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Old 11-08-2009, 02:26 PM   #1
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Need help figuring out rafter lengths.


I do not know anything about framing but I am also not scared to get in there and try it. Because of what I have read on here I will try to give as much info as I can. What i am going to attempt to build is a shed still roof attached to my existing house. The house is a single story ranch with a garage that is attached at a 90 to the left of the home. The measurements of the porch will be 31'x10'. This guy is doing the same thing I am wanting to do.Shed Porch roof tying in.... Can anyone help me find the common rafter length or what other info is needed to help with finding the length?


I link his page because the drawing shows what I am wanting to do, but I may not be able to go all the way up to the garage ridge because I have a sky light that is siting in the front entry hall and 1/4 of it sits below the garages ridge height.


Last edited by CleIndiana; 11-08-2009 at 02:32 PM.
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Old 11-08-2009, 04:34 PM   #2
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Need help figuring out rafter lengths.


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Originally Posted by CleIndiana View Post
I do not know anything about framing but I am also not scared to get in there and try it. Because of what I have read on here I will try to give as much info as I can. What i am going to attempt to build is a shed still roof attached to my existing house. The house is a single story ranch with a garage that is attached at a 90 to the left of the home. The measurements of the porch will be 31'x10'. This guy is doing the same thing I am wanting to do.Shed Porch roof tying in.... Can anyone help me find the common rafter length or what other info is needed to help with finding the length?


I link his page because the drawing shows what I am wanting to do, but I may not be able to go all the way up to the garage ridge because I have a sky light that is siting in the front entry hall and 1/4 of it sits below the garages ridge height.
What is the pitch of the existing roof and what size rafters are they? Also, what is the length of the existing rafters and the measurement from the existing wall to the bottom of the skylight measuring up the rafters?

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Old 11-08-2009, 04:44 PM   #3
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Need help figuring out rafter lengths.


sorry i knew i would forgot info. its a 4/12 pitch with 2x6 rafters. from the bottom of the sky light to the edge of the facia is around ten ft and the length of one full length rafter is around 14ft. sorry only rough estimates. cant really tear anything apart while in the planing. sorry i added the over hang, so subtract that and its 8' 6"

I should also mention that I am going to tie into the side of the existing rafters, unless the ledger on the roof is a better idea. it seems to me youll be able to get sheathing to lay more flush by tting into the side of the rafters.

Last edited by CleIndiana; 11-08-2009 at 05:03 PM.
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Old 11-08-2009, 06:13 PM   #4
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Need help figuring out rafter lengths.


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sorry i knew i would forgot info. its a 4/12 pitch with 2x6 rafters. from the bottom of the sky light to the edge of the facia is around ten ft and the length of one full length rafter is around 14ft. sorry only rough estimates. cant really tear anything apart while in the planing. sorry i added the over hang, so subtract that and its 8' 6"

I should also mention that I am going to tie into the side of the existing rafters, unless the ledger on the roof is a better idea. it seems to me youll be able to get sheathing to lay more flush by tting into the side of the rafters.
What size rafters are you using on the porch rood and will the top of the porch roof header be flush with the top of the existing top plate?

As far as the rafters running along side the existing rafters or sitting on top of the existing rafters with sleepers and getting the sheathing to lay flush, it doesn't matter, it lays flush both ways. Ledger is easier.
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Old 11-08-2009, 06:55 PM   #5
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Need help figuring out rafter lengths.


I was going to use the same as the house 2x6 nd i would like the new top plate match with the existing. by using the spleepers would i nail them perpendicular to the existing rafters?
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Old 11-08-2009, 06:56 PM   #6
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Need help figuring out rafter lengths.


Sometimes it's easier (for me, at least) to put up the beam or wall that the roof will sit on at the outside edge, then put the ledger where I want it. After those 2 items are in place you can figure the rest out with strings and straight-edges. You can nail a block up on the outside edge of the beam/wall with a nail set at the desired throat height and set a tight string from that point to the top of your ledger. You can then measure exactly what length and angle you need in 3 dimensions. After you get the straight "common" section in you string/straight-edge along the top of the rafters to determine where your valley lays and measure those rafters in as well. More often than not (but not always) you find something is not exactly right when you try to calculate from an already built roof.

You can put the ledger right on top of your shingles in many cases, which helps if you can't get the whole thing built and dried-in quickly. Also, getting the sheathing down flush doesn't really make a difference, as the plane remains the same and flashing will bridge the gap, or you can just cut a bevel into the decking so it sits flat on the roof.
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Old 11-08-2009, 07:19 PM   #7
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Need help figuring out rafter lengths.


