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Roof truss and envelope dimension questions

jdchess | Posted in General Questions on

I understand that from an energy efficiency standpoint, less exterior surface area is better. So assuming same square footage as you lengthen the building and make it narrower you are increasing the exterior surface area. 40’ x 50’ has less exterior surface area than 32’ x 62’6”.

 

Assume a rectangular slab foundation with a footprint of 51’6″ x 40’6” with a gable roof using trusses that clear span the 40’6” dimension. 8/12 or possibly 10/12 pitch. Metal roofing. No interior load bearing walls. Exterior walls would be 2×6 24” OC. Probably a wrap around porch that would add additional load to the load bearing exterior walls. Envelope boundary / air barrier at ceiling. Unconditioned, vented attic. Insulation on attic floor.

 

What are the potential problems/issues building a roof this deep with clear span trusses and no interior load bearing walls?

 

Does it make sense to build the house this deep if the floor plan works otherwise or would it be better to build something longer and not as deep, maybe 32’ or 28’? I assume the trusses would be cheaper per but you’re obviously using more trusses overall. Other benefits that may outweigh the negative of the additional surface area? Effects on overall construction costs or labor?

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Replies

  1. charlie_sullivan | | #1

    Where's the envelope boundary at the roof or ceiling? Is there an unconditioned attic, or is it cathedral ceiling, or a conditioned attic?

    1. jdchess | | #2

      Charlie,

      Thank you for the reply. I should have specified those details. Envelope boundary / air barrier at ceiling. Unconditioned, vented attic. Insulation on attic floor.

  2. Expert Member
    AKOS TOTH | | #3

    Nothing wrong it, keep in mind there are usually truss height limits, which might limit your slope.

    Generally, it is harder to get light into deep buildings, long rectangular ones with windows along the longer sides is easier on that front. With a square footprint, a light well (skylight, clerestory, popup) in the center of the house helps.

  3. jdchess | | #4

    Akos,

    Good point on the height limit. One truss manufacturers in the area did tell me that anything taller than about 12' would have to be "piggyback" trusses because they couldn't trailer and deliver anything higher than that.

    An 8/12 truss with a 40'6" span would be about 13'6" high at the peak, and a 10/12 would be about 17' high. So these would have to be "piggyback" trusses. I do realize that this would be a BIG roof and that's one of my reasons for asking. I wanted those with experience to weigh in on whether its feasible to build something this size or if it makes more sense to go narrower even if its at the expense of creating additional exterior surface area.

    On a different note, good point on the natural light issues with a deep building. Definitely something that I'm aware of and making conscious efforts to ensure it isn't a problem. Natural light is obviously one of the reasons to consider building something narrower.

  4. Expert Member
    MALCOLM TAYLOR | | #5

    Jason,

    There are things I would not include in a design for efficiency reasons, like complicated rooflines, just as there are things like low-slope roofs, or no overhangs, that I would not include in most climates because they make the envelope more prone to failure.

    However I think it's a mistake to make efficiency the primary driver of house design. That was what the early energy efficiency movement did in the 60s and 70s, and while some important technical innovation came from those houses, they were difficult to live in and a lot of them frankly a bit hard to look at.

    Houses are primarily cultural objects that reflect how we wish to live, how we want to relate to the land and other buildings around us, and enable that. Design is a dance between many competing aims and requirements. Using the volume of the enclosure as the primary driver of a house means short-changing a number of other things that may ultimately be a lot more important. Remember that if you or other future occupants don't like living in your house, it won't end up lasting very long - and that's not very efficient.

    1. jdchess | | #6

      Malcolm,

      Thank you for the reply. Your thoughts and input are certainly appreciated. Your point is well taken...a house that you hate to live in, no matter how efficient, is still a house that you hate to live in.

      My intention isn't to make efficiency the primary driver, as I agree with you that that would be a mistake. I do believe there is always a balance that must be struck...in nearly everything we do in life.

