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This question is in regard to interpretation of IRC:2012 R302.11 for a single family dwelling.

It is clear that junctions between vertical and horizontal framing need to be fireblocked (e.g. between a wall cavity and the bottom of an interior soffit). However, I am less clear on fireblocking requirements for junctions between what could be considered two separate horizontal concealed spaces that run perpendicular to one another (e.g. between floor joists and the top of an interior soffit that runs perpendicular to those floor joists). Please see the diagram in this answer for a visual depiction of the example described: https://diy.stackexchange.com/a/66942/36011.

This question is with regard to interpretation of the statement I have put in bold from R302.11 of IRC:2012:

R302.11 Fireblocking.

In combustible construction, fireblocking shall be provided to cut off all concealed draft openings (both vertical and horizontal) and to form an effective fire barrier between stories, and between a top story and the roof space.

Fireblocking shall be provided in wood-frame construction in the following locations:

  1. In concealed spaces of stud walls and partitions, including furred spaces and parallel rows of studs or staggered studs, as follows:

1.1. Vertically at the ceiling and floor levels.

1.2. Horizontally at intervals not exceeding 10 feet (3048 mm).

2. At all interconnections between concealed vertical and horizontal spaces such as occur at soffits, drop ceilings and cove ceilings.

  1. In concealed spaces between stair stringers at the top and bottom of the run. Enclosed spaces under stairs shall comply with Section R302.7.

  2. At openings around vents, pipes, ducts, cables and wires at ceiling and floor level, with an approved material to resist the free passage of flame and products of combustion. The material filling this annular space shall not be required to meet the ASTM E 136 requirements.

  3. For the fireblocking of chimneys and fireplaces, see Section R1003.19.

  4. Fireblocking of cornices of a two-family dwelling is required at the line of dwelling unit separation.

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    Item 1.2 might be the only applicable requirement for horizontal cavities. ...if the soffit and joist cavities combined add up to more than 10ft – Paul May 31 '15 at 14:54

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