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Building My Own Gambrel Truss

JaySun | Posted in General Discussion on January 5, 2005 05:53am

Hey,

I am building a 16X24 storage shed / workshop.  I would like to build my own gambrel roof trusses to increase the head space in the loft for storage.  I don’t want to pay for manufactured ones.  I have seen many sheds with homebuilt trusses and they look simple enough to build.

But, I need to know:

*How do I determine what angles for the top and bottom rafters, and the angle for the connection to the ceiling joist.

*How do I determine what pitch the roof should have for a 16 foot span?  What will the lengths of the rafter segments be?

*I am spanning 16 feet.  Do I need 18 foot ceiling joists for a 12″ overhang on each side?  I have seen sheds that rest the bottom rafter directly on the sill plate.  The rafter is cut wide at the bottom to create the overhang, but I want a loft for storage.

*What size lumber do I need for the rafters?  2×6 or 2×8??   I have seen a 12×16 that used 2×8 for joist and 2×6 for rafters.  Ok for 16 foot span?

I plan on using plywood gussets at each joint, glued and nailed.   3/4″ plywood with subfloor adheasive???

16′ is a common span.  There must be a number of plans for these trusses out there but I can’t seem to find any.

Thanks!!!

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Replies

  1. FNbenthayer | Jan 05, 2005 07:57pm | #1

    You might also look at Lee Valley, they have gambrel rafter brackets that appear to make things easier.

    YMMV

     

     

     

     

    The awful thing is that beauty is mysterious as well as terrible. God and the devil are fighting there, and the battlefield is the heart of man.
    - Fyodor Dostoyevski

    1. frenchy | Jan 05, 2005 08:19pm | #2

      I dislike brackets on two grounds!  first there is an inherant conflict between the rate of change between steel of the bracket and the wood!  the steel expands and contracts totally based on tempurature while the wood shrinks and expands totally based on moisture in the air!  Second depending on the type of wood involved there may be a chemical reaction between the steel and the wood.. especially when you consider how damp wood used in construction is.. plus or minus 2 points of 19% of the  wood is water!

        I do agree that toe nailing is a poor method to fasten wood together, but it is so easy to fabricate nice brackets from scraps and off cuts!

        for my roof I made the brackets a design element, fabricating them out of boxelder and running  a router around their outside edge.

       the contrast between the darker Tamarck  and the near white with vivid red streaks in the boxelder makes a nice statement..

           I'll grant you I'm pretty anal about it since they are up in the attic that few will ever see let alone notice.  But considering how little effort it was to do and the fact that hopefully this house will be here long after I've passed on, it's my little signature of immortality.

        As for angles!

            why not lay some boards out on the ground and see what angles you like?  there are no "rules"   the span and the other restrictions can affect the desgn much more than a particular angle.   For example if you need too long of a board or the over hang would hang down restricting opening a door etc.. could cause you to change the pitch of either angle

      Edited 1/5/2005 12:26 pm ET by frenchy

  2. MikeSmith | Jan 06, 2005 01:03am | #3

    ham.... you got a lot of questions..

    the normal response is " hire an engineer ".. but it's a shed , right ?

     16' huh ?...

    if you want a 30# / sf load... i'd use 2x10 .. assuming you are going to set your trusses at 24"  oc

    .. then you decide what you want for headroom..

    get some graph paper  and draw it out to a nice scale.. like 1/2" = 1 ft

    use 18' 2x10 for your base

    the 5 points of the gambrel traditionally fall on a half circle

    if you want it to be traditional... make it so..

    warning .. it's very easy for a gambrel to look like a mushroom..

     anyways  you can push or pull the front pitch and the top pitch to your heart's content

    for gussets, i'd use  1/2" underlayment ply.. it has no voids.. and i'd  glue and nail them with 6d common hot dipped galv. nails

    when you get it to where you like it on the graph paper.. lay it out on the deck of the shed.. build one.. stand it up.. walk under it .. like it ?

    lay it back down.. and nail blocks in place for a template.. use the first one as your pattern.. now build 12 more just like it

    do you need a permit ?

    have fun.. post some pictures when you get done

    Mike Smith Rhode Island : Design / Build / Repair / Restore
  3. rocco jibraltar | Mar 19, 2014 10:53pm | #4

    gambrel angles are standard, no matter what size the building...

    ...and they're all identical, as well!

    It's really just simple geometry.  All you have to remember is that a circle contains 360º and that a half-circle is 180º.

    So, with four faces needed to construct a gambrel roof, you need four angles at the center of your circle.  Divide 180º by 4, which gives you equal angles of 45º each.

    For the classic gambrel, here is a shortcut way to diagram this roof, and i've attached a diagram of my work:

    1. in scale, draw a circle with its diameter equal to the width of your building*.

    2. draw the horizontal diameter

    3. from the center point of the diameter, draw three radii: one each at 45º angles to the left and right, and one straight up to the top of the circle

    4. connect the points where those three lines intersect the circle, as well as from the left- and right-most ones to the diameter, and you have your gambrel roof line.

    5. measure these lines to arrive at the dimension along the outside face of your roof joists.

    Yes, there are other gambrel designs, but they will give you "flattened" or "peaked" gambrels... the method described above will give your roof a "classic" gambrel shape.

    You can make the side walls whatever height you need, but be aware that the height added by the classic gambrel roof will always be one-half of your building's width. [remember all radii of the circle are the same, whether left, right, diagonal or straight up!.] so if you build eight-foot side walls on a twelve foot wide building, its overall height will be fourteen feet.

    All joist ends will need to be mitered at 67.5º to meet flush with one another and at the side walls.  [Speaking of the side walls, i'm neither a professional carpenter nor architect, but i do know that allowances would need to be made for an overhanging eave:  measuring longer joists for the bottom half of the roof; notching them to sit on the top plate; lifting the roof, albeit slightly are all considerations to overall building height which must be accomodated.]

    I hope this helps.

    r j

    *You will still need to offset this circle inward by the thickness of your intended roofing material, be it plywood, composition board, flake board, etc., to arrive at the exact height measurement you want.

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