Unfortunately you can't do this yourself.
I suggest that you get a structural engineer to size this beam. He'll be able to eye ball it in minutes if he's any good. To get a building control calculation, he'll need to spend an hour. If you're lucky, a steel supplier might offer a service that does this kinda stuff if you promise to buy the beam. Unless head height is extremely important ,do not go for a column (UC) section as that will cost a disproportionate amount extra. You can double up the beams (we don't typically do this though), but it complicates the calculation even further so don't attempt this yourself (or this forum, unless you're paying someone).
Most of the numbers in this thread are inaccurate or just plain wrong. If you're going to make important project decisions based on this advice, you'll regret it later and have to pay for it. Or your ceiling will crack. Don't try to use an on line calculator for this as I'm not aware of any that can size a beam for free. Just working out some plywood loadings won't get you very far. I'm not being nasty - I have some experience of this stuff.
It's a bit complicated to design steel work. You can probably short cut the point loads onto the beam and just assume a uniform loading, but the simplifications will have to stop there. All the other stuff is harder. Your beam will be sized according to other criteria than just load. You will need to calculate the deflection (as this might be the deciding factor). You will need to consider how the roof joists connect onto the beam as that affects the beam size /strength. Also, don't forget what's holding up the beam. And it depend on what type of roof it is too.
For estimation, go with what the people here are telling you but don't try to refine it any further and allow plenty of room for a deeper beam.
PS. For what its worth, don't forget that roofs often fail by being sucked off, not falling down...
Last edited:
