Prescriptive codes are usually based on historical usage, good experience with reasonable success, engineering design and testing. But, they also tend to be somewhat conservative in their intent and approach. This is because the code writers understand that these codes will be used, applied and interpreted (or maybe misinterpreted) by a bunch of people (builders?) who probably don’t have much engineering knowledge and experience as part of their basis for their interpretation or misinterpretation of the needs and intents of that code. Usually, a well engineered solution will involve lighter members, fewer connectors, longer spans, etc., or alternatively, that the prescriptive solution can carry more load than the basic code tabulation or approach would indicate or allow. Prescriptive codes usually imply that you can build this particular element or structure, this way, reasonably safety, as long as it all falls within these bounds of loading, span length, etc., etc. There are usually a number of engineering considerations which the prescriptive codes brush over rather lightly. One example is that the rim board must be well fastened (nailed) to the top pls. of the wall below, and to the fl. sheathing above, or it can be pulled, ripped, right off the bldg. The upshot here, is that the lateral and vert. loads from the deck or new roof must be adequately taken back into the fl. diaphragm, or bldg. structure beyond/behind. These types of ledgers should not be applied over siding or foam insulation, and the like, but rather let-in to bear on the bldg. wall sheathing, or the rim board directly. And, great care is needed to detail and provide adequate ledger flashing tied into a proper weather, vapor, water plane system.