Reinforcing existing header

carpentry

My house has an existing header, which I believe is somewhat undersized. For context, it's two-ply 2x10s, 12ft long (but I don't want to get into the sizing calculation here).

I would like to reinforce the header by putting two braces at each end. These would be pieces of 4×4, about 3ft long, which would be attached at a 45 degree angle at each end of the header, going between the jack studs and the header (attaching to each roughly 2ft from the upper corner).

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Questions:

  1. What is the name of a brace like this ? (I can't seem to google something comparable).

  2. What becomes the effective span of the reinforced header ? It's gotta be better than the original 12ft, but I doubt it's as good as the 8ft between the two braces.

  3. What's the best way to attach the braces to the jacks and header ? I'm imagining large lag bolts (perhaps 5/8" x 8"); install them horizontally and vertically, or straight into the bottom face of each brace (so 45 degree angle to the jacks and header) ? Some other kind of fasteners ?

Best Answer

  1. this is called a knee brace
  2. You mean the span not the bearing width. Assuming the brace to be fully effective, the effective span is less than the span between the knee braces. The reason is too complected to explain in detail here. For design, you could be conservative and just use the distance between the braces
  3. I usually suggest lag screws. The screws will be in shear, not withdrawal, so the length only needs to be long enough to get through the brace and into the main member a few inches

This isn't a very good solution. You'll create bending in your trimmer and king stud that will reduce their design strength. Knee braces generally aren't used to reduce the span, they are for lateral (sideways) strength so that it doesn't fall over. Think about a gazebo

Your best solution is to replace the header with a stronger one. It can be of the same(ish) size and therefore not reducing the opening under it. Good options include glue laminated beam and manufactured lumber such as LSL, LVL and PSL.