Which of the following would not contribute to rafter spread?

The choice indicating that adequate knee walls would not contribute to rafter spread is accurate because knee walls provide additional vertical support at the eaves of a roof. These walls effectively help stabilize the rafters and prevent them from spreading apart under lateral loads. By placing vertical load-bearing structures near the rafter's base, they resist the outward force that could cause rafter spread, which typically occurs when the roof structure experiences excessive outward pressure from snow loads, wind, or the weight of the roofing materials.

In contrast, the other factors listed can lead to rafter spread. The lack of collar ties can allow the rafters to move apart without any lateral restraint, creating a risk for structural failure. Over-spanned rafters are more prone to flexing and bending under load, which can also lead to spread. Similarly, having too many layers of roof coverings adds weight to the rafters, increasing the potential for them to spread if there are no supporting features like knee walls or collar ties in place to counteract this added load.