# 12/12 Roof Pitch - cutting rafters



## Joe Carola (Apr 14, 2006)

JustAHack said:


> Now... where to go from here...
> I planned to get my ridge board up so that the top of the board is 6' 1/2" high off my wall. That should give me my 45 degree sloped roof - right?


Dan,

Do not set your ridge at that height because it's wrong. You can't do that because you not figuring for the HAP cut which is the "height-above-plate" after you cut your birdsmouth and deduct 1/2 the thickness of the ridge.

Do you know how to figure out the rafter length? If so, there is no need at all to pre-set a ridge. You nail your front and back rafters to the top of each side of the roof. You then lift the ridge up in between the plumbcut of the top of the rafters and slide the ridge flush to the top of the rafter. That sets your ridge height.


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## JustAHack (Apr 13, 2010)

Hello Joe - 

I see your point about the height being wrong... obviously not the right way for me to do it. I understnad the right way that you explained. Looks like I will have to get a second set of hands to place the ridge board in place. 

So... how do I calculate the length of the rafter, and placement of the birds mouth, after I cut the plumb cut? My shed is roughly 12' wide.

I am heading out now to get the siding on.

Thanks...
>Dan


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## Joe Carola (Apr 14, 2006)

JustAHack said:


> Hello Joe -
> 
> I see your point about the height being wrong... obviously not the right way for me to do it. I understnad the right way that you explained. Looks like I will have to get a second set of hands to place the ridge board in place.
> 
> ...


Are you using a framing square, Construction Master Calculator or Scientific Calculator to figure the rafter lengths? What is the overhang?


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## Joe Carola (Apr 14, 2006)

Here's the rafter length with a 1' overhang for now.


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## joed (Mar 13, 2005)

A 12/12 pitch is a simple 45º angle. Use a speed square.










Here's a link that might be useful

http://www.construction-resource.com/calculators/roof-frame.php


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## JustAHack (Apr 13, 2010)

Don't get much light outside this time of year. It is not fun hanging plywood on the walls by yourself. Also with the back of the shed elevated about 2' - the front is level. Kind of a funny thing... I framed the walls in 2X4 PT - the 2X4 PT was on sale for $1.90/piece and the KD was $2.60/piece. The PT was all pretty straight too. At least for now when it is wet, we shall see what happens when it dries out.

Anyway - both Joes - thanks for the response. Joe C - the picture you put in there is great - thanks ! Let me ask this though... how did you calculate all of that? I may end up having to re-calculate a little bit depending upon what my final with is... it will probably be between 12' 1" and 12' 2". I checked out the calculator that Joed provided. When I put in 145" for the width, 12/12 pitch, and a 1' overhang - it came out with "Wall to Ridge Rafter Length" of "-9 ft 5-8/16 inches" and "Overall Rafter Length" of "-8 ft 10-7/16 inches". I don't know what the "Plan Distance" is - so I left it at 0. What is that?

Overhang is simply preference - correct? I figured I would just leave them long (since I have to use 10' stock anyway) and then cut them to the length that looks the best after. I assume it is just easier to cut them to the correct length before installation. Personal preference I assume.

I have a few other Qs if you dont mind... 
- I plan on using architectural roof shingles. Do I just use 3 tab shingles (cut apart) to do the top cap?
- I will use drip edge on the roof. Does the tarpaper go over the drip edge? 
- Years ago, I shingled a shed roof using 3 tab shingles. I put the first row of 3 tab shingles upside down with the tabs pointing toward the peak. On the left vertical edge, I put the tabs pointing to the right and on the right side I put the tab pointing to the left. Basically I made a "U" out of the 3 tab shingles. I then shingled normally. Should I do this same thing, making the "U" under the architectural shingles? Did I explain this well or are you confused?
- I am using 1/2" 4 ply plywood for my walls installed horizonally. I am going to install "SmartBoard" - that T-111 composite over the 1/2" plywood on the sides and back of the shed. On the front I am going to do clapboards with a 5' door in the center and 2 - 24X24 windows, one on either side of the door. I am going to build the doors out of T&G pine. Maybe I will try that cement clapboard. Is that cheaper than cedar? My question is... should I tar paper (since I dont have Tyvek) over the 1/2" plywood? Or am I just wasting my time putting on tarpaper? Is there a point to do this?


