Over Framing on Roofs - Additional Dead Loads 1

[medeek]

I don't really have a set method for dealing with the additional dead load associated with framed over sections on a roof. Typical situations are with valley sets or sticked framing over trusses as shown below:

Usually this doesn't affect my numbers too much but I am wondering if others neglect this additional roof load or perhaps use some set value or increase to the dead loads at the reactions into the wall, beam or girder below.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[JAE]

You shouldn't neglect it. Usually this overframing is only about 4 to 6 psf so simply include it in your calculations.

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[jayrod12]

I've been told by a major Truss engineer here (He sealed almost every wood truss shop drawing around here for a while) that there is no additional load accounted for in your typical valley set.

 

[XR250]

I neglect it on new construction as it is usually covered sufficiently in standard dead loads. On existing structures, I check it, but it is rarely and issue. In reality, the overframe probably strengthens an existing roof system as it will tend to act as deep cantilevered beam coming off the addition. If I am doing a large stick framed overframe onto trusses, I add stiffknees coming off each truss up to the new rafters. Otherwise, the california valley can concentrate too much roof load on the chords.
No use sharpening your pencil too much on wood framing as there is so much unintended redundancy and composite action.

 

[KootK]

I can confirm Jayrod's comments from the truss manufacturer side of things.

Keep in mind that you get an extra layer of sheathing in addition to the valley trusses themselves.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[msquared48]

Normally I ignore it too.

Frequently, for cross venting, a portion of the lower sheathing is removed anyway, lowering the additional load due to the overframing.

Mike McCann, PE, SE (WA)

 

[BUGGAR]

I've heard that called "California Framing". We usually allow some extra psf to cover it. It's frequently shown shaded on roof framing plans to clarify that there is framing over framing.

How many design for a re-roof.

 

[msquared48]

I use 15 psf that allows for one reroof.

Mike McCann, PE, SE (WA)

 

[KootK]

Frequently, for cross venting, a portion of the lower sheathing is removed anyway, lowering the additional load due to the overframing.

Not too much I hope as that's the "diaphragmy" part of the roof.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[medeek]

Now that we are talking about dead loads and roofs. I've noticed that most of the truss companies I work with will assign a 7 psf dead load for the top chord and a 10 psf dead load for the bottom chord. I'm not sure exactly how they arrive at these numbers. The 10 psf on the bottom chord is probably the gypsum ceiling however the 7 psf top chord seems a bit skinny for asphalt shingles if you figure on a re-roof or two.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[XR250]

Gypsum is about 2 psf and insulation is less than 1 psf so the bottom chord number is high.
They use 10psf around here for the top chord but I can see 7 being ok. 2 for sheathing, and 2 for each layer of shingles and 1 for self weight?