replacing a structural span
 

replacing a structural span
i am looking at moving a span header into the ceiling in my home. the span only supports an attic (with no storage) in a small ca. 1900 home. i would like to support the ceiling joists while i install a new header over top, cut the joists and tie them to the new header using joist hangers. i will eventually have a more structurally competent engineer in to view the design, but is there a good rule of thumb for redesigning a header.
first, is the current header size adequate for an equivalent span if the joists are hung, as opposed to overlapped on top? second, i have heard that there is a general rule of 1 inch of header depth for one foot of span width - is that generally the case?
we don't use rules of thumb. we use formulas. it is actually a violation of registration laws to provide specific designs (i.e., tell you you need a 4x10) over the internet, or in this forum. it is legal only to hire a professional engineer licensed in your state.
i can answer your general query - it is ok to hang the joists, given the spans or loads are not changed.