Abstract: A method is invented of sealing through penetrations in fire rated barriers (walls or floors/ceilings), caused by metallic pipes, metallic conduits, metallic cable trays with cables inside, cables, metallic ducts and electric busways, by placing an external firestop seal comprising of cementitious liquid material, which upon curing becomes rigid and heat absorbing. In order to obtain the same fire rating with the same rigid, cementitious fire stop material for an external seal as compared with an internal seal, the total depth of the external seal, on both sides of the fire barrier, was determined to be at most the same as the depth of the internal seal.