Fixing a window framing mistake

When framing a window under a load-bearing wall, you need to put in a header to carry the roof load. Typically it's built out of 2x lumber on edge, with plywood sandwiched between to make it the same thickness as the 2x4 wall. (2x6 walls are done a little differently.)

I screwed up and put the header in sideways, with the lumber laying flat. This is far, far weaker.

I tried taking it apart, but it would have meant tearing up the king stud to pull the nails out.

Will it still be strong enough? Here's the math:

The roof design load is 10psf dead load + 25psf snow load = 35psf total. Rafters/studs are 2' O.C., and rafters are 8' long (I designed to use them full length.) Tributary area is 2' x 8' = 16 sq. ft. Design load = 16 sq. ft. x 35 psf = 560 lbs.

The weight limit for a span is normally Fb * b * d * d / (9 * L), where:


Fb = Fiber bending stress of the wood you're using. My 2x4s are Douglas Fir "#1 or BTR". According to WWPA, that wood has a fiber bending stress of 1200. (I have no idea what unit that is!)

b = breadth of the beam, in inches. A 2x4 laid flat (oops) is 3.5" wide.

d = depth of the beam, in inches. A 2x4 laid flat is 1.5" deep. Notice it's squared - this is why lumber on edge is so much stronger.

L = length of the span, in feet. My window rough opening is 2'.

Put it all together: 1200 * 3.5 * 1.5 * 1.5 / (9 * 2) = 525lbs. Not enough.

That formula is for an evenly distributed load across the span, which assumes that the strain on the wood varies linearly with the distance from the point of support. To put it another way, it behaves like a point load on the center of the beam that's 1/2 as large. So, divide my max load by 2.

However, I have 2 pieces of wood here (twice as strong), so multiply my max load by 2. So we're back to 525 lbs. Still not enough.

There's one more factor to consider: there's a double top plate that can carry some of that load, too. If you assume it's just strong as the misplaced header, then I'm fine, but I've never seen that described before, so I was wary of counting on it.

However, we're close. A little more wood could do the trick. And I had the perfect scrap of wood - a piece that was intended to be a window header, but it was 1/4" too short. If I lay it flat across the top, it increases the strength of the header by 50% more - enough to be certain we're strong enough. And I can end-nail the cripple on to this extra header *before* I install it, avoiding the hassle of toe-nailing.

That went together really nicely.

As I've been learning to build, I have asked myself the question: what is the difference between me (a novice) and an experienced pro? There are several obvious answers:

• Pros cost money, I just cost time.
• I'm way slower, because I spend a lot of time figuring out *how* to do this, and because I make mistakes I have to recover from. Also, I get tired because I'm not used to the work.
• I waste material due to mistakes.
• Pros are good at this, and know how to produce high-quality work.
• I care about the outcome far beyond my concerns for reputation and callbacks, so I'll take care when it matters to me.
• Pros cut corners to save money & time (which is also money to them) if they think they can get away with it.

But there's one more point that I have become aware of: pros are really good at hiding their mistakes. A novice can do work that's just good (or just as bad) as a pro, but a pro will make it look like they did a good job, even if they didn't.