I have an enclosed back porch with a concrete block foundation that must be replaced. It`s foundation is separate from the main home`s foundation as it ajoins at the ends where the main foundation exists and accessed with a door cut out from main foundation. The porch is 8 feet wide by 18 feet long, single story with pitched roof,asphalt shingles containing nothing more than a deep freezer, closet spaces. If I assume the structure`s weight as 50 lbs per square feet (?) what size steel I Beam is safely adequate to use to support the weight of the porch while I remove the blocks and install solid form concrete walls? W6X20, W8X28?