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Old 06-03-2019, 07:05 PM   #1
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Attaching porch overhang rafters to house rafters with 150lb snow load? Doable?




Im building a porch overhang very similar to the attached picture (minus dimensions), and was wondering what the best way to connect the porch rafters to the current house roof would be. We have a 150lb snow load and the snow will definitely be piling up several feet above the overhang as it slides off, and will be accumulating several feet on the overhang structure. My current porch rafters are 2x8 which span 16ft spaced every 16inches, and attach to a 2x8 plate on the roof which is nailed onto the house rafters which are 2x6 with 16" spacing and span roughly 12ft. The slope of the house roof is 11/12.

Will the 2x6 house rafters be able to support the weight of this porch roof as snow builds up on it as the snow slides off and accumulates on top of it? The snow load for my area is based upon having 15ft of snow on the ground at one time back in the early 1990's. We regularly have 10ft of snow on the ground at one time. Also, what is the best way to attach the porch rafters to the 2x8 plate so that the metal roofing of the porch overhang and the house come together? Is there a simpson tie that anyone recommends for attaching the porch rafters to the 2x8 plate on the roof of the house? Any advice or tips are greatly appreciated. Thanks
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Old 06-03-2019, 07:24 PM   #2
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Re: Attaching porch overhang rafters to house rafters with 150lb snow load? Doable?


This is an engineering question, and I am not one.
I would add a soul plate to land the rafters on the house roof to compensate for weaker rafters.

Adding wall above the house wall reduces the weight applied to the original roof and ceiling joists in the porch stops the tendency for everything to move away from the house.
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Old 06-04-2019, 04:11 AM   #3
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Re: Attaching porch overhang rafters to house rafters with 150lb snow load? Doable?


https://www.admoyer.com/builders/spantables.pdf


Something like this should give you some direction. It goes up to 100 lb snow load. Without an engineer who will figure out what you have and what you can add, layman should double everything he knows, my opinion. So you should study how houses are built in your area.
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Old 06-04-2019, 04:27 AM   #4
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Re: Attaching porch overhang rafters to house rafters with 150lb snow load? Doable?


Your snow load is a bit over the top and does not allow for any scratch pad estimates. Whether you are pulling permits or not this needs some serious engineering. Pretty simple job foe an engineering firm that is familiar with your local requirements and gives you a piece of paper to answer any concerns in the future, like the questions an insurance company might ask if it falls down.

15' of snow, where are you??

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Old 06-04-2019, 08:26 AM   #5
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Re: Attaching porch overhang rafters to house rafters with 150lb snow load? Doable?


I suspect the snow on the roof will be sliding off onto the porch overhang before the snow on the overhang slides off. So you will be getting extra snow on the overhang.
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Old 06-04-2019, 11:54 AM   #6
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Re: Attaching porch overhang rafters to house rafters with 150lb snow load? Doable?


Quote:
Originally Posted by huesmann View Post
I suspect the snow on the roof will be sliding off onto the porch overhang before the snow on the overhang slides off. So you will be getting extra snow on the overhang.
Well said.

If the roofing on the home however is steel, and snow trim is used, the snow won't slide off. But you just can't add snow trim unless the homes roof structure is designed for it.

To correctly design this one would need to know:
  • the pitch of the roof on the home,
  • the home roof design data (rafters or trusses & their rated capacity),
  • what type of roofing is currently on the home (shingles or steel),
  • the size of the new porch roof,
  • environmental conditions at the site (Snow and wind loading data).
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Last edited by HenryMac; 06-04-2019 at 11:58 AM.
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Old 06-05-2019, 08:34 AM   #7
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Re: Attaching porch overhang rafters to house rafters with 150lb snow load? Doable?


Just my opinion.

I would make it an a frame.
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Old 06-05-2019, 08:45 AM   #8
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Re: Attaching porch overhang rafters to house rafters with 150lb snow load? Doable?


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Originally Posted by ron45 View Post
Just my opinion.

I would make it an a frame.
Do you mean, go right up to the ridge?
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