Unfortunately I only have the AWS D1.1, I see that D1.3 relates to sheet metal welding and requirements. I will have to further research. Maybe AWS D1.1 will address issue.
Are you looking for design strength of the weld, or details and requirements for the weld, or something else? If you want design strength of the puddle weld itself, you can calculate it using the AISI cold-formed steel code.
zennis...interesting definitional issue. Welding position is usually the weld electrode relative to the piece. In this case, since your roof slope is greater than 45 degrees, a "flat" position weld could only be done on certain features, and the deck would not be one of them.
If you are welding sheet metal, the welding and procedures should conform to AWS D1.3, Structural Welding Code-Sheet Steel. The flat position is pre-qualified and must have a 1/32 inch concavity on the weld. If you want to do otherwise, it would require qualifying a procedure first, then qualifying welders to that procedure.
It would be easier in this case to use mechanical fastening.