Common Rafter Layout: How to Measure, Mark, and Cut Rafters for a Gable Roof

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Reducing roof framing geometry to framing lumber with a calculator, a speed square, and a circular saw
A gable roof is almost the simplest roof you can frame aside from a shed roof, which is basically half of a gable roof. The gable roof frame is a series of identical rafters spaced equally for the length of the house held together with sheathing.
For a simple regular gable roof, the rafters on the left side are identical to the rafters on the right. So it all boils down to laying out one rafter.
The rafter is the hypotenuse of a right triangle. We can measure the run, and the roof pitch is noted in the plans, but we need to figure out the rise and the rafter length.
The Pythagorean theorem (a2 + b2 = c2) describes the relative lengths of a right triangle. You can calculate that with a pencil and some OSB, but because we’re paying you by the hour, you’d better use a construction calculator, which you can download on your phone.
How to calculate the length of a common rafter
To lay out the common rafters, we will need to know
The building width (30 feet exactly)
The width of the ridge (1-1/2 inches)
The overhang (8-inchers, including the subfascia)
The roof pitch (6/12)
If there were no ridge, the run would be half the width from the center to the outside of the wall. In reality, you’ll need to subtract half the ridge from the measurement.
So, after marking the center line, also mark each edge of the ridge. And don't forget to include the exterior plywood wall sheathing in the dimensions. It doesn't always make it up to the top of the wall plates when someone is pulling numbers for rafter measurements.
Hook the outside of the building with your tape and measure to the inside of the ridge. That’s the run, 14 feet, 11-3/16 inches in this case.
Pitch is shown on the plans and is indicated as a fraction, usually with 12 being the denominator. The first number is the rise; the second is the run. A 6/12 roof rises 6 inches for every 12 inches of run.
So how do you get from expensive new lumber on the saw horses to a perfect fit on top of the walls?
With a calculator and then a saw. Punch this into your calculator.
14 [feet]
11 [inch] 1/4 [run]
6 [inch] [pitch]
[diagonal] gives us the rafter length (excluding tail): 16’ 8-7/16”
Marking and Cutting a common gable roof rafter
That’s the calculator part, now comes the saw part.
In order to measure the rafter accurately, you’ll need to make the plumb cut on the upper end so that you can hook your tape on it.
Use a speed square to draw the correct angle. You can use the degree scale along the outer edge, but the common rafter scale is much faster and easier. Rotate the square until the edge of the rafter lines up with the 6 on the common rafter scale and scribe the edge.
The speed square in the video is one of the small ones that fit in your nail bags, but you can buy bigger ones for laying out wide rafters.
After making the plumb cut, hook the long point and measure along the top of the rafter to 16’ 8-7/16” and mark that plumb line. The rafter should sit on the full width of the wall, so square a line off the plumb line that is six inches long; this is the seat cut. Collectively, this triangle notch is called a bird’s mouth.
The overhang is 6.5 inches past the plumb cut (8 inches minus 1-1/2 inch subfascia), so mark that plumb cut. Square the soffit cut by squaring over from the bottom of the subfascia.
After making those cuts, you’ll have a perfect full-size pattern for laying out the rest of the rafters.
A rafter jig is lightweight and easier to handle
To make the process of cutting a roof full of rafters a little easier, make a rafter jig with a leftover piece of subflooring, and a 1x fence nailed into the edge. It is a miniature rafter you can use to scribe the plumb cut and the seat, bird's mouth, and soffit cuts.
The fence stops just short of the plumb line so that you can line it up to the measurement mark. With this little and a little knowledge, you can make quick work of that pile of rafter stock.
How do you get them all up there?
That’s the ruff part.
(It's also the next part in this series)
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[@james4wd236]

I've watched like 15 videos about how to make rafters and cut the birds mouth... this is by far the best rafter video on planet earth. Thanks!!

[@ProTradeCraft]

Wow. Best on the planet? Thanks. I mean, I'm not sure how many rafter cutting videos there are on the other planets, so...

[@james4wd236]

@@ProTradeCraft I was just eager to give praise...

[@ivangonzalez5425]

I agree ! Super detailed and couldn’t be described better ! Thanks definitely saving

[@KewellXi]

This is the best video about the rafter!

[@ProTradeCraft]

Thank you. I like it too.

[@istabraq4]

Best video ever seen on common rafters , great work my man 👍

[@ProTradeCraft]

Thank you. I am glad you liked it.

[@waltercalderon5762]

This is such a great video! Clearly explained, down to the cut! I would love to be that good at carpentry.

[@ProTradeCraft]

Glad you like It, Walter, I will expand beyond roofs to walls and floors in the coming weeks and months.

[@AmishHitman73.Archive]

thanks!@@ProTradeCraft

[@joshsager-gk8ly]

I don’t know what it is but carpentry/framing is difficult for me to understand. Even with videos like this, I’d have to watch it multiple times (fyi, I’m not in the construction trade, just want to learn about it).

[@ProTradeCraft]

It is even harder at the top of a ladder with a saw in your hand!

[@genollanas2110]

Really great video man, well done

[@ProTradeCraft]

Thank you.

[@aeronez6447]

How to get them up there that's the ruff part haha 😄

[@ProTradeCraft]

glad you liked it.

