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I want to support a 20 ft. span of my roof where two roofs come together. My house is was originally 46'by 20'. A 40'by 12' addition was added, making an L shape, and when they put the beam in they put 2 supports in middle of room that I want to elimanate.

Roof is 2x6 rafters. From top of wall to peak is 32". Was wondering what kind and how big of beam I should put under old beam and be able to support on just the 2 ends, not in the middle.

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Do NOT attempt to DIY this. Employ a qualified structural engineer. (This is more or less @keshlam's answer, but I wanted to state it again. Strongly.) – AndyT Feb 12 at 15:37

You need an engineer's advice on what the load is and what is required to support it.

And no, you really can't make a mixed materials i-beam.

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But you can make a mixed materials I-Beam. It's unlikely to be necessary or cost effective for a domestic roof, but it is most definitely possible. Bridge decks are often steel beams with a concrete slab, that concrete slab effectively acts as the top flange of the composite beam. – AndyT Feb 12 at 15:31
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I think that within the context of this question, you can't. You can not practically equate a composite bridge deck with a small lintel in someone's house. There are engineered wooden rafters but they're only rafters, not beams. You'd need professional advice anyway in selecting even these though, For the application in question, he'll be looking at a steel beam. – Paul Uszak Feb 13 at 1:15
    
Engineered wood beams exist - a large opening in one of my load bearing walls is spanned by two in parallel with supports at each end transferring loaf straight to foundation. Hightech cousin of plywood. There are wooden I-beams for other kinds of load, but again the engineering to make that work is nontrivial and I don't see any good way to transfer the stresses if you tried to go mixed material. – keshlam Feb 13 at 1:27
    
The key point is that you need to know what the stresses are and how much safety margin to design in... and that takes engineering training to do properly. – keshlam Feb 13 at 1:29
    
@keshlam Interesting. What are your beams supporting and what is the span? We don't really go in for this in the UK - we have no trees. – Paul Uszak Feb 13 at 1:45

Span tables are available online for any material just google around. Like here.

It is not clear by your question what distance you wish to span.

The other thing is, there are different requirements depending on where you live and what, if any, the snow loading will be and the material of the roof. Roof support requirements are different in California than Montana and if you have cement or slate roofing versus asphalt or steel.

Most companies that engineering roof trusses have software that can calculate all that for you if you are having them bid the roof package or LVL's.

Good luck with your project!

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I wouldn't bet on a calculation by span alone unless you are certain that isn't going to be loadbearing. – keshlam Feb 13 at 1:34

Whilst this is not an full answer, it's to give you some ballpark estimates.

Your (steel) beam will have to be approximately 9 times the strength of the one you have at the moment. This is based on a 6m span compared to 3 no. individual 2m spans. The supports will have to carry 3 times what they carry presently, so you might have to consider the bearing capacity of your walls. It's common to build small brick piers out from existing walls, but for a wooden house you might need some more steel as vertical columns. Do you Yanks do these things in wood? You might get away with some form of wooden post or lattice. It'll also be significantly deeper than the existing one, so can you stand that?

Without a detailed drawing of your current and proposed arrangements, we cannot provide a more detailed answer. You don't need a structural engineer actually, as such lintels are very simple and an experienced builder should be able to size them. He'll have to visit you though.

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