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16×24 Open Beam Porch Roof Framing

DIwhYer | Posted in Construction Techniques on December 27, 2022 06:13pm

Hello everyone.  I’ve read some great discussions here!

Let’s say this is to be built in the coastal Southeast, with lots of humidity and storms, and minimal frost depth.  I’m planning a 16×24 elevated deck/porch with an open beam/cathedral ceiling.  The deck will be freestanding from the brick veneer home, and the roof will connect to the existing 7:12 roof, forming a gable.  So far so good, I hope.

Where I’m getting confused is in figuring out how to frame (spec and cost out) the roof, because I’d like to have 48″ OC T&G exposed sheathing (likely topped with OSB), and because I’d rather not have rafter ties or collar ties or trusses, I realize I’ll need a substantial ridge beam and rafter beams (48″ OC) to support the 24′ span and 16′ depth.

Despite lots of googling, reading codes, discussions, changing my designs, looking at builds, I just cannot find information for anything beyond 4-ply 2x12s for ridge beam applications (like southernpine.com tables).  LVL is now much more, maybe twice as, expensive as dimensional lumber.  My span will be around 17′, so that the inside is supported by a 6×6 to the existing house framing, the outside by a 6×6 to the deck.

So, my theory is 5+ ply 18′ 2x12s glued and bolted will be great for the ~17′ ridge beam, with 2-ply 2x10s for the 7:12 rafters (48″ OC).  The walls would be 6x6s on the 16′ sides (48″ OC) and 24′ outside end (72″ OC), with 2×6 plates.  Decking joists would be 2x10s (16″ OC) with (12) 6×6 footings diagonally braced and on standoffs from poured reinforced concrete above grade, due to stormwater/tidewater conditions.  I’m concerned about LVL delamination over time, too.

In short, can I use 5+ ply glued/bolted 2x12s for a ~17′ ridge beam and have low deflection?  Sorry for the long post!  I tried to anticipate your questions.

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  1. User avater
    unclemike42 | Dec 28, 2022 07:49am | #1

    I am having a hard time with the idea you will make the porch freestanding from the home, but connect the roof.

    The ridge beam support at each end needs to be carried direct to the foundation, which may need some help with these additional point loads.

    How high is this going to be?

    You may want to consult with a local engineer to work out the details, and make sure the design meets wind loading as well as flooding design requirements.

    1. DIwhYer | Dec 28, 2022 10:26am | #2

      Thanks, unclemike42.

      The overall height will be ~18' 5", with a 3' 6" elevation matching the home and 8' walls. The roof would tie-in perpendicular to the existing 7:12 slope, and the porch would open up from the kitchen. There is a standard door opening and a double window opening just below where the support would need to be. So, the ridge beam support would not realistically be carried direct to anything but the kitchen outer wall (2x6 framing, 2x6 rafters), though the load could be spread out, perhaps. Foundation is 2x10 framing on blocks, and I don't know if the blocks are core-filled, but I suspect they are.

      I take your point, though. Maybe I'm forced to go with rafter ties or trusses so the only weight on the house itself is the tie-in roof section?

      I love the look of a Howe truss, but wanted to get as much natural light into the porch as possible, without resorting to skylights that would notoriously leak or get damaged by deadfall limbs from trees on the property. Ties would mean no beam, and maybe 2-ply 2x6 rafters with 24ft ties, or perhaps shorter ties closer to 1/3rd high? Possibly I could even use wire ties. Hmmm.

      As you can tell, I've been all over the place with ideas of how to do this ;)

      Edit: As for wind/flood, yes, there will be hurricane ties and straps at every connection and every level, and the raised concrete footings (one foot+ below and ~one foot above grade) prevent the wood from standing in water in all but the worst hurricane-generated flooding.

  2. User avater
    unclemike42 | Dec 28, 2022 05:32pm | #3

    The support of the ridge board, (or even the extra roofing you plan to install on the house) has to be carried to the foundation. if it has to be carried via headers, they need to be sized for the job, and the path from the header ends to the foundation need to be understood.

    the foundation for the house needs to be up to the job. If it is marginal, adding more to the roof is not a great idea.

    the added structure needs to be supported as the house is, and tied to the house, or separate from it. not half and half.

    I repeat my free advice to have a local engineer who understands the specifics engage on the details.

    1. DIwhYer | Dec 28, 2022 10:21pm | #4

      Well, that's all fine and well in very general terms, but I posted my questions on a forum like this for a bit more detail than "hire a local engineer".

      I've given a lot of detail above, so why exactly is it "marginal", or "half and half", and what does "sized for the job" really mean? Those are details I was looking for here from some experts, because Google searching came up with lots of generalities and/or specific details for different situations than I face. I'm not saying I haven't engaged a local engineer, nor am I saying I'm sticking with the ridge beam solution.

      For example, why exactly wouldn't a 5-ply or 6-ply 2x12 ridge beam work for the span indicated if the home-side load is well distributed over a number of members, such as adding strongbacks against a number of the existing 2x8 ceiling joists above the kitchen? Why exactly is a common freestanding deck for a brick home somehow not right for a tie-in gable roof regardless of roof framing? Why exactly is the foundation not up for the job? I could go on.

  3. User avater
    unclemike42 | Dec 30, 2022 07:15am | #5

    I may have been mislead by your description of the house on blocks. This may be a proper CMU wall on a suitable footing capable of supporting not only the house sitting on it, but also the deck you plan to add.

    If you want to use a different ridge beam structure for a different application than a published table applies to, you need to do the calculations (or better yet, have a qualified engineer do so)

    Tables I have seen have lots of drawings and notes that all need to be understood and complied with. They usually assume uniform loads. (roof rafters and decking). Ridge beams on 4 ft centers is just enough different to call for custom calculations. (or contact with a technical support department of your supplier)

    A point load (such as the end of a ridge beam) needs to be carried through whatever it sits on to the foundation, and into the supporting soils. Along the way, diversion for window or door opening would call for a header calculated to handle the weight above, and a path for those forces to the foundation from the supported ends of those headers. These calculations need to be specific to the calculated loads above.

    When you build a free standing deck, the assumption is that it is not tied to the house or any other building it may be near, so that if it shifts, settles, or moves for other reasons (say moving water or high winds) it does not move the adjacent building.

    When you tie the roofs together, you loose this independence, and risk damage from any movement. Prudent design calls for the deck to be firmly attached to the house if the roof is attached.

    You may want to look into treated laminated beams. (if you are worried about delamination)

    And the process of iterating "what I want" with "what the stuff I can buy can do" is a normal part of the building design process. Enjoy the journey.

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