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floor / ceiling / rafter tie framing

John Fulton | Posted in General Questions on

Hi,  My name is John.  I joined GBA for information and advice on the house that we will be building this year.  It will be our first house.  We have some general construction experience, mainly through a barn with a loft apartment that we have built on our property over the last 3 years.
We will be doing most of the work ourselves, including electric and plumbing, but probably hiring out the monolithic foundation.  It will be off grid solar.
I attached elevations and floor plans.   There is ample southern exposure and our views of the Sangre De Cristo Mountains are to the South and West, with rocky outcropping views to the east.
We want to stick frame walls and roof, looking at zip r sheathing for the walls and a super insulated roof.  Finished concrete floors on the 1st floor with in floor heat
In our county you need an engineered foundation but you can do your own engineering on the structure.
My first question is what the best practice / options might be for the floor/ceiling framing.  It also needs to serve as rafter ties.
Thanks and sorry its so long for a simple first question!

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Replies

  1. Expert Member
    Akos | | #1

    Doing rafter ties as in your drawings means either the floor/celing joist or the rafters need to pass through your air tight layer. That is a lot of sealing that needs to be done right and even then it might fail as the building moves. If you are OK with spray foaming the sloped roof section all the way down to the top plate, this might work well.

    Doing a structural ridge and avoiding rafter ties would make these details much easier.

  2. John Fulton | | #2

    Do you know how to figure out the engineering / what size or type of structural ridge beam to use and how to make wall strong enough to bear the load? And is a structural ridge usually one beam (36' in this case) to be set with a crane or something?

    1. Aedi | | #3

      Those kind of calculations should be performed by an engineer, but to get a general idea you can consult the span tables provided by the Southern Forest Products Association:
      http://www.southernpine.com/span-tables/headers-beams-size-selection-tables/

      Based on your plans, it looks like the longest clear span is just shy of 17 ft as designed, but you can easily throw a column in by the staircase to reduce that to ~14 or even ~10.5 ft. It is a good idea to make the ridge beam a single piece if you can (of laminated 2xs, that is), but not strictly necessary to my knowledge.

    2. Expert Member
      Akos | | #4

      John,

      You would have to get a structural engineer to spec a beam for something like that. My feel is that you wont be able to span that with an LVL without a column support somewhere along the way. You can definitely span it with a steel beam but that would mean a crane.

      If you want to avoid the crane, go with the LVL option. I've installed a 22' 14" triple ply LVL without too much problems, install it one ply at a time. An exposed LVL with nice veneer on top adds a nice touch to a room.

      If you want to avoid the ridge beam with and stay with the rafter tie construction, one option might be a soffit extensions like bellow:
      https://i.pinimg.com/originals/56/75/dd/5675dd2d6dc4e744e5aa2b24d468690e.png

      It does mean an extra layer of plywood for your roofing but now the air barrier can simply go from your wall wrap over the rafters/floor joists and up the roof.

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