Help With Rafters

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Old 06-23-11, 03:24 PM
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Help With Rafters

Hi guys...new to the forum.

I am getting ready to build a patio cover in the back yard. I'm a rookie carpenter, but I can swing a hammer and turn a wrench. However, I do not know how to read a framing square.

I originally planned on a simple shed type roof under the eaves and it has kind of grown from there. I quickly realized that head room issues would severely limit me if I went under the eaves, not to mention for just a little more money I could do it better.

With that said, I now plan to tie into the plate of the exterior wall and put in a 5/12 hip roof. This is going to tie into my existing 5/12 gable. I will use double 2x12's on notched 6x6 posts for the beams. The dimensions will be 16' wide (to outside of beams) and 14' deep (outside of beam to exterior wall) with a 16" overhang.

The first thing I need to tackle will be the ridge board and common rafters. Since its not simply a square room I don't know how to figure out the length of my ridge. I plan to use the step-off method on the rafters. The biggest question I have here is how do I determine where to cut my birdsmouth once I make my plumb and seat cuts for the soffit and fascia?
 
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Old 06-23-11, 03:40 PM
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What you need to do is go down to HD or Lowes and buy a Swanson Speed Square,it will set you back about 10 bucks.With the speed square you'll get a little blue book with it that will tell you in the simplest way to cut a roof in and also give you the exact cuts with the dimensions from your room.I've taught many a rookie carpenter using this little book to give them and it works every time.Also remember that when you make a hip cut,always remember that you use a 5/17 pitch cut not a 5/12 in this case .Like say for instance you were using a 7/12 pitch you would make your hip cut 7/17.
 
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Old 06-23-11, 03:45 PM
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Now I normally like math and I'm sure there is someone out there that can come up with the pencil and paper answer, BUT trial and error is faster for me. When it comes to compound angles and the normal construction variables, just make a good guess, cut it long and hold it up to see how you did. A little here, a little there, and you have an acceptable fit. If it is not as good as you want, use it for a pattern, make adjustments and cut a new one. If you were in the business it would be worth learning the details of how to make these cuts. But for one ridge board, play with some scrap, or even patch some scrap together to simulate what you want. 30 minutes later you are moving on. Hope this work around helps.

PS, you are in a wind prone zone so be sure that roof is extremely well anchored.

Bud
 
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Old 06-23-11, 03:52 PM
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Originally Posted by multitim56 View Post
What you need to do is go down to HD or Lowes and buy a Swanson Speed Square,it will set you back about 10 bucks.With the speed square you'll get a little blue book with it that will tell you in the simplest way to cut a roof in and also give you the exact cuts with the dimensions from your room.I've taught many a rookie carpenter using this little book to give them and it works every time.Also remember that when you make a hip cut,always remember that you use a 5/17 pitch cut not a 5/12 in this case .Like say for instance you were using a 7/12 pitch you would make your hip cut 7/17.
I have a speed square that I've had for years...can't say I ever looked at the book, or even remember it coming with a book. Maybe I'll invest in a new one.
 
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Old 06-23-11, 03:56 PM
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Originally Posted by Bud9051 View Post
Now I normally like math and I'm sure there is someone out there that can come up with the pencil and paper answer, BUT trial and error is faster for me. When it comes to compound angles and the normal construction variables, just make a good guess, cut it long and hold it up to see how you did. A little here, a little there, and you have an acceptable fit. If it is not as good as you want, use it for a pattern, make adjustments and cut a new one. If you were in the business it would be worth learning the details of how to make these cuts. But for one ridge board, play with some scrap, or even patch some scrap together to simulate what you want. 30 minutes later you are moving on. Hope this work around helps.

PS, you are in a wind prone zone so be sure that roof is extremely well anchored.

Bud
I'm not necessarily looking for someone to spoonfeed me some numbers. I like to learn the reasoning behind things. In other words, I want to learn how to figure it out myself. I understand how to mark all of my plumb and seat cuts, the overhang is what's throwing me off.

As for the wind, I will be bolting the posts to the slab using stand-off brackets and will use hurricane ties on all rafters.
 
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Old 06-23-11, 03:58 PM
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Just make sure its a Swanson and it will have that little book.You'll be glad you did.If you try and guess at it and have to recut a little here and keep guessing at it it will be very aggravating and time consuming.I've done this for years and this is the best way to do it.No disrespect to Bud.
 
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Old 06-23-11, 04:06 PM
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ps. The book also will tell you how to make your seat cuts and explain the overhang in simple terms.good luck
 
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Old 06-23-11, 05:41 PM
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None taken multitim56. I remember getting a book with one of my squares and was amazed at all of the cuts one can accomplish with just a simple triangle. But I had a building to get up so went back to the old habits.

