Definition
Seen when the home has open tail rafters, a bird block is a wood or manufactured blocking member installed between rafters or roof trusses at the top of an exterior wall near the eaves. It closes the space between adjacent framing members and most often contains screen provides protected ventilation openings.
Explanation
Bird blocks are commonly fitted between the roof-framing members where they pass over an exterior wall. Without blocking, the spaces between rafters or truss heels can provide an opening from the exterior into the attic.
A bird block may be solid or may contain ventilation openings. Vented versions typically incorporate holes or openings with screening intended to permit airflow while limiting entry by birds and other pests. Construction drawings sometimes identify these components as “bird blocking” and specify their ventilation capacity as part of an attic-ventilation design. Source: Kirkland Permits
Bird blocking can also have a structural function when it is incorporated into the roof or wall-framing design. Depending on the construction, blocking at the truss or rafter heels can provide support for sheathing edges or contribute to load transfer. Its structural role should therefore be distinguished from its use simply as an eave closure or vent.
The term should not be confused with a bird stop used with tile or profiled roofing. A bird stop generally closes the shaped openings beneath roofing material at an eave or ridge, while bird blocking refers to blocking installed between roof-framing members. Source: Interlock Metal Roofing
Why It Matters
Bird blocks help close otherwise open areas at the roof perimeter. Where the blocks are part of the attic-ventilation system, their openings must remain capable of providing the intended airflow while limiting pest entry. Missing, damaged, obstructed, or improperly altered blocking can affect either of those functions.
Example Usage
I observed deteriorated screening at several bird blocks that could allow pest entry into the attic. The affected areas should be repaired while maintaining the intended attic ventilation.