Hi,
I just got a list of corrections from the Bldg. Dept. for my room addition. They indicated that my intended headers are overspanned. According to the tables I've seen, I should be golden. My longest span is over a window 8' wide. (This is a one-story building with a composition roof.) The walls will be 2x6, so I planned on double 2x10's with a 2" spacer between (to equal 5 1/2" total), using Doug Fir #2 lumber. Does anyone have any info to support my plan, or is it wrong? Will 2x12's be necessary? Thanks for your help.

Sue

No way to tell without knowing how the roof will be framed, and without knowing the total depth of the house.

The roof will be framed with trusses. The window in question will be in a load-bearing wall holding up one side of a gable roof. The total depth of the house will be 18 feet, and the wall with the window is 24 feet. (I can never seem to give enough info the first time!)

Thanks for your help.

OK, 18 feet divided by two is 9 feet. Hmm, now I need to know the design live load for roofs where you live. Where the heck is Palomino? Do you get snow there? See? You thought designing structures was easy.

OK, let's work backwards. A double 2x10 has a Section Modulus of 21.39 in.³. Assume a Fiber Strength in bending for #2 Douglas Fir - Larch of 1250 psi when used non-repetitively. Multiply the two and we get 26,737.5 in/lbs, which is the maximum Moment allowed. Multiply that by 8, and divide by the span in inches (96) twice, and we get 23.21, which is the maximum load on the header in pounds per inch. Multiply that by 12 and we get 278.5, which is the maximum load the header can carry in pounds per linear foot. Divide that by 9 feet (half the house depth), and we get 30.9 pounds per square foot, which is the maximum allowable roof load that the header can support.

Now, assume the ceiling dead load is 10 PSF and the roof dead load is also 10 PSF, and assume there will be no attic storage of any kind in the trusses. Add them and we get 20 PSF, which when subtracted from the total allowable load on the header of 30.9 leaves 10.9 pounds per square foot as the maximum roof live load the header can support. Compare that with what is required in your area for roof live load, and you'll have your answer.

If it's short, you can add another 2x10 to the header, making it a triple, but I'm tired and I don't want to do the calculation all over again for that.

Sue,

Just to reiterate what Richard said, I looked it up in a span table and assuming you have 30 psf snow load then your maximum span with a double 2x10 is only 8'-5". Again like Richard said if you add another 2x10 then the span reaches 10'-6", which is more than the 10 ft you need. If you are in a 50 psf snow zone then that bumps it up to (3) 2x12's. Call your local building dept and ask what zone you are in, if they are really nice they will tell you what size the header needs to be.

I think Jay forgot some loads. My figures demonstrate that clearly. It also shows why attempting to do structural engineering in a forum such as this is not acceptable. There is no substitute for full knowledge of all conditions which may affect the design, and we have no assurances that we have such knowledge.

Thanks Richard & Jay,
We are in an area that requires a live roof load of 20 (just north of Reno, NV). I'm going into the building dept. today & get more info. Maybe they'll tell me what I need. I guess I can use a triple 2x10 over the window, but still use a double 2x10 over the 6' French door (on the opposite wall) so I can insulate it. Does that sound o.k? And, Richard, believe me -- I do not think designing a structure is easy. Far from it. I'm just trying to save some money so we'll actually have enough to do the project. So you're expertise is greatly appreciated.

Sue

Richard I didn't forget any loads. As I stated before, I looked this up on the span table (incidently it's table R502.5(1) in the 2003 IRC) for headers in a single floor with a roof above it. In fact why would you assume that I forgot loads when my conclusion completely agreed with you?

A live load of 20 PSF, plus dead loads of 20 PSF is a Total Load of 40 PSF. Multiply by half the depth of the house, and we get 360 pounds per linear foot on the header. Divide that by 12 to get 30 punds per inch of header length (w). The formula for calculating moment is M=(w)x(l)x(l)/8. The length of the header (l) is 96 inches. The calculated moment is 34,560 in/lb. Divide that by the Allowable Fiber Stress of 1250 psi for #2 Douglas Fir non-repetitive, and we get 27.648 which is the Section Modulus required for the header. A double 2x10 has a Section Modulus of 21.39.OMG I made a mistake last night!!! I am so sorry. The Section Modulus of a DOUBLE 2x12 is 42.78 in³!!! The number I gave was for a single. YOUR 2-2X10 HEADER SHOULD BE FINE!!!

I told you I was tired. Next time, I'll answer in the morning!

Hi, Richard & Jay,
Sorry -- I didn't mean to start something between you two! I met with the Bldg. Dept. & they said I either needed an LVL or I could go with a triple 2x10. Based on the cost, I decided to go with the triple 2x10. Thanks for all your input!
Sue

If I hadn't made a stupid mistake, Jay and I would have agreed. All the calculations in the world can't make up for reading a chart wrong.