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Firestop has been tagged to identify certification information, thus enabling proper maintenance.
Packing with inherently fire-resistive materials, such as rockwool or ceramic fibre is intended to protect sealants that would, on their own, be consumed by the fire. Lesser packing, such as foam backer rod or fibreglass are used simply to hold up materials that can survive fire testing on their own. In both cases, the packing is placed in such a way as to control the exact depth of the materials that top off the seal.
Proper packing is particularly important with self-leveling sealants. If the packing is not tight enough, the expensive caulking will drip out of the hole. If the packing surface is not straight enough, the thickness of the caulking will be wrong.
If firestopping takes place on plumbing pipes that have not yet been tested, packing can move the pipes and interfere with the plumbing. It is thus important for the work to be co-ordinated between plumbers and firestoppers.