Roof rafters

[OlJarhead]

Just trying to wrap my head around my rafters.  Here is a drawing I did:


Let me know what I need to do/think about etc to make these work right.

I'm trying to build with a 12" overhand so my plan is to cut 2x6x12's at 45 degrees at the top and then notch them initially at 1 1/2" on 45 degree angles to create the notch, then fit them and adjust until the notch allows the rafter to sit on the wall and meet the beam flush with a nice 45 degree pitch.

What do you think?
Thanks!

[OlJarhead]

Also I assume nailing straight in from the beam into the rafter is possible?  At least one side?  Then toenailing the rest.

Or is it just better to toe nail the whole thing and put hurricane clips on the wall?

Figure 24" bird blocks between rafters -- do I also need other blocking?

Collar ties at 4' except I'm wondering if they are required in the loft since that might hurt head room a bit.

Of course in the vault it would be fine.

Floor joists in the wall at 7'6" (2x8's) will be the rafter ties and set on 16" centers for the sleeping loft.

[MaineRhino]

I used a program called Easy Rafters. It had a free 30 day trial, enough time to play with the dimensions and print what I needed. It will makes things much easier!

[OlJarhead]

I used a program called Easy Rafters. It had a free 30 day trial, enough time to play with the dimensions and print what I needed. It will makes things much easier!

Shazaam!



Looking at this I'm thinking I now have the plans   Just need to cut them and nail them on!  WhooHoo!
Thanks for the program info!  AWESOME!

[OlJarhead]

I calculated this with 14'7/8" width (14' cabin with 7/16th sheeting on both sides.

Was surprised a little becuase the base of the ridge beam will be at 6'10 13/16" rather then the 6'8" in the plan.  Did I miss something?

[OlJarhead]

One question I have is whether or not lookouts will work with 2x6 rafters?  I'd have to notch the rafter for the 1 1/2" 2x4 framing to provide the 12" overhang.  Seems stronger then a simple ladder that would need support anyway but that means the rafters on the end walls will be 4" thick at the notches...probably ok but I'm wondering.

[devildog]

I went to the lirary and checked out three dvd's by a guy named Larry Haun. There called framing floors, walls ,roofs or something like that. Check them out they show and explain in great detail how to do this

[John Raabe]

That diagram looks fine to me. Most builders would snap out chalk lines on the deck using real dimensions and then cut a master rafter to test the fit before duplicating it for the other side.

The plan dimension is to give an idea for the head height, the exact position of the ridge board depends on the notches, deck width, etc. You will determine this position when you set the first rafter pair. It is unlikely to be exactly what the program states.

[Don_P]

Your birdsmouth in the easyrafter sketch must be somewhere around 2-9/16" to yield the 5-3/16" HAP (height above plate).

I'm coming up with 89-3/8" light, overall rise above plate (for a 12/12 its easy, rise (which is the same as span here)+HAP). Plumb depth of the ridge is 7.778". Bottom of plumb cut, or ridgeBOARD, 81-1/2" above plate.

COS of 45* is .7071
Line Length Ratio is 1.414, imprint those numbers in your noggin, they will get you out of a jam.
Example: 5.5/.7071= 7.778 or 5.5x1.414=7.777 plumb cut height
84.125/.7071=118.97 or 84.125x1.414=118.95 rafter length

[OlJarhead]

Your birdsmouth in the easyrafter sketch must be somewhere around 2-9/16" to yield the 5-3/16" HAP (height above plate).

I'm coming up with 89-3/8" light, overall rise above plate (for a 12/12 its easy, rise (which is the same as span here)+HAP). Plumb depth of the ridge is 7.778". Bottom of plumb cut, or ridgeBOARD, 81-1/2" above plate.

COS of 45* is .7071
Line Length Ratio is 1.414, imprint those numbers in your noggin, they will get you out of a jam.
Example: 5.5/.7071= 7.778 or 5.5x1.414=7.777 plumb cut height
84.125/.7071=118.97 or 84.125x1.414=118.95 rafter length

haha ever watch cartoons when one of the characters shakes his head back and forth really fast and it makes a funny (blubba blubba blubba) sound and then there are little stars and things twinkling above his head but he starts to think again?

I was doing that a moment ago! hahahha 

[poppy]

I don't think Don was making a joke.  He does write some funny things, but when it comes to framing calcs. he is usually fairly serious.

I respect his posts in this forum; they are usually thoughtful and helpful.

[OlJarhead]

I don't think Don was making a joke.  He does write some funny things, but when it comes to framing calcs. he is usually fairly serious.

I respect his posts in this forum; they are usually thoughtful and helpful.

Don't worry I realize that he was being very precise and serious.  I was just looking at the calcs and had to clear my head becuase while I work with technical stuff all day his post was just, well, very technical!  It had me trying to decide if I could absorb it all and print it and keep it handy becuase lord knows I won't remember it without some help.

