Expert: Dan Griffin - 3/12/2008

Question
I have a garage door header that is sagging about 3/4". It's a 15 Ft span. The header is made of two 2x12's. No fitch plate. I've got the header exposed because I am replacing the siding on the house. The home is 55 years old. The header shows no signs of water damage. I attempted jacking it up and it goes fairly easily. My plan is to jack up the beam (slightly overjacking to compensate for settling)and bolt a steel fitch plate onto the header. I discussed the problem with a structural steel place that sells fitch plates. He took dimensions from my house plans and calculated that I need a 1/2" thick plate. I asked if I can use a 1/4" and he said I have to tell you to go with the 1/2". Is this overkill? I know he over engineered it a bit. I can get you garage dimensions and other details if you need more.

Answer
This is truly a structural engineer question.

This company makes and sells flitch beam girders and headers:
http://www.betterheader.com/

To span 16 feet they require a type 21 beam:
http://www.betterheader.com/beamlegends.asp

This beam is made from 2 each 2x12 with 1/2 plate.  You will also notice their use of grade 5 bolts.

You did not indicate whether this thing was carrying roof live loads or other contributory loads.  Many garage headers are carrying roof rafters, second floor and roof, or second floor only.

Unless you have a strong need for a flat plate, you could probably come out better for purchase price, weight, and carrying strength by using a Cchannel with its back laid flat against the header.  A good steel man should be able to help you.