If the edges meet over the floor joist, what subfloor material is recommended?

The recommended subfloor material, in this case, is plywood. Plywood is favored because it provides substantial support and structural integrity when installed with edges meeting over floor joists. This orientation helps distribute weight evenly, reducing the risk of sagging or failing underload.

Plywood offers several advantages over other materials. It is known for its strength and durability, which are particularly important in construction where the subfloor must withstand various stresses. Plywood also has excellent moisture resistance compared to materials like particle board, which can swell and deteriorate in humid conditions. Furthermore, plywood's layered construction gives it flexibility along with rigidity, making it less likely to crack or warp.

The other materials listed have limitations that make them less suitable for this application. For example, while concrete is strong, it is typically used in specific applications such as slabs rather than as a subfloor material. Particle board, although often cheaper, is less durable and more susceptible to moisture damage. Lastly, the term "3S3grinders" does not refer to a recognized subfloor material and is not applicable in this context. Overall, plywood stands out as the best choice for ensuring a strong, reliable subfloor when edges meet over joists.