Sheet Metal Screws vs. Wood Screws

I will second the use of well nuts, as they were designed for this type of application. It is best practice to fasten into a rafter, if possible. Use a stud finder to locate where they are. Use dicor around the well nut to seal against the wood (redundant as the rubber will seal, but for secondary backup). Use a poly butyl tape for under the bracket and tighten so that it squeezes out. Complete the job with dicor sealant over the bolt head and around the (trimmed excess of butyl tape) outside. Make the sealant rounded over, so that there are no sharp edges, it functions better that way. JMO