When are washers required for snug tightened joints?

Washers are required for snug tightened joints under specific conditions to ensure the integrity and performance of the connection. In the context of the question, washers are necessary when the connection features a slotted hole in the outer ply, as this helps distribute the loading more evenly and prevents damage to the material at the interface. Furthermore, when the outer face of the joint has a slope greater than 1:20, washers are also required. This slope can lead to an uneven distribution of forces through the joint, and using washers provides a wider bearing surface that can accommodate these conditions, enhancing the overall stability of the connection.

Thus, the correct answer encompasses both scenarios: situations with slotted holes and those with a slope greater than the specified ratio. This comprehensive approach ensures that the structural integrity is maintained under various conditions that can affect the snugness of the connection.