No disrespect meant at all!  Just trying to make light of the fact that he's very far in advance of my limitted knowledge and experience.

Erik

[Don_P]

None taken. One thing forums do is allow different voices to explain the same thing in different ways. As long as one way is understood and gets you where you're going you're in business. When we teach another, do we not also teach ourself? Sometimes something unforseen

I also answer questions as I did here, perhaps not for the questioner, but for a reader who might be following along looking to pick up another tool. Or to clarify something that might be muddy so another reader doesn't continue down an ever muddier road. This is often what I read forums for, gleaned knowledge. In that light I'll break this down further.

Take a framing square and measure from the 12 on the blade diagonally to the 12 on the body, 16-31/32". Divide 16.97" by 12"= 1.414. The ratio of span to rafter length is 1.414 or, to put it another way, for every inch of horizontal span the rafter run is 1.414". Knowing the ratios of the sides of the triangle of your roof can come in handy since that triangle appears in many parts, the plumb cut, the heel cut, the overhang, etc.

Still there? Now divide 12" by 16.97", just another way of expressing the ratio this time setting the rafter run to 1 and the sides are 70.71% the length of the rafter. For every inch of rafter length the horizontal run is .7071"

I did so well at algebra/trig I took it 3 times just to show everyone else how easy it is 

[OlJarhead]

Pythagoras Theorem

A²+B²=C²
(12x12)+(12x12)=288
Sqrt of 288=16.97

It's funny that when I read your post the first time I was looking at numbers knowing that I should see it clearly but for whatever reason I did not.  I took them to be correct but decided I'd had the rafter figured (DOH)

So, it's clear really that reading your post again was wise of me   becuase I then drew out a triangle, use the theorem and saw the same results -- now it makes sense.

Funny how sometimes people thing differently!  Drawing the right angle triangle and then doing the math made sense.

Funny thing also, it's one of the few things I remember of my higher level math classes!  Hmmm.....I'm probably not smarter then a 5th grader I'm guessing 

OK let me take a crack at this so I fully comprehend (ok maybe partly) the math.

7' 7/16" from center to outside of wall.
7' 7/16" - 3/4" (1/2 of beam width) = 6' 11 11/16"

So to make a 12/12 roof with that dimension then it must rise that as well as span that right? So..

A=6' 11 11/16"
B=A

A²+B²=C²
48.5809' + 48.5809' = 97.1618'
sqrt of 97.1618' = 9.857'

So the length of the rafter from the edge of the wall to the top of the rise should be 9.857 feet or 9 feet 10 1/4" give or take a 32nd or two

But if I want to ad a foot of overhang I have to account for that and it's not actually just another foot of rater but using the 12/12 rule it should be easy really since for each 12 inches of width I will have 12 inches of rise therefore I would need a total of 16.97" of rafter to get to one foot out from the wall (right?)

Or my rafter should be a total length of:
9' 10 1/4" + 16.97"
11' 3.824" or 11' 3 13/16" (give or take a 32nd).

Which darn near works out to what the drawing shows    

[Don_P]

There it is, method 3. Using the same methods and adjusting the birdsmouth to desired you should be able to figure the length of the prop.
Here are some simple backchecks;
http://windyhilllogworks.com/Calcs/rftllclc.htm
http://windyhilllogworks.com/Calcs/ridgehgtclc.htm
John's method of lofting it on the deck is also tried and true, that's how my Dad showed me.
One more set of lookouts and we should be throwing ply today 

[Don_P]

Off on a side journey...
Since we started playing with trig and line length ratios I thought it might be useful for some to show how the load traces though the rafters. Using this simple truss calc;
http://www.jhu.edu/~virtlab/bridge/bridge.htm

I drew a 12/12 rafter/ceiling joist triangle. Set it on supports at the ends and hung a 200 load from the peak. The black line where the 200 load is also describes a right triangle, notice the reactions of ~100 each. Each right triangle side carries a 100 load. The bottom chord has 100 units tension in it, So each adjacent side of the triangles carries a 100 load. The hypotenuse carries a 141.4 unit compression load. For every pound of vertical load there is a 100 lb horizontal thrust load delivered by the rafter at an angle. Remember the ratios of the triangle as we drew the rafter. Here are the identical ratios being used to develop the load path.

Carry that concept one example further. A 6/12 pitch has double the tension load in the bottom chord... look at the ratio of the 2 sides 6/12. A² +B²=C²..(6X6)+(12X12)=180. Square root of 180 is 13.41...13.41/12=1.118. The line length ratio for a 6/12 pitch is 1.118.

