RSJ - advice on choosing size pls

I have a car port which is about 4m wide and has 4.5x3" and 5x2.5" wooden beams every foot or so, topped with a board+felt flat roof. Over time (and with the extra weight of the newish roof) one of the 4.5" beams has a sag such that one of the full-height gates into the carport now fouls on it. So I'm wondering about what size RSJ etc I could use to replace it? It'll need to be as strong as or stronger than the timber, but a bit of deflection would still be ok if it were, say, only 3" in depth - as long as there's more clearance for the door. Oh, and how heavy would it be roughly? All advice welcomed! Thanks. Colin
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You could fix a length of angle steel to the beam. That will support a lot of weight as the wood stops it twisting.
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I did wonder about that as an option. There's not going to be enough room to 'wrap' the angle around the underside of the beam, and I wasn't sure how well it would attach alongside (i.e. sticking out to the side) - would need bolting at pretty regular intervals I imagine.

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