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Old 10-24-2010, 05:46 PM   #1
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Doubling up rafters


We live in a 1920's house that has a 22.5' x 13.5' porch that has been covered and turned into a family room. It has single slope roof with 2x6 rafters with 12" centres. The ceiling is not vaulted and it is supported by 13.5' 2x4s nailed to the house's original outside wall with metal brackets (joist slings?) on one side. There is about a 2 foot rise.

The ceiling is very low, just over seven feet and we wanted to open it up and have a vaulted ceiling. We wanted to know if we could remove the ceiling joists, extend some of the rafters to 2x8 to increase space for insulation and spray foam insulate.

We brought in a contractor to have a look and he said we could just double up the rafters with 2x6s and we're good to go. He said we could just do it ourselves and wouldn't need a permit. He didn't do any span calculations and was just eyeballing it.

I wasn't feeling confident about the recommendation but I have had a hard time getting another contractor or engineer to come take a look. (One guy was supposed to come buy this week and didn't show 3 days in a row)

To my question(s):
-How does doubling up the rafters affect the span calculation?

-Does the recommendation sound sane or reasonable? I'd think we'd at least need to go to 2x8s for some of the rafters to allow for enough insulation.

-If the no permit recommendation is not reasonable and if we do manage to come up with a drawing and submit it for a permit, can we feel satisfied that we're being safe if it gets approved?

-Could/should the structure be reinforced with plywood on the end walls?

Thanks in advance for any feedback!

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Old 10-24-2010, 06:30 PM   #2
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Doubling up rafters


I am not certain I fully understand your post, pictures and a diagram would help immensely. If I do understand it correctly, you are asking about removing the 2x4 ceiling joists completely, and doubling up the rafters. If I understand you correctly, you will be effectively removing the lateral bracing from your walls, and they will immediately begin to bow outward.

You did not say where you are located, but if in snow country this would be a very serious problem. If you are not in snow country, it would take longer for your walls to bow out, like until the first good wind storm.

You also indicate you want to "extend some of the rafters to 2x8". I don't exactly understand what you mean by the word extend, do you mean replace the 2x6 rafters, or are you thinking about nailing on a 2 inch wide strip onto the bottom?

The proper size for the rafters depends on the roof load, which is a function of code, which also depends on where you live. In any case, I would not put a lot of stock in a contractor who eyeballs the job and says you don't need a permit, again I don't know where you live, but where I live conversion of a standard ceiling to a vaulted ceiling would require a set of plans and a permit.

Anyway, post some pictures and clarify if you want to remove the ceiling joists. If so, you are going to need to do more than increase the rafter size, you would need to install a ridge beam, install trusses, or go to more complex framing such as purlins or knee walls in order to stabilize the outside walls against lateral forces.

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Old 10-24-2010, 09:16 PM   #3
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Doubling up rafters


I agree with the previous post, we need a diagram or -backyard facing it- picture. Sound like a shed roof off the back of the house dropping 2' in 13-1/2' or a 1-3/4" per foot cut, that is 22+' parallel to the house back end.

Is that correct?

The problem I see is a converted porch: slab on grade- with no thickened footings or stem walls, or deck/porch without full bearing under the bearing back wall. The rafters span is acceptable, depending on ground SNOW load, as mentioned.
Where are you located that you would only need R-19 in your cathedral ceiling plus the vents?

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