how to stop a barn door from twisting a 2x8 header

Question

I have two barn doors covering an 18' foot opening. The opening has wooden 4x4 at each end, and one in the middle. There are two spans of 2x8, 9' long as a header at the top of the 4x4s.

After hanging the ~9ft barn doors which weight about 100lbs each, the middle of each 2x8 is starting to bow out in the middle, at the top of the header where the barn door hardware / railing is attached.

It seems the weight of the door on the railing is causing too much torsion on the 2x8 header causing the bowing.

To rectify this I was wondering if anyone had thoughts on the best / cheapest course of action. Some ideas we were considering:

1. Add a 2x4 bracing on the inside of the header to the next parallel member approximately 3ft away. The logic is this will pull the header inwards to counter the force of the barn door pulling the header outward. The problem is putting a screw through the header to the end of the 2x4. The 3.5inch #10 screws did not grab enough into the 2x4, and simply drilled out a hole. I was hoping there was some metal bracketing we could attach instead between the 2x4 and header, though haven't found a suitable piece yet.

2. A stronger header. I thought about doubling (or tripling) the 2x8 header boards. this is guess work though, I looked around and could not easily figure out the torsional strength of a 2x8 to figure out what was needed. Over time it seems an uncountered load (e.g. from 1) will eventually warp it. Perhaps there is a metal beam which could work, though this is pricey and heavy.

3. reinforce the header with a metal L beam. I was looking into a long strip of 1.5" x 1.5" L beam and screwing it to the top of the beam. However, the bowing is slight, and I'm not sure the L beam is going to remain rigid enough to counter the load.

Other ideas, or thoughts on what we could do? It's for an old bike shed, so we would like to minimize the costs if possible.

Thanks for your thoughts and insights!

Answer 1

If I understand correctly, your single thickness 2x8 header is sitting on a 4x4 post at each end (addressing each door's header as a single entity, not the entire 18' double-door span).

This means that you've got a 1.5" thick piece of wood (the nominal dimension of a 2x8) resting on a 3.5" thick post.

Normal residential header construction requires that the 2x material be doubled with a 0.5" piece of plywood or OSB sandwiched in between. This will give you a 3.5" thick header (1.5" + 1.5" + 0.5") to sit on your 3.5" wide post. This gives you better bearing on the post and, more importantly, significantly more strength in the header.

You don't indicate if the barn is build with residential-style 16" OC stud walls (unlikely) or is post-and-beam construction (much more likely). If it's post-an-beam, you probably don't need the load-bearing capacity of the doubled header, but, obviously you need it for torsional strength.

• Purchase a single 4x8 sheet of 1/2" plywood or OSB
• Purchase 2 @ 2x8 x 10'
• Rip 3 strips out of the plywood/OSB, each 8' long by 7.5" wide (the nominal width of your 2x8)
• Put one full length strip of sheathing against the existing header board, tight to one end of the header
• Cut the 3rd strip into the appropriate lengths to cover the rest of the header (approximately 12" - measure to get the exact dimension)
  • Cut each to the precise length necessary to match each of the existing headers
• Push each new 2x8 into the pocket above the door it was measured for
• Nail thoroughly
  • 2 rows of 3" 16d nails, spaced 6-8" horizontally should be more than adequate