I have a Concret patio with a wood frame structure over it. It is 20'by 20' with a 4" hang over. I would like to put a metal cover on it also remove the post in the middle of the room. Can it be done with metal beams and side's I thought it would be less weight if done this way. It wood be a span of 20ft. Can I get beams that long if so where can the materials be purchased.with the post in the middle it is useless and too hot with a cover. Big problem
Local building codes in your area will dictate what kind of beam (material/size) you will need to span that distance. Same goes for where to source the beam.
Also depending on where you live you may need to factor in snow-load if you are adding a roof to a structure that currently doesn't have one (e.g. pergola).