R311.7.5.4

Section R311.7.5.4 is titled 'Exterior plastic composite treads' and covers the requirements for exterior treads made of plastic composite materials. The purpose of this section is to clarify that exterior treads of plastic composite material are intended to follow the requirements of this section of the code, and that they must also adhere to the cross-referenced section. The reason for this clarity is because plastic composite materials come in many different molded configurations, which can make it a bit more challenging to design and build stairs, (especially where rip cuts have to be made in the planks), and installers might assume that they don't have to adhere to the rules of this section when using plastic composite materials.

The model shows a few short sections of plastic composite planks which are mostly hollow with only a few small vertical supports between the top and bottom. If a plank had to be rip cut near the edge of a hollow section, that edge would be weak and it couldn't be used as the nosing of a tread, as an example.