What can cause sagging of the top plate on a knee wall?

Sagging of the top plate on a knee wall is primarily influenced by the structural integrity and design of the wall assembly. When the top plate is a single plate, it lacks the necessary support and distribution of weight that a double plate design would provide. A double plate, which consists of two parallel pieces of lumber, is more effective at distributing loads, resisting sagging over time due to factors like the weight of the roofing material, live loads (like snow accumulation), and any other forces applied to the wall.

In contrast, if only a single top plate is used, it can become weakened or compromised, leading to sagging, especially if significant loads are applied. This occurs because a single plate does not adequately distribute the weight and loads, making it more susceptible to bending and deflection. Therefore, the structural choice of having a single plate is critical in assessing why sagging may occur in this context.