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I am not an engineer and I am using a small beam calculator for this information, so use at your own risk.
first to figure a beam one should have some idea of the weight it needs to carry, and the possible additional loads, such as snow loads and such, so with out that information all one is doing is guessing, also one should know the allowable deflection that is acceptable after loading,
But if that is a large enough beam for your needs I can not answer, as there is way to much information missing to say yes or no to it, as there is nothing even suggesting as to what it holding up, (my guess is it a roof deck) but how much, what is your snow loading in your area, type of roofing, will there be a ceiling and so on,
I have a little slide rule calculator and using it, an 18 foot span, beam made 4x10 Douglas fir lumber with a deflection of 1/360 would be able to have about 88 pounds per foot of capacity,
a lesser strength species beam of 4x12 on 18 foot span, my calculator says slightly over 100 pounds of evenly loaded per foot, of beam, same 1/360 deflection,
if it was the same in Douglas fir (the calculator has larch in the same line as there are two Douglas firs on the calculator the other is Douglas fir -south, which would about in the middle of the two examples,)
would have a load rating 4x12, 18' for about 158 pounds per foot of span, same deflection,
also my calculator is stating a 4x10 or a 4x12, not necessarily 2 2"x10" or 2 2"x12"
I would suggest you contact your local building inspector and see what he would approve before you start,
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Thank for both answers. Here is more information. Existing deteriorated back yard porch, only roof, not snow (Arizona). Rafters 8" - 24" on center. 20' large by 11' deep. Concrete slab with solid footers. Would like to eliminate the center post for viewing purposes using 20' beam with two post, each one with an 1' overhang and clear/spam 18'. Will check and reinforce footers if necessary. I wouldn't like steel, can I use a 4 x 12 x 20 beam? We only find in stock here Douglas fir #2
Ain't no engineer only reference I ever used was BOCA which I hear is no longer in use. But hear is my two cents. Header or beam consists of 2 peices of dimensional with 1/2 ply core. 2X10 header is good only for 16 feet 2X12 doubled, 1/2 ply core crowned up will carry it but please use sense I have seen 2 2X12,s splintered and knotty used over a 20' garage door and had to cut them out and replace. A glue lam (paralam) beam is better and should be reasonable this a standard size.
Don't use 2x10s. An opening that size requires special lumber.
It would be worth getting you hands on a copy of the building code for your town/state. Codes are there for safety not inconvenience.
I am pretty sure you can get what you need out of an LVL (laminated veneer lumber) without having to use a steel beam. The only issue I can see is that the size you would need for the span is going to be heavy. Might want to think logistically how you would install it, unless you have a machine available to lift that kind of weight. You are probably looking the the range of a couple hunderd pounds.
Reference the attached spec sheet from GP. You will get an idea of what they suggest you install based on the rough opening, roof pitch and snow load.
More info here http://www.gp.com/build/product.aspx?pid=1392 . Look through the 3rd PDF file on that link, it gives complete details on using their lumber. The first and second PDF files will give you lbs/ft specs for specific LVL's.
I am not an engineer so use this data at your own risk. If you do this work without the suggestions of a licensed engineer I would err on the side of putting in lumber that is oversized rather than undersized.
The general consensus is to contact an engineer or architect. No one wants to commit to providing what they think is an accurate answer, to such a project in case it fails. I will say that my garage has 3 2x12's as a header for a 16' garage door. I live in the north and snow loads are a big factor. I would suspect you'll need a steel beam (as already suggested) or an engineered laminated wood beam.
If the span is only 20' and u are coming in a foot on both ends, then u should use 2 lvl lam. beams 1.75" thick and 14"high. They should be glued together and nailed every 12" on both sides. I have worked with engineered wood for a long time and it is great stuff. Hope this helps u out good luck.
In my township the local building department has plan inspectors and they have been more that helpfull with my past building projects, even going so far as to run my data thru a span calulation program and telling me what size beams to use.
When you apply for a permit they calculate these things anyways to make sure your plans are up to snuff.
They charge nothing.