Dual Pitched Raftered Roof


#1

I am adding a 2-story garage addition perpendicular to my existing garage forming an L. The existing garage is raftered at about 12/12 pitch. I want to change the pitch on the back side of the new garage to something like 4/12 but keeping the 12/12 on the front side. I understand the sizes of the rafters needed and the use of collar and rafter ties on the typical rafter roof where the pitch is identical on both sides and thus just needing a 2x12 as a ridge. The question that I have is now there is no continuous floor joist for the rafter ties since the backside rafter is now sitting on a 8’ wall. Does this type of design require a ridge board to carry a considerable amount of weight? This new garage addition is 50’ long, 28’ is the actual car pad and the remaining 22’ is the depth of the old garage. So this 50’ length is where my new ridge would be. I then plan on extending the ridge of the old garage to this one and then putting in valley rafters to tie the two perpendicular roofs together.


#2

Im having trouble picturing what you described. Dont you have a structural engineer that can answer your concern? Send a Pic or a scan of your plan and Ill try to answer your question. The purpose of rafter straps are for shear drag, ridge boards are for keeping your roof a unit and collar ties are for sway and intermidiate bracing as well as reducing the likeliness of the dead weight of your roof pushing your walls out of plumb and pulling your rafters away from the ridge.


#3

You might want to consult an engineer if you want to do this right. What you are doing is taking aeay the opposing force of the pairs of rafters. I would highly reccomend using a double LVL, What size COnsult your engineer. The LVL will have to span the full distance NOBREAKS IN THE MIDDLE UNLESS SUPPORTED UNDERNEATH AND THE LOAD IS TRANSFERED TO THE GROUND. THis willstop the downard deflection, thus causingthe back wall to bow outward and the 12 pitch rafters to push it outward. So if I understand you will be spanning 28 ft unsupported. From my framing experience, which is what I do everyday, you will need 2 28ft x 1 3/4" x 24"tall


#4

Thanks for the replies, I had a feeling that when I wanted to change the pithc of one side of the roof compared to the other that this would require a ridge to carry the span. I haven’t consulted an engineer on the roof question but have on the flooring spans, which has a 24’ span. This requires a triple 18" LVL. So it sounds like I have a decision to make, put up a massive ridge with a support at 28’, which is kind of in the middle of the my wood shop space or keep the pitches the same thus not requiring a support pole.