Wooden beam bowing and cracked. How serious is this?

Hi mac Bow alone is not as serious , many purlins " beams running horizontally " bow a little in old homes . They even tend to have horizontal cracks ( shakes ) that rant that bad BUT a four inch crack vertical is very serious . There are a number of ways of sorting it . 1 flitch plate ! this can be a piece of steel or sometimes ply bolted to the side and sandwiched between another large piece of wood / purlin. 2 replace with steel . probably the best way as these days its virtually impossible to get a structural calculation for a timber purlin . 3 for a quick immediate fix please put a spreader plate on the floor 8"x2" 6 ft long and prop the purlin up asap before whom ever put the dormer on cause you a massive problem . and if i was you ring a loft convertor , post it on here or structural engineers asap . thanks woody at apex lofts barnsley

need to fit a steel beam underneath this beam and fit stud wall up to faulty beam to transfer weight down on to beam will need a structural engineer to calculate beam