Installing 8" hardwood flooring parallel to floor joists that are only 13" apart can potentially lead to issues with the stability and durability of the flooring. In general, it is not recommended to install wide hardwood planks parallel to the floor joists with such a narrow spacing. Doing so may result in the following problems:
Sagging and Bouncing: The flooring might not have enough support, leading to sagging or bouncing when walked upon, especially in the areas between the joists.
Cupping and Buckling: Wide planks are more prone to cupping and buckling due to changes in moisture levels. Installing them perpendicular to the floor joists provides more stability and helps prevent these issues.
Floor Noise: With insufficient support, the flooring may make creaking or snapping noises when weight is applied.
To avoid these problems, it's generally recommended to install wide hardwood planks perpendicular to the floor joists, as it provides better support and stability for the flooring. If you want to maintain the same flooring throughout the space where the joists run in different directions, you could consider using a transition piece or a threshold to separate the two areas with different plank orientations.
If you are set on installing the 8" hardwood flooring parallel to the joists, it would be wise to consult with a professional contractor or a structural engineer to assess the specific situation and ensure proper reinforcement and support are provided to prevent potential issues in the future. Additionally, the contractor can recommend appropriate measures to minimize any negative effects of installing wide planks in this configuration. Safety, stability, and durability should always be a priority when making such decisions about flooring installations.