Bud
 
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Old 06-24-11, 04:54 AM
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One more tidbit. If you have a small Swanson, buy a 12" one. I carry the small one for framing, but the larger one can't be beat for tracing your angles on wider boards, where the smaller one falls short. It is also nice for 12" tile markings. Just a good combination of tools to have.
 
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Old 06-24-11, 06:37 AM
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Roof



Here is my visualization of the view from above of the old and new roof configuration. The original roof is a gable roof and the proposed patio cover is a hip roof connected to the existing gable roof. The roof pitches of the old and new are the same pitch. Is this close or am I way off?
 
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Old 06-24-11, 09:14 AM
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Yes, that is exactly what I'm planning to do. Attached is a pic of the back of my house. Obviously, the slab has since been repoured. The cover will go from just to the leeft of the back door to just before the kitchen window underneath the roof jack.

 
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Old 06-24-11, 02:02 PM
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Okay, so I went down and picked up a 12" Speed Square and read the little blue book. Man, my head hurts.

If I understand things correctly, I will need to mark a plumb line for my ridge cut, then 8' 8" down I will need to mark another plumb line for the birdsmouth. Then, for a 16" overhang, I will need to mark one more plumb line 17-5/16" down from there for the fascia cut. That would make my common rafters 10' 1-5/16" long, but I would still need to make my ridge cut 3/4" shorter to allow for half the width of my ridge. So, all in all, my common rafters will be 10' 0-9/16" long. Sound right?

I'm still not clear how long my ridge needs to be. The book says leave it 3" long and use a common rafter to determine where to cut it.I don't know where to start to add the 3".
 
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Old 06-24-11, 04:10 PM
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Ridge Board

The ridge board will be same length as the 2 sides of the roof addition. Look at the upper and lower horizontal lines in the gray area of the diagram above. Hope this helps.
 
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Old 06-24-11, 04:54 PM
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Originally Posted by Wirepuller38 View Post
The ridge board will be same length as the 2 sides of the roof addition. Look at the upper and lower horizontal lines in the gray area of the diagram above. Hope this helps.
That's what I thought, but I wasn't sure. Anyone care to check my math above?

One other question (at least for now)...if I want my overhang to match the existing overhang exactly, won't the top plate of the patio cover have to be at the same height as the top plate on the rear of the house?
 
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Old 06-25-11, 02:55 PM
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Patio Cover

.if I want my overhang to match the existing overhang exactly, won't the top plate of the patio cover have to be at the same height as the top plate on the rear of the house?
Yes, and I agree with your calculations.
 

Last edited by Wirepuller38; 06-25-11 at 03:35 PM.
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Old 06-25-11, 05:36 PM
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In my experience you will need to adjust the pitch of the new roof to match the ridge of the old roof. you do want the ridges the same and tied together do you not?
If the new addition is 16 feet wide with a 5/12 pitch then the top of the ridge board will be 80 inches tall. Let's say for guess that the old is 24 feet wide with a 5/12 roof. That means its ridge is 110 inches tall, so if all the walls are the same height then the new roof will fall 30 inches lower than the old ridge.
Now I am not saying that is not good just that you need to be aware of it before you start building and then are confused when the ridges do not match.
I am a bit confused on the hip roof to gable roof thing as a true hip roof would only touch the new roof at the existing exterior wall. Meaning the peaks will be quite a ways apart and I see a potential for snow and ice to build up in this area. Or am I totally off base there?
 
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Old 06-26-11, 11:01 AM
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JTupper - I'm a little late to the party but my recommendation is to use an old school framing square - no book, no math, no headaches.
All of the info you need for framing rafters is already on the square.
As a plus, once you understand the square it is simple to use.

There are lots of sites on the internet that explain how to use it. Here's one.
 
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Old 07-19-11, 11:37 AM
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if the span is 16' then divide that by 2 + the 1'-4" overhang for a total 9'-4" then divide that by the cosign of angle (22.6199)associated with 5/12 which comes out to be 10.1111' or just short of 10'2". This assumes that the end cut will be plumb. If it is a square cut then you need to add that to the length
 
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Old 07-19-11, 12:08 PM
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Seriously?

I have a four year degree in math and I wouldn't advise someone to use that method.
 
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Old 07-19-11, 05:02 PM
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And I thought conduit bending was complicated by sines and cosines. It works, but what a PITA.
 
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Old 07-19-11, 06:45 PM
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And some people think construction workers are dumb!
Stick with me Chandler, I'll show you everything you need to know on conduit bending.
 
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Old 07-19-11, 08:07 PM
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My wife is a cardiac nurse and was amazed that a lowly construction worker was knowledgeable of sine waves. My 3 phase waves are curvy and hers are sharp, only difference. Mine go out of sync it makes "noise"...hers go out of sync, well not a pretty picture.
 
 

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