[@AmishHitman73.Archive]

I am stunned this video is superb

[@ProTradeCraft]

Thank you. It sucks that this is stunning, because that I cannot possibly be that much smarter than all of the other people making roof framing videos.

[@BG-uf8kh]

Great stuff n soo easily understandable 👍

[@ProTradeCraft]

Glad it helped. I just had another person saying I was talking too far over most peoples' heads.

[@Gigidy1]

Awesome video! The comments cracks me up 😂

[@ProTradeCraft]

Glad you like it. Gotta keep from getting bored somehow.

[@bvisbuilds]

Great, informative video!

[@ProTradeCraft]

Thank you.

[@Vasen1983]

Best video tutorial!

[@ProTradeCraft]

Best? Thank you!

[@JB-mh6xz]

Great video

[@ProTradeCraft]

Glad you enjoyed it

[@gogo12able]

brilliant illustration 😂😂😂 not as hard as the rough part to layem up😅😅😅 there with muscles

[@ProTradeCraft]

thank you, and yup.

[@davidholmes7275]

I’m Amish, I’m not allowed to use a calculator, can you explain that method please?

[@ProTradeCraft]

I feel like you're not being truthful with me. Are you allowed to use google?

[@swav100]

😂

[@BobPeterson-rw9yw]

Yeah first you go somewhere else, then you pound sand.

[@davidholmes7275]

@@BobPeterson-rw9yw That must be what you do when your calculator doesn’t work.

[@davidholmes7275]

No I’m not! I feel like someone is watching me and I’m breaking the law. But I gotta go figure out these rafters in the sand

[@sixpotshot]

Excellent!!! ✨✨✨✨✨

[@ProTradeCraft]

Correct!! (thank you)

[@ARomashchenko]

I've heard framers don't prefer to cut rafters without overhangs ("monopoly framing") Why is it preferable in the big workflow picture to frame a roof with tails?

[@tderozan3868]

To creates eaves which have the water flowig out side the building line

[@ARomashchenko]

@tderozan3868 of course eaves shed water. The question was not about the usefulness of eaves. The question was about the framers' workflow. Monopoly framing involves the extra step of framing eaves on top of the air barrier layer after. (And looks like I may have answered my own question.) But I was still curious if tails are used maybe to help line up the fascia board or something along those lines.

[@tderozan3868]

You certainly did . It's less work to incorporate the tails with the rafter than to do it as a separate task

[@ProTradeCraft]

Adding overhangs after the air barrier is established makes it a zillion times easier to get the air barrier right. Otherwise, you must air seal around every rafter, bridging the inside and out. It is easier for framers to add the overhang to the end of the rafter instead of returning and adding overhangs after the roof and walls are sheathed and sealed-and usually covered with continuous insulation. Many builders do not understand your question because most builders do not build very efficient houses. Your method is best for continuous insulation because it not only makes the air barrier continuous but also eliminates a thermal bridge every 16 or 24 inches on at least two sides of the house. Here's an old animation showing that process in a retrofit situation, but it is transferable to new construction: kzbin.info/www/bejne/gJ7WoKaBZ7KsbNE

[@ProTradeCraft]

It is much harder to detail the air barrier is framing bridges inside and out. Rafter tails on the rafters is the traditional way of framing a roof, but if you want to build superinsulated structures that use very little energy, skip the tails until the air barrier is established and then add rafter tails, like this: kzbin.info/www/bejne/gJ7WoKaBZ7KsbNE

[@chuck-wv9nh]

Who set the ridge height? God?

[@ProTradeCraft]

No. Pythagoras: kzbin.info/www/bejne/bHbcl6yqgseMZqc

[@chuck-wv9nh]

@@ProTradeCraft Thanks

[@chuck-wv9nh]

who determined the layout along the wall? The holy Spirit?

[@ProTradeCraft]

The plywood determined the layout. Well, the person who is going to nail plywood to the walls determined it.

[@TT3TT3]

So hard to understand

[@ProTradeCraft]

16 minus half the thickness of a 1-1/2 inch framing member (3/4 inch). If your engineered floor joists have wider flanges (they will), subtract half of that width from whatever number you want the joists to be centered on. make sense?

[@rykvanderpol4775]

might as well title the video "how to use calculator"

[@ProTradeCraft]

not as easy as it looks.

[@davidholmes7275]

Hahaha

[@averageslam]

Are you trying to teach this to journeyman carpenters who should already know all this? Because you're firing off terminology without explanation of those terms as if your target audience are 4th year apprentices and not youtube viewers looking for pointers. This is coming from a journeyman plumber

[@ProTradeCraft]

It is meant c=for building professionals who are not framers. They are typically familiar with the terminology, and sometimes offednded if I "dumb it down," which I always tru to explain is "clarifying," but it doesn't always get heard. What terms are tripping you up?

[@The2006200]

Im a carpenter and this video makes perfect sense. so it worked for me

[@moeb4348]

Quick question. You use run from the outside of the wall to the center(offset by half the ridge width). This will determine, in conjunction with the roof pitch, the angle of the cut at the rafter ends. But, when you cut out the "birds mouth" you effectively are lowering one end of the rafter thus changing the cut angle where it meets the ridge, so you would have to lower the ridge end the same distance to maintain pitch and angle. Therfore shouldn't one measure run from the inside of the wall so that angle and pitch are unaffected by the cut out for the "bird's mouth"?