BS 9999: 2008
“Fire precautions in the design and construction of buildings”.
BS 9999: 2008 is a British Standard, and an Approved Document for compliance with the Building Regulations (England & Wales).
This Document addresses the need for sealing the threshold of a fire resistant door assembly, stating that if the gap at the threshold exceeds 3mm, then a threshold seal must be used.
You may also find our FAQ on threshold sealing helpful (Fire, Smoke and Weather Sealing for Door Assemblies section).
This link to British Standards Online may be of interest.
Relevant Lorient links: