What fire resistance rating is necessary for fire blocking materials?

Fire blocking materials are designed to restrict the spread of fire and control smoke movement within concealed spaces of a structure. The requirements for these materials do not specify a fire resistance rating, meaning that in many building codes, particularly under residential construction, fire blocking can be made from materials that don't require a specific fire resistance rating at all. Therefore, having a fire resistance rating of "nil" is appropriate for fire blocking materials as long as they effectively serve their intended purpose of blocking drafts and limiting fire spread.

This is because the primary function of fire blocking is to inhibit the passage of flames and hot gases through concealed spaces, such as cavities or gaps in construction, rather than to withstand fire for a specified duration. Many common materials, such as lumber or non-combustible materials, can be used for fire blocking without needing an official fire resistance rating, allowing for flexibility in construction and adherence to code requirements.