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## BigJim (Sep 2, 2008)

If you have installed the 1/2 inch plywood and the T 1-11 to the top of the top plate before you put your rafters on you will have to re-figure your rafters as your building is now wider by the thickness of the T1-11 and 1/2 plywood on each side of the building. 

The way I figured my rafters is this. If you look at the gable end of a building you will see the rafters and top plate across the front form two triangles back to back. Dealing with one side you have a triangle. You can figure the hypotenuse of the triangle and have the length of your rafter. Formula is A squared + B squared = C squared. 

Example a building with a 20 foot span will have a run of 10 feet. 10 feet is the base of the triangle. The pitch is 12/12, 12 X 10 = 120 inches, the vertical leg is 120 inches. So "A" squared is 10 feet so we convert it to inches which = 120 inches "A" squared is 120 X 120= 14,400. "B" squared is 120X120= 14,400. Now add "A" and "B" together = 28,800. Find the square root of 28,800 which = 169.70562 inches long. Take 1/2 the thickness of the ridge board off that measurement and there is the length of your rafter. 

Now remember this is the line length of the rafter which is from the tip of the rafter at the top (after the plumb cut) measured down the top edge of the rafter. Make a mark on the top edge of the rafter at 168.955 inches (providing the ridge board is 1 1/2 inches thick) this is where your birds mouth is, take your square and make another plumb mark from that mark all the way to the bottom edge of the rafter, that will be the back edge mark of your birds mouth. Now all you have to do is make the level cut mark of the birds mouth and you have it. Don't forget the 1/2 inch plywood and T1-11 on each side of the building if it is installed before your rafters.

The way I made my level mark of the birds mouth was to take a 2X4 (if the top plate is 2X4) place the 1 1/2 edge against the plumb line you just made for the back line of the birds mouth and slide the 2X4 up the line until the top inside edge of the 2X4 is flush with the bottom rafter edge. In other words, you want your birds mouth level line to be the same width as the top plate. I hope this helps.


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## Joe Carola (Apr 14, 2006)

JustAHack said:


> Anyway - both Joes - thanks for the response. Joe C - the picture you put in there is great - thanks ! Let me ask this though... how did you calculate all of that?


I used a Construction Master Calculator. What were you planning on using?



JustAHack said:


> Anyway - both Joes - thanks for the response. Joe C - the picture you put in there is great - thanks ! Let me ask this though... how did you calculate all of that? I may end up having to re-calculate a little bit depending upon what my final with is... it will probably be between 12' 1" and 12' 2". I checked out the calculator that Joed provided. When I put in 145" for the width, 12/12 pitch, and a 1' overhang - it came out with "Wall to Ridge Rafter Length" of "-9 ft 5-8/16 inches" and "Overall Rafter Length" of "-8 ft 10-7/16 inches". I don't know what the "Plan Distance" is - so I left it at 0. What is that?


You have to have the exact run of the rafter minus half the hip thickness to the outside of the plate, or if you have your sheathing on as was mentioned to try one of those calculator sites. What will your exact width be, and are you sheathing the walls before you put the rafters on?


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## JustAHack (Apr 13, 2010)

What was I planning on using? I didnt quite know as this is my first venture into building a roof. 

I plan on sheathing the walls first - so I am not sure what the exact width will end up being quite yet. I will let you know once I figure it out... maybe you can plug the numbers in for me again?


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## Gary in WA (Mar 11, 2009)

With that siding installed per instructions, you don't need any plywood under it. Water resistive barrier is required: http://www.icc-es.org/reports/pdf_files/ICC-ES/ESR-1301.pdf

Gary


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## JustAHack (Apr 13, 2010)

GBR in WA - thanks much for that info. Looks like I spent a little bit more than needed on the shed by installing the 1/2" plywood first - oh well. This shed will outlive me - that is for sure. The install instructions on the SmartBoard was very helpful. I was wondering how to install it and now I know. I probably never would have put the batt down the seam - but now I will. It has been raining here in MA the last few days... hopefully this weekend will be dry so I can get back out there and work on this thing.


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