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I'm going to build a single story addition to my home with a width of 14ft. I plan on using ceiling joists every 16" but I'm not sure which size to get. The roof will be asphalt shingles and the attic will not be required to hold storage. I've looked at span charts for both rafters and ceiling joists but I'm not sure of the dead load, live load and deflection limit. Another factor is ground snow load. My location is Northern Virginia.

What size joists and rafters should I use? I'm guessing 2"x6" for both.

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A 14ft span and 2x6 will equal a very bouncy floor. –  The Evil Greebo Feb 8 '13 at 18:33
    
Do you mean "ceiling joists" of the room below (the floor of the addition) or are you refering to the collar ties? Is "ceiling joist" the same as a "rafter tie"? –  auujay Feb 8 '13 at 19:27
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If it is a walkable attic space then look in the charts for at least 60PSF. 2X8 min. Forget engineers, just talk to your local building supply. They can help you understand the span/load charts for your application. local code officer can be a good resource. –  shirlock homes Feb 9 '13 at 0:25
    
@shirlockhomes is right about talking to locals, as the snow load varies widely by area. –  Jay Bazuzi Feb 9 '13 at 4:30

2 Answers 2

There seems to be some confusion in terminology. I understand the ceiling joists to support the ceiling finish, support no live loads, and serve to tie the rafter ends together to form a triangular truss like structure. The ceiling joist size will depend on whether you will be installing knee braces and purlins or not. Such a ceiling joist you can figure a 10 psf dead load for span tables, but with knee braces, you can't use span tables. The roof dead load depends on your roofing material. For regular asphalt shingles, figure 15psf dead load, much more for tiles. The roof live load will typically be some portion of the ground snowload, depending on roofing material and pitch. Some jurisdictions specify a roof snowload that is not reducible.

Snowloads can vary drastically by small changes in location. You should inquire with your local building authority for this kind of information. For deflection limits, 1/240 the span for ceilings is OK. For rafters with no ceiling finish in the typical attic situation, you can use 1/180 span under combined dead and live loads.

Unfortunately, there is not enough information to answer your question. If I understand your ceiling joist application correctly, a 2x6 should be adequate if no knee braces are involved, but I couldn't say for sure, it's just my gut feeling.

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Thanks for your quick replies. I think the consensus is to check with my local codes department. I'm not even sure if I need purlins or collar ties. I hear that collar ties are necessary or at least good to have to brace against wind. Aren't purlins more of brace for very wide spaces? –  Rich Feb 9 '13 at 13:27

As @TheEvilGreebo mentions, 2x6 is not enough for a floor that will be used as "habitable space". I know because when I remodeled my attic to convert it to a bedroom, the existing 2x6 floor joists (over a very similar span) were not enough and I had to sister them all.

At a minimum you will need to check with your local (municipal or county) building department. They should be able to tell you what you need.

It might be easier to get a structural engineer or architect to come out and tell you what to do. If they are not drawing up plans you might be surprised that their prices are quite reasonable (they should be able to give you a quote for what it would cost for them to come out and generate a report up front). It will also help when you get your building permit to be able to submit the report written by a certified professional to the building department.

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