      Let me try to explain my reason for asking the question to begin with. Our design ideas and sketches have gone through many, many iterations and I have one particular design, rough as it may be, that I like overall. It has issues that I still need to work out, but I thought that one of those issues was the potential difficulty in building an enclosure as described (40'6" deep). So what I really wanted to do was get some input as to whether it was feasible to build such a large roof structure using trusses as described, or if it would create difficulty or potential costs that warranted moving the design in a different direction (i.e. narrower design). And I would really like to know your thoughts on the matter, considering your extensive experience.

      1. Expert Member
        MALCOLM TAYLOR | | #7

        Jason,

        The only issue I can think of is that at some point it becomes much cheaper to provide intermediate support for trusses - but I can't remember what that width is. A friend of mine is a truss designer. I'll ask him and relay any useful insights he has.

        Standing those trusses will be a bear! Probably easiest to rent a crane truck for the day.

        1. jdchess | | #8

          Malcolm,

          I assumed that trusses that size would need to be placed with a crane. Probably wouldn't really consider any other way.

          Any insight or advice your friend could provide would be greatly appreciated.

          1. Expert Member
            MALCOLM TAYLOR | | #9

            Jason,

            My friend has weighed in.

            Intermediate supports would only become a factor with a much lower slope on the trusses.

            As you and Akos have already discussed, the only thing that appreciably alters the price is having to go to Piggy-back trusses. Under 50 ft the chords are still 2"x4", and there isn't much to be gained by altering the shape of the house if it has a similar volume but is narrower.

            One thing that would offset the cost of the piggy-back trusses is that a crew could set them off scaffold without needing a crane, except for the initial roof-top delivery.

            Save for potential aesthetic concerns, there is no advantage to going to 10/12 I can think of. 8/12 sheds debris just fine, and is walkable by crews when sheathing and roofing (and by you if you need to perform periodic maintenance) the way a 10/12 isn't.

  5. jdchess | | #10

    Malcom,

    Thank you so much for taking the time to check with him. Please let him know I appreciate the information.

    One question though...you said that intermediate supports would only become an issue with a much lower slope. Could you help me understand why this is?

    1. Expert Member
      MALCOLM TAYLOR | | #11

      Like all structural members in horizontal applications, trusses rely on their depth for their strength. For instance wood joists can carry four times the loading if you double their depth, and deflect only 1/8th as much.

      Shallow trusses with long spans require more lumber with more connections to perform the same function, making them more expensive.

      Good conceptual way to look at trusses is that they are a beam of the same shape, that someone has removed all the extraneous material from, leaving voids in its place.

      1. jdchess | | #12

        Malcolm,

        I never thought about a truss that way in comparison to a beam or joist. Very insightful.

  6. Expert Member
    ARMANDO COBO | | #13

    I would like to bring a different issue. If your building dimensions are 51’x40’6”, you may not be using Advance Framing techniques, where you could save anywhere from 15-25% of framing material, at the same time, allowing you to install more insulation. You also can count of savings in labor and energy bills, designing the building on a 24" grid..
    Here is some free information about it:
    https://www.greenbuildingadvisor.com/article/the-pros-and-cons-of-advanced-framing
    https://www.greenbuildingadvisor.com/article/three-easy-and-essential-advanced-framing-techniques
    https://www.energy.gov/energysaver/energy-efficient-home-design/advanced-house-framing
    https://www.apawood.org/advanced-framing
    https://www.buildingscience.com/documents/insights/bsi-030-advanced-framing
    Must Builders who use 100% of its guidelines, are usually building smaller homes. For large homes, I still use double top plates and the required number of jackstud bearing at openings per code. Also, you are required by code to frame 16” o.c. if you have three floors.
    In my work 40’ trusses are common. In general, you have no issues there at all, as previous responses stated.

    1. jdchess | | #14

      Armando,

      Thank you for the input and I also appreciate the links, even though I already had all of them bookmarked :).

      I do intend to use some Advanced Framing techniques...2x6 exterior studs 24" OC, 2-stud corners, and ladder blocking for interior wall / exterior wall intersections. I'm with you on still using double top plates. The single top plate seems to create way more issues than it solves. Doesn't really make sense to me.

      And while I understand your point about the exterior dimensions and the 24" grid, I feel like trying to fully implement a 24" grid into a design is too restrictive from a design and layout standpoint, but your point is certainly not entirely lost on me.

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