BS 8214:1990 ( Incorporating Amendment No. 1) Code of practice for Fire door assemblies with non-metallic leaves states that:- 13.6 Threshold sealing Fire test experience has shown that intumescent seals do not have to be fitted across the bottom of door leaves. The pressure conditions evolved during a fully developed fire are the main reason for this. The test given in BS 476-31.1 is designed to measure ambient temperature air or smoke leakage, and test conditions are such that the entire face of a doorset under test will be under positive pressure. However, for test purposes, the threshold gap is taped and some form of effective threshold seal should be used if complete smoke control is to be achieved, unless the threshold gap does not exceed 3 mm at any point. BS 8214:2008 Code of practice for fire door assemblies states that:- 9.5.3 Under-door (threshold) gaps Under-door (threshold) gaps should be in accordance with the manufacturer’s installation instructions for the particular doorset design. When fitted, smoke seals should give an even contact with the floor but should not exhibit significant increased frictional forces that could interfere with the closing action of the door (see BS 5588-11)). 12.3 Smoke seals NOTE BS 5588-11 recommends that when installed, the threshold gap should where practicable be sealed by a (flexible edge) seal either with a leakage rate not exceeding 3 m3/h per metre at 25 Pa or just contacting the floor. Where this is impracticable, the threshold gap should not exceed 3 mm at any point. The seal manufacturer’s recommended installation instructions should be followed whether the intumescent and smoke seals are separate or combined.