steel gurder length and pricing advice

Hi all, here goes another post and look forward to the replys I am after a steel gurder to go from gable to gable in my loft to support the roof purlin as i want to take the struts out to open the loft space up, I have had loads of advice on here and toying with a few things to support it, a stud wall to the new joists is the other option open to me but a bit hesitant on the weight of the roof on the joists and want to experiment with another method. The span of the gurder needs to be roughly 18-20 foot long what depth i am unsure of due to the lack of knowledge on these beams. I am looking for an H shape for the shere strength that it has. Firstly does anyone know what depth i need the gurder to be to support its own weight/length and at what price range am i roughly looking at? secondly as it would be a right faff on getting the straight length up to the loft without having to hire a crane could the length be divided into 2 and bolted together once up there, if this is visable would the strength be reduced due to the join. Thanks in advance Ian

I think you need the services of a structural engineer.

You really do need the advice of an SE for this one, but also bear in mind that the floor will still need beefing up.and the difference between it just supporting the floor and the live load and supporting the purlin is negligable, when compared to using steel for the purlin.
As I said before I do almost exclusively loft conversions and I can tell you from experience that almost without exception purlins are supported off of the floor joists or a steel at floor level, which then carries the new floor joists.
as a rough guess you would probably be looking at 2 £4-500 steels and as I said these would usually be placed at floor level with a purlin wall built up off of them.

Depends on how much floor and roof load is going on it, but for that kind of span, you're most likely looking in the 203UC range, probably 52 or 60kg/m. It can be divided, not advisable in the middle though, better as 1/3: 2/3 split. If the joint is designed correctly, then it's as strong as the uncut beam; if you use normal bolts, though, there's a little bit of slip to take up, increases the deflection slightly.