A.Jordan Truesdell, a structural engineer in Blacksburg, Va., responds: In a simple gable roof, the rafters carry live and dead loads that push both down and out against the top of the supporting walls. This horizontal load — or thrust — can be considerable, especially on a low-pitched roof. To resist thrust, the IRC calls for a structural ridge (required for any roof with a pitch less than 3/12) or for each pair of rafters to be securely connected to each other by a continuous ceiling joist (R802.3, 2006 IRC).

Code does allow joists to be installed above the top plate, but only under certain conditions. Previous building codes permitted rafter ties to be placed as high above the plate as two-thirds the distance between the top plate and the ridge, but the 2006 IRC now limits this height to one-third the distance between the plate and the ridge (see...

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