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Author Topic: Help on roof design  (Read 3141 times)

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Offline mmhailey

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Help on roof design
« on: December 14, 2010, 11:55:33 AM »
I need to order some timbers for a porch addition to a small cabin. The porch is 16' wide and 8 foot deep.  I need the design to look something like this.


I think I can figure the load on the rafters which are 4X6, but my question is on the thrust from the ridge beam transferred to the tie beam. I have a 10 dead/35snow load, and I was hoping to get by with a 6X6 tie beam. Everything is 6X6 with the exception of the braces, and the rafters.

Does this look feasible? The reason I am in kind of a hurry is my sawyer is leaving town, for about 6 months, and I would like to get these cut and stacked before he leaves.

Thanks is advance for the help......

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Offline Jim_Rogers

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Re: Help on roof design
« Reply #1 on: December 14, 2010, 12:24:29 PM »
First of all let me help you with some terminology. Thrust is the outward pressure of the rafters at the plate from the vertical load, see your drawing I marked up.

So with a bunch of rafters being supported by your ridge beam you have somewhat reduced the thrust on the plates, which is a good thing.
But what you have done is placed the vertical load down onto the tie beam.

In order to understand what size tie beam you need; you need to do some math work.

The area half way from the plate to the king post on each side of the king post will be supported by the ridge. The area from the half way point to the plate will be supported by the plate. So your distance is from half way point to half way point from one side of the ridge to the other side. Basically that would be 8'. Next half of the load will be on one king post in the front and the other half will be on the back one. So that's 4'. Your roof area now being supported by the front king post is 8'x4' or 32 sqft.

Multiply by your load. You say it's 45lbs (10+35) so that's 32 sqft x 45 lbs per sqft =1440 lbs.

Use this number for the vertical load and enter it into DonP's point load calculator.
You'll have to also enter the "unsupported" span of the tie beam. Usually engineer's figure it from post to post without the braces but you can also try it with the distance from brace to brace as some of the load will come down those braces, as well.

Then you have to enter the values for the type of wood you are going to use, as well as the size of the timber (6x6) and see if it will pass the tests.

If the 6x6 doesn't then keep making it deeper until it does.

Let me know if I can help you more.

Whatever you do, have fun doing it!
Woodmizer 1994 LT30HDG24 with 6' Bed Extension

Offline mmhailey

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Re: Help on roof design
« Reply #2 on: December 14, 2010, 01:44:04 PM »

Alaskan Yellow Cedar is always hard to find in the property tables. I used 950/1.42/115

Looks like a 6X6 is marginal, if I figure only one point load. I am going to increase the tie beams to 6X8, and that seems to make it work.  I probably could have gotten by since the struts are there, and the rear tie beam will be mechanically connected to the existing structure, but no use taking the chance.

Thanks Again.
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Offline krusty

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Re: Help on roof design
« Reply #3 on: December 14, 2010, 02:19:15 PM »
If possible I would go with a 12:12 pitch and metal roofing. where I am code allows you to reduce snow load significantly with high pitch and snow shedding roofing material. My load is 40 PSF and reality is there is never any buildup on the roof more than an inch.

Offline Thehardway

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Re: Help on roof design
« Reply #4 on: December 14, 2010, 02:24:00 PM »

Although the design in your drawing will work it is a little bit backwards or upside down to be more accurate for a kingpost design. It is not historically correct nor as structurally sound as a true Kingpost truss arrangement.  In a true kingpost design, the kingpost is used to support the tiebeam at mid-span and prevent it from sagging (it is a tension member not compression).  The tie-beam has no vertical load and serves only to control outward thrust of the rafters at the plates as Jim indicates in his drawing (another tension member not compression). The Kingpost is held upward in place by the pawl action of the rafters and its own weight (it actually dangles).  In some cases they were not even pegged.  The ridge beam is either omitted or placed between the bays to support common rafters or as a purlin in a principle truss/purlin arrangement.  All downward thrust is carried downward through the top chords and to the plate/post. 

Your design demands that the tie beam, which is the weakest, longest span, carries all the weight of itself, the "kingpost" the ridgebeam, and a large portion of the rafters as well as the outward thrust of the plates.  Can you see the single point of failure potential here?  Additionally the struts would act negatively or at best neutrally in the weight transfer process here.  They only weaken the tie beam and rafters near their weakest points.

In my opinion and from a historical standpoint, the struts should be received by the kingpost rather than the tie-beam, the ridge beam should be omitted or placed between kingposts and the kingpost should receive the rafters rather than the ridgebeam.

Going with your present drawing would require a significant upsizing of your tie-beam

Here is a picture of a true Kingpost truss design as built at George Washington's Ferry Farm Pavillion.

It used principle truss with common rafter design.  The span is approx 24' but all the factors apply the same for your project just scaled down.  There are many more kingpost pictures in my gallery that you may find of interest.

The idea that the tie beam supports the weight of the kingpost and supports the roof is one of the most common misconceptions and illusions in timberframe design.  It is very pervasive to find this mistake and it is typical to see tiebeams failing or sagging where the wrong approach is used. 

I hope this is helpful, please don't take it as a mean criticism.  I think your drawing is very well done and the kingpost design is one of the most beautiful, elegant, and functional forms ever conceived just widely misunderstood.
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Offline mmhailey

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Re: Help on roof design
« Reply #5 on: December 14, 2010, 03:04:17 PM »

I like the 12:12 idea, but I think I will follow the existing 9:12 pitch of the rest of the roof. Always metal roofing here in rainy SE Alaska. Too much wind, and Asphalt Shingles grow moss. Cedar shakes work, but you also get growth.


Thanks for the input. I took your ideas, and Jim's and put the two together somewhat. Since I had to get the order in today. I ordered beams, that will work for two different designs.  I then can take my time, and decide which one I like better.


Since I just cut 3 Kingposts for the main cabin, I was kind of thinking of trying something different on the porch. However I think I can take my time now, and draw up several different options.... I'll post some drawings as I work on them....

Thanks as always.
Give a man a fish, feed him for a day. Teach a man to fish, and turn him into a liar.

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