup.com/ProductDetail/?pid=000000000030332501 with regard to specification of door assembly components.

Best to check with local authority building control on fire door requirements then. Protected escape route from upper floors, Approved document B (dwellings) Annex C Fire Doors.

Does the work you are doing fall within the scope of Building Regulations?

Yes, timber packers should be used at all fixing points so that you can get a secure fixing. They can be full width of the door frame. Ensure the mineral wool is tightly packed to fill all gaps and leave no voids then cap the wool on both sides with a 10mm depth of intumescent mastic.

Yes, Oak is a good timber for a fire door threshold.

The regulations in terms of access and use of buildings is https://www.planningportal.co.uk/info/200135/approved_documents/80/part_m_-_access_to_and_use_of_buildings I would advise you to check with local authority building control as to whether your proposal is in scope of Part M requirements for dwelling houses. There are limits to how narrow a fire door leaf can be in terms of its fire resistance performance but as you haven't said how narrow the door would be, I cannot comment further.