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What design load factors for rafters should be used when you are building a PERSIST type roof?

Irishjake | Posted in General Questions on

The roof from the outside in, consists of standing seam, plywood, vent channel (cold roof) 2×4’s, 9″ ISO, ice and water, and plywood are used above a rafter?

there is wood cladding or sheetrock on the cathedral ceilings….
I have snow loads of 75-80 lbs/sf per CCREL…zone 6A.
What if PV or hot water Solar panels are added to roof?

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  1. GBA Editor
    Martin Holladay | | #1

    Calculating the design load factors of a PERSIST house is no different from calculating the design load factors for a conventional house. If the calculations are beyond your abilities, it's time to hire an engineer.

  2. Irishjake | | #2


    Beyond ability - think not.....would like to know how to figure it, rather than just asking for an answer. So it is a calculation available in a book...where is the book I'll find the info or I'll pay a fee to and ask a question with the hope that I can get the calculation....not just the answer

  3. Expert Member
    ARMANDO COBO | | #3

    Here is a NEW idea... how about reading the CODE BOOK!!! Capter 8 will give you all the answers.

  4. GBA Editor
    Martin Holladay | | #4

    There are many factors to consider -- the span of the rafters or trusses, the weight of the sheathing plus insulation, the weight of the snow, wind loads, and perhaps the prevalence of earthquakes.

    Here's a document to get you started: Design Loads for Residential Buildings.

    In most cases, an engineer's fee is well worth paying.

  5. gusfhb | | #5

    What kind of beam is it? is it a simple beam? Is there a cantilever? What pitch is it? Flat roofs need a bit more load than a pitched roof, an engineer can build a steep roof[0ver~12/12] lighter than a lower pitch IIRC.

    80 PSF is a lotta snow, hell that is like 16 inches of water.

    On a PSF basis, foam, plywood solar panels etc weigh almost nothing. They fall under the standard dead load. You would want to make sure you are not point loading a big old solar panel in the very middle of a span if you had not planned on it, but even then it may only cause a little sag, maybe.

    find a span table for the loading and wood species you are using, the work has already been done.

    double check the snow load it seems high, snow load is your dominant load

  6. Irishjake | | #6

    Armando thanks for keeping to the rules, by at least keeping it brief! I am going to stay productive though and address the actual intelligent and articulate answers........
    I am concerned about the point loads from the solar specifically....I realize there is all the other stuff to consider......
    I live in a "CS" area in NH where thanks(!!!!) to CCREL and other engineering firms who volunteered (more thanks!!!!) a great amount of time to study the whole state (every town!) and figure out our specialized snow loads for this region. The only areas that still need specific tests are above treeline where data changes dramatically. It is an interesting study and here is the link for those that are info geeks like myself Armondo.......

    I actually only have a average snow load of 75 lbs, (using elevation factor) but with the design of the house, there will be some secondary lower roofs, and a valley on the north side that may see drifting, so I am allowing for 80 lbs.....
    I shovel 690 hydrants every year....the year before last, we shoveled 18 storms worth! So I can tell you all about how 80 PSF is a lotta of water!
    I understand an engineer's fee is worth paying, but that doesn't give me intimate knowledge about the building (my house, that I will sweat, cuss, and bleed over), and anything worth having isn't I continue my search for the the way Armondo what page EXACTLY would that info be on in chapter 8????

  7. Expert Member
    ARMANDO COBO | | #7

    As Martin, Keith and you have brought up, there are too many issues to consider in your roof design. See 2009 IRC Chapter 8, pages 372-431. If you are not familiar on how to use the code book, as Martin said, you should hire an engineer. $300-$500 fee will save you money in the long run and give you a piece of mind.

  8. Irishjake | | #9

    I have the code books, the IRC, the IBC, the IFC, etc., etc., etc..... - but just as I use the IFC, I like asking professionals or other likeminded individuals for their help when I need clarification, or opinion (consensus standards), etc., etc., etc.. Doing this has always been a way to network, find a path to an answer faster, or more efficiently - and it helps me to critically think through my process for checks and balances......thanks all for your help once again.

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