What problem is associated with running the weave direction parallel to the step in stair carpeting?

The issue of having the weave direction of stair carpeting run parallel to the step primarily leads to the "smile effect." This phenomenon occurs when the carpet lays flat on the step but begins to curve upwards at the edges, resembling a smile, particularly when the step is subjected to foot traffic. When the carpet is installed with the weave direction parallel to the step, the tension caused by stepping on the carpet can exacerbate this effect, making it visually unappealing and potentially compromising the integrity of the installation.

In addition to aesthetic concerns, the smile effect can create safety issues, as the raised edges may snag on shoes and increase the risk of slips and falls. Therefore, proper directionality in carpet installation is essential for both functional and visual reasons.