So your saying i can put the ledger any where i want and then figure out my rafter lengths?
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Old 11-08-2009, 08:22 PM   #8
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Need help figuring out rafter lengths.


That's exactly what I'm saying. I'm not saying you can't figure it out with a calculator, I'm just saying you don't have to do it that way. I've built many-a-roof both ways. When building the whole thing from scratch I use a calculator to make a cutlist for the whole thing and cut 95% of the rafters before setting the first ridge, but when adding onto an existing roof I usually calculate the lengths just to get a material list, then I physically measure to make sure I get a tight fit. I believe you were referring to another post that had a picture of a string in place just as I described.

A ledger is installed perpendicular to rafters.

If somebody gives you a rafter length all you'll have to do is set it on top of the beam and go from there, either way it's essentially the same thing. You'd use the rafter to determine where to put the ledger, or use the ledger to determine what the rafter is.
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Old 11-08-2009, 09:58 PM   #9
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Need help figuring out rafter lengths.


So from what your saying I dont have to go that far up my existing roof in order to get the same pitch. I can set the ledger board say about four ft up from the top plate then figure everything out from there. i guess i see that picture of that other guys roof and just assume I have to go that far up but maybe he was doing it just for aesthetics.

That is why I;m asking all the questions because in my county i need to have a scaled drawing for them before they will issue me a building permit. and it would help me on figuring cost for wood. Also i dont care how much roof damage is done becaus ei plan on also putting new roofing on after this is completed.

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Old 11-08-2009, 11:36 PM   #10
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Need help figuring out rafter lengths.


You won't get the same pitch. What I'm saying is if you know your general rise and run, you can place your ledger and measure an actual length for a rafter. You have to have a minimum pitch for shingles to work on a roof. The industry standard anywhere I've lived or built is 4/12, but I've seen older homes with 3/12 shingled roofs.


Best thing to do would probably be to post a simple drawing, even a clear picture of a hand-drawn plan with dimensions of your roof and location of the skylight, etc... I've submitted plenty of hand-drawn plans for permit. A cross section of the roof/ceiling framing would be most effective.
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Old 11-09-2009, 05:18 AM   #11
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Need help figuring out rafter lengths.


Joe is the man he helped me with my rafters for my porch. Do a search on porch roof I have pictures there.
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Old 11-09-2009, 08:46 AM   #12
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Need help figuring out rafter lengths.


Yeah I have read you post you kinda have it easy being that you can put the ledgers against a wall, I'm tying into a existing roof. Back to nailed it once I get the new top plate up for the porch would just be easier to figure out what I would need to do for a 4/12 pitch and run numbers over and over until I get the length for the rafters for that pitch. I'm afraid that I'm going to have to settle with a 3/12 or 2/12 because of the sky light. Also saw that Joe uses sketch it so might try downloading that and try to give you guys a picture to look at
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Old 11-09-2009, 09:26 AM   #13
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Need help figuring out rafter lengths.


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Originally Posted by CleIndiana View Post
sorry i knew i would forgot info. its a 4/12 pitch with 2x6 rafters. from the bottom of the sky light to the edge of the facia is around ten ft and the length of one full length rafter is around 14ft. sorry only rough estimates. cant really tear anything apart while in the planing. sorry i added the over hang, so subtract that and its 8' 6"

I should also mention that I am going to tie into the side of the existing rafters, unless the ledger on the roof is a better idea. it seems to me youll be able to get sheathing to lay more flush by tting into the side of the rafters.
With those dimensions keeping the shed roof top plate/girder at the existing plate height and using a 2x6 for the shed roof, your pitch will only work out to be 1-11/16:12.

Here's a quick sketch. Can you drop the girder height of the porch so that you can a least get a 3/12 pitch?
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Need help figuring out rafter lengths.-porch-shed-roof-diy.jpg  
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Old 11-09-2009, 09:40 AM   #14
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Need help figuring out rafter lengths.


What is the height from the top of your patio to the existing top plate? What are you planning on using for the girder/headers? Is this a closed in porch or an open porch with a girder and posts?
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Old 11-09-2009, 10:09 AM   #15
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Need help figuring out rafter lengths.


Thanks joe for the drawing, its not going to be closed in I'm going to put in 6x6 post trying to utilize only 3 post. I plan on using 2x12 for the girders that way I get more span with less post. The porch doesnt have to be exactly the same hieght as the existing hieght but I would like to stay around 7ft for the hieght of the new porch. I'm not at home to measure but I would say the existing wall is aroung that same hieght once the pad is in place.

I believe code in my county requires a 2/12, unless I can squeeze the 1 11/16 /12 past them, hell its close enough to 2/12 to me.


Last edited by CleIndiana; 11-09-2009 at 12:43 PM.
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