I found the 10"x6" header over my garage door has rot in the center of it over the garage door, I want to sister 7 2x12's under it and also sister 3 2x10's to it to go under the floor joists in the garage that is below my kitchen and dining room! I then want to support the new header with 2 2x12's under the new 2x12 header and use 2 2x10's under the 2x10 portion of the header. is it possible to get 16"-17" lag bolts to secure the new header together? I will be cutting drilling and pre-fitting each board for 1/2" lag bolts and gluing each board after cutting them to fit, also what diameter lag should I use 1/2? and how many? to make this work? I would just replace the header but it is one solid beam running the width of my home about 35' and this would be a major project that I cannot afford, so my plan it to over build and auxiliary header to take the load! as far as code this is unlikely going to be a issue were I live the town really does not care I just want to make sure the house is safe anyone have any ideas for this project? I can attach photos of the area in need of help if needed.
Thank You
Ken

Pictures would help - it sounds like you have some of your framing sizes and terms out of wack.

Sistering is joining a new structural framing piece alongside the existing. If your current beam is a 6x12, then you really don't need to go larger than that. 7 - 2x12's making up a 10-1/2" x 11-1/2" beam is a bit of overkill. For cost and size, you'd be far better to sister a steel plate - something that's 1/2" x 10". The thin steel will work much better than your planned 10" thick addition. Plus it would require shorter bolts that could be 1/2" in diameter. Going to the extreme thickness of all those 2x12's will require some massive bolts - probably 3/4" minimum threaded rod.

It would be money well spent to get someone with structural experience to come in and give you some advice. Even if you choose to do the work yourself if you do it correctly without needless overbuild you will likely save the money it may cost to get an experts recommendation.

I'll keep my comments in this thread but I've seen the pics in the other.

It looks like you have room to add another header directly under the existing. 7 - 2x12's is still overkill and not needed. The other issue is that all your joists are attached to this now rotting existing header. Also consider that any header you add will need to have support going all the way to the slab on either end - or some very strong brackets bolted to the existing jacks on each side.

If it were me, I would:

1. Remove that horizontal 2x4 that is under the existing header.
2. Have a metal fab shop make you up a steel plate - 1/4" thick and sized so that it can be screwed to the underside of the header (flat) and have it's outer edge project under the leading edge of each joist by about 2".
3. Install a new header directly under the existing one, with the steel plate sandwiched in-between. Header size needs to be at least the same as the existing.
4. Triple 2x jacks on either side to support the new header or a thru-bolted heave gauge steel bracket / plate combination.
5. Sister each joist with same size as existing. New joist ends should rest on the edge of the steel plate.

An alternative method would be to build a temp. support for the joists and remove the existing header completely. Then replace with similar. This would cost you less in materials as you could use joist hangers to support all the new and existing joists but there would be a lot of complicated demolition to get the header out without causing other damage. Still, it would be nice to remove the rotted wood and start fresh.

I would also like to replace the header but it runs the length of the house, the part of the header you see is only 1/4 of it's full length, short story on the house it was only one floor when is was built as a carriage house pre 1900 the original floor is now the basement and garage, it now has 2 floors and a small attic on top of the original structure, the back and sides are a field stone foundation the front it about 16" brick then wood up from there. I attached a pic of the side of the house were the garage door is and showing the length of the house.

I uploaded the pic to the existing album I am still trying to figure out how to use this forum sorry