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Structural Roof Design-Basics Please?

| Posted in General Discussion on December 4, 2000 07:36am

*
Someone once told me that on a roof with a steep pitch (say 12:12), most of the load is transferred to the walls. More specifically a roof system of rafters with a structural ridge beam. I would like to be a little better educated in the basics of roof design.
Would someone tell me how to calculate the loads at the sidewalls and ridge?
Does the loading change as the pitch of the roof changes? When calculating the total roof load is the square footage based on the span of the roof (measured horizontally) or is it based on the actual square footage of roof surface? Any good Rules of Thumb? Thanx for any input!

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  1. The_Tennis_Court_Builder_...on_t | Nov 27, 2000 04:25am | #1

    *
    Structural ridges carry half the weight of each half of the roof...The pitch matters not....

    Non structural ridges are different...All the load ends up on the walls pushing down and out.

    Next question?

    near the stream,

    aj

    1. Mike_Smith | Nov 27, 2000 04:26am | #2

      *don.. basically .. everything you need to know is in the building code... the rest is basic engineering ( very basic).. and geometry .. and / or trig...

      1. The_Tennis_Court_Builder_...on_t | Nov 27, 2000 04:29am | #3

        *In my snowy area...steep pitches were built with full size 2x4s for years in the construction of camps on the lake...All that I have been in are doing fine. So I don't worry much about strength these days as most size rafters so as to fit insulation and use at least 2x10s and sometimes more.near many old camps,aj

        1. Francis_Voignier | Nov 27, 2000 04:47am | #4

          *The ridge beam is the essential thing here. The span merely adds dead load and increases the potential for live loads, 2X12's at 16" on center can span a way.fv

          1. Davo_ | Nov 27, 2000 06:51am | #5

            *When calculating the roof load, use the actual square footage of the roof itself. Remember, the load takes into account the actual real weight of the framing members, the sheathing, the roofing materials, etc;(this is referred to as the "dead" load) not to mention that you must also take into account the proposed wind and snow loads as well(for your particular area) when designing a roof structure.I'm really not sure how much roof pitch affects the loading; I would think the steeper the roof, the more the load is transferred straight down to the walls....and likewise, the more flat a roof is, the more weight that must be borne by the rafters themselves; though I may be all "wet" in this theory.Also, the more steep, the more that roof will be subject to wind loads. The more flat, the more its subject to snow loads. Sounds confusing?Davo.

          2. Boss_Hog | Nov 27, 2000 01:40pm | #6

            *Live and dead loads are always figured based on a horizontal projection, not along the slope of the roof. (Although it seems to me that the dead load should be figured along the slope, I've never heard of that being done)AJ is right about a structural ridge beam carrying half the weight of the roof. Remember that gravity pulls straight down, not along the slope. Are you asking about a specific case, Don ? It's a lot easier to discuss a specific case instead of gerneralities........

          3. Darrell_Hambley_PE | Nov 27, 2000 08:13pm | #7

            *What AJ said. That's all you probably need to know.It could get real messy however; ie, with high snow loading in a seismic zone, you use different loading depending on the slope. Maybe what your friend meant is that there's more stress on the joists for a low slope roof, so it's better to use a ridge to bear the load. DH

          4. Don_Mallette | Nov 28, 2000 12:54pm | #8

            *O.K. So I guess what might be useful here is to know how to calculate the loads, down and out, in the absence of a ridge beam. If I use a ridge beam then half the load is at the ridge cause gravity pulls straight down. But if I don't use a ridge beam then all the load is on the walls. So, is what A said still all I need to know?Don

          5. Francis_Voignier | Nov 28, 2000 05:36pm | #9

            *If you don't use a structural beam, you will have to use collar ties or proper trusses. Of course, you end up with a flat ceiling with trusses. You could also use cable and turnbuckles which can be exposed with the added industrial look.fv

          6. CaseyR_ | Nov 28, 2000 08:05pm | #10

            *Francis, while most readily available trusses will give a flat ceiling, trusses can be designed with a variety of ceiling shapes. You would probably have to have them custom engineered, however. Take a look at the little slide show on the following site to see some trusses giving other than a flat ceiling:http://www.woodtruss.com/

          7. doug_hubbard | Nov 29, 2000 02:47am | #11

            *don the forces are simple- gravity as in down- and how to counter.in a roof system gravity pulls down creating a load. the peak of a roof is a fulcrom- pivot. the peak or ridge either counters that load or does not.if it does not the load will cause a moment- an action. the rafters acting in opposition will be forced down and away. that action must be countered or the roof will fail.the walls must be tied together or the ridge must support the load- it is that simple

          8. Francis_Voignier | Nov 29, 2000 05:31am | #12

            *Casey.Thanks for the great pictures. I actually build my own trusses and I have scissor trusses in my shop. It just didn't come to me when I wrote my last note.Thanks again!fv

          9. Jim_Tredeau | Nov 29, 2000 05:37am | #13

            *Yes, the Dead Load weight will increase, as the roof pitch increases. To figure out the amount of increase you divide the Dead Load by the cosine of the angle of the roof (e. 10# Dead Load divided by .707106781 which is the cosine of 45 degrees for a 12:12 pitch, equals 14.142# Dead Load) If it where a 6:12 pitch it would only be 11.180# Dead Load.

          10. christoph_smith | Dec 02, 2000 08:14pm | #14

            *I can't find an old issue reguarding roof overhangs. My roof is 5:12 (trussed)with the last truss on the gable end sitting 20 inches inside the outside edge of the wall. I want to extend the non-existent overhang to 3 feet. Any suggestions re: structural/construction details?

          11. PE_Alicia_McCormick | Dec 04, 2000 07:36pm | #15

            *To Don Mallette regarding roof framing: If there are no horizontal members (floor or ceiling joists, or properly designed collar ties)to create a triangle with the rafters, the ridge will almost always need to be "structural" and will carry half the gravity roof load and the exterior bearing walls will each carry 1/4. This is somewhat simplified, but generally adequate assuming a symmetrical roof. If there is a "triangle" formed with horizontal members, the "ridgeboard" simply acts as something to nail the rafters to and transfer the compression from opposing rafters. Please note that the forces at the connection from the horizontal part of the triangle to the rafters are more than you may think. Let me say that the nailing indicated in the CABO fastening schedule is not generally enough to couteract the thrust. Use a lot more!

  2. Don_Mallette | Dec 04, 2000 07:36pm | #16

    *
    Someone once told me that on a roof with a steep pitch (say 12:12), most of the load is transferred to the walls. More specifically a roof system of rafters with a structural ridge beam. I would like to be a little better educated in the basics of roof design.
    Would someone tell me how to calculate the loads at the sidewalls and ridge?
    Does the loading change as the pitch of the roof changes? When calculating the total roof load is the square footage based on the span of the roof (measured horizontally) or is it based on the actual square footage of roof surface? Any good Rules of Thumb? Thanx for any input!

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