Subtreads refer to the blocking mounted above or between the stringers to support the decking used for stair treads.
Subtreads can be installed to add additional support for treads on stair stringers, where stringers are set further apart than code or testing allows. For example, where stringers are set 14″ apart, subtreads can be installed beneath Azek decking treads to completely support the decking, since Azek decking mounted directly to the stringers is not allowed to have the supporting stringers to be more than 12″ on-center.
