A. The most common reason for installing collar ties is to prevent rafters from spreading apart under load. However, in a conventionally framed peaked roof, like the kind you describe, collar ties would probably serve little or no function, since the attic floor joists serve as ties to prevent the rafters from spreading. Note that the connections between the rafters and the joists must be adequate, and that the overlapping joists at midspan must also be properly nailed (see Practical Engineering, 5/96).
There are some exceptions, however, when collar ties might be useful even in a conventional attic roof. For example, very long rafters in a relatively steeply pitched roof (slopes above 6/12, for instance) may benefit from a stabilizing effect if adequately connected collar ties are installed on every rafter pair. In this case, the collars serve...
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