Which intumescent strip size does your fire door need? Most 30-minute doors take 15mm x 4mm and most 60-minute doors take 20mm x 4mm or two 10mm strips, but the door maker decides. This guide explains fire-only versus fire and smoke strips, the 3mm gap rule, how to fit the strip in the frame, and a costed eight-door HMO example using real Astroflame prices.