200 units of loadX1.118=224units

So geometrically for every foot of horizontal run the rafter is 1.118' long.
Engineeringwise for every unit of vertical load the horizontal thrust is doubled.. 6/12. For every unit of horizontal load the compression load travelling down the axis of the rafter is 1.118X horizontal load.
You can see how both disciplines are using the same geometric information to follow their respective paths in designing the rafter. On the most basic level also notice how dramatically the horizontal thrust increases as the pitch lowers.

[MountainDon]

That's a good exercise to run through to gain an understanding of the mechanics of design.

That takes me way back to Mechanics (of Engineering) 101.   

[georgevacabin]

So a few questions from us non-engineering, liberal arts types . I'm looking at the 12/12 clipped roof and would like to use 2x4 @ 24" oc  (btw, thanks for the easy rafter tip)

1.  How do I determine the run?  Is it 7ft for the 14x24 cabin (half the cabin width)?

2.  If I use 2x4 rafters do I also use a 2x4 ridge beam?

3.  How do I determine heel and seat?

4.  Is there a detailed visual on how to actually cut the clipped end of rafter?  (not clear in plans or my framing book)?

Thanks ahead of time!

[MountainDon]

Here's a link to a website with an online rafter calculator

http://www.blocklayer.com/Roof/GableEng.aspx

Many of your questions will be answered by entering data and seeing what the program draws. This program doesn't give any afvice on whether or not the rafter is properly sized.

For sizing go to...

http://www.awc.org/calculators/span/calc/timbercalcstyle.asp
or
http://www.awc.org/calculators/span/reversecalc/reversecalc.asp

The ridge would usually be one size up; 2x6 rafters and 2x8 ridge board for example.

[georgevacabin]

Thanks MD!  I will take a look at the links.  I've spent all day thinking about the cabin roof that I forgot I have to get ready of tonight's shindig!  Happy New Years!  Have fun/Be safe.

[Redoverfarm]

George I don't want to sound critical but you should rethink the 2X4 rafters.  I would not go anything less than 2X6.  Even then so your insulation space will be comprimized with that to about an R-17.  Not to mention that you do get snow.  Not that much in the past but occassionally you get dumped on and I would want the little bit of added security.  If it were mine I would also go with 16OC.  The added cost of the roof would be very little in comparison to "peace of mind".  Sort of like the Advantex on the deck.

In regards to setting the rafters against the ridge board you could always make youre self a jig to set them in at the top.  I have a little something in mind but I don't know how to describe it.  I got a new prim=nter for Xmas that also scans.  If I can figure it out I will sketch up something and send it to you.

[John Raabe]

I concur with Redoverfarm, 2x6 would be a minimum rafter size.

If you are using my Little House plans kit to build your 14' wide cabin, see the Steep Gable Roof detail on page 5 of the plans. We do not use 2x4 on even the smaller buildings (plans include cabins of 10' and 12' width).

The plans detail above has additional information about how to tie the rafters together at the bottom for a loft or open ceiling configuration and how to vent and strap the top. These are also important to get right.

[Redoverfarm]

George excuse the crudeness but I think you can get the jist of what I thought of.  You can start on one end and just slide it to the other stopping at your layout( 16-24"OC).  This will hold one end of the rafter at the ridge.  You could also use a 2X6" for the bottom of the jig to give you more of a foot to hold the rafter.

[Don_P]

If you are doing what I call bobtail rafters, I just temporarily nail a horizontal 2x6 on the outside of the wall sticking above the plate. The tailless rafters sit on the wall and slide out till they bump into the 2x6 and it's hammer time. Joist hangers nailed laying on their back to the plate would also do the same thing and although not a code use it would work well for this size building. You are squeeking the numbers on a 2x4 roof but I'd use 2x6... I'd also use an overhang, keeping the walls dry is money well spent.

[OlJarhead]

I used 2x6's on ours with a 2x6 ridge beam which allows a nice 1" gap above the ridge beam for venting.

I calculated the width as half of the width of the cabin as measured to the outside of the walls.  So a 14x24 with 7/16" sheeting (x2) divided by two gives the center line.

Then you take half the width of the ridge beam and start your rafters there.  Easy Rafters did this nicely for me

Once I'd gotten that far and screwed up I reread Johns plans and took his advice by laying out the rafters on the floor with a block for the ridge beam and a 14' 2x6 as the wall top.  Once I'd done this I could see my mistake clearly and was able to correct.

One other thing I did was make my rafters only 2 or 4 at a time.  This is becuase no matter how much I tried my cabin walls were not perfect (maybe it's just me ) so this helped a little as I had to adjust a little a couple times so I had a nice even ridge (within 1/4" anyway).

Once I was nailing them down I found it got easy enough to slide the top in place then bump the birds-mouth onto the wall.  I sometimes also used hurricane ties laying flat on the wall and nailed into the top of the wall and the rafter as an added strength thing -- don't know if that's good or bad though.

Erik