Expert: Dan Griffin - 7/2/2008

Question
I am converting my carport into a garage and will be using a 20' wide by 7' high steel door (ceiling height is 8').  I have been told to use 2 - 2X12 beams w/ 1/2 inch plywd. between and that the beam should be approx. 20'-6' to span the entire garage door opening.  In order to get a beam that long, I would have to special order it and was told it would be expensive and difficult to handle due to the weight.  Are there any other alternatives you would recommend that would be less expensive/heavy (without compromising the strength of the header)?  Do I need to use a header that is long enough to span the entire opening of 20' or is there another alternative?  Thanks!

Answer
Harold,  there is no substitute for a continuous header for the size opening you create.  Alternatives:
Use a normal 16' garage door, I would consider a 20' door unusual.
Use two smaller doors with a load bearing narrow wall section between the doors.

There are variables about what the building is made of, what loads are imposed on the header, etc.  What has been carrying your carport?

I don't think your 2each 2x12 with plwood will work, I'm pretty sure it will sag noticeably within 5 years of its own weight, sooner if carrying a load.  16' headers of your design are prone to sagging.
Header materials to consider:
LVL
flat or gabled truss
flitch beam
steel beam
boxed beam
boxed metal stud beam would be excellent if you are ot carrying a major load

The bigger issue will be controlling shear on the face with the door(s) in it.  How are you planning to wind brace that face?  Will there be an inspector on this project?