Hey folks, today we're going to show you how to cut a rafter and build a roof. Now we want to do it quickly, but do it clearly. So I've got the lovely Anna here with me to help me make sure I do that, because I've done this a thousand times, but we need to explain it clearly to somebody that's doing it for the first time. So if I say something that I have not explained, Anna's going to jump in here.
Hey wait, what's that? All right, let's get started. Roofs are made of rafters, and rafters are really nothing more than... boards on a slope so the water will run off and to cut a rafter we really only need to know two things how to make these cuts on either end and how long to make the rafter now to cut a rafter we first have to know the roof pitch this is an 812 pitch and you see how these cuts fit the wall but if we had a flatter pitch say like this they don't fit anymore so you have to know the pitch to make the cuts and we're going to show you how to do that so For a standard gable roof... Oh, wait, wait, what's a gable?
Good question, Anna. Let's take a look at this drawing. Okay, Anna, here is a gable roof. It's just a simple two slopes on either side of a ridge.
You know, you just want to think Snoopy's Doghouse. Now, for a couple other types of roofs, here's a shed roof. It's just a single slope off to one side. And here's a drawing of a hip roof.
And like a pyramid, it slopes on all sides. Down here is a barn or gambrel roof and you know there are a lot of different roof configurations and very often they will combine them on different parts of a single structure. But today we're just dealing with this simple gable roof right here.
Okay the first step in building a gable roof is to lay out and cut the common rafter. Oh wait hold on what is a common rafter? Sure let's take a look. Here we've got a hip roof with a gable sticking out on this side. And in this drawing all the common rafters are highlighted in blue and the ridges in yellow.
Now a gable like this will have a common rafter that will run up this side and then down on the other. And between them they'll pinch the ridge and determine the height of this ridge. Now we also have other types of rafters.
We've got a valley rafter on each side of this gable. We've got a hip rafter over here and we've got jack rafters. But we don't have to worry about those today because a gable end like this is comprised entirely of these common rafters.
So what again is a common rafter? Any rafter that runs from the wall up to the ridge. Okay to cut a common rafter we first have to determine the slope or pitch of the roof.
Now all rafters slope but that slope will vary and to understand it we need to keep clearly in mind the concept of run and rise. And what is that? We take this rafter we set it on the wall here. Run is the horizontal distance covered by the rafter. So that's from right here to the end of the rafter here.
Rise is the amount that it goes vertically in that same distance. So let's take a look at this drawing. This drawing will show you the basics of run, rise, and roof pitch.
Now if we have a rafter that's indicated by this sloping line here, it will begin here and it will run horizontally 12 inches. It will rise vertically 8 inches. And these two points here and here will determine this slope or roof pitch.
And we call that an 8-12 pitch. But what if it's a 4-12? Still beginning here, it's going to run horizontally 12 inches, but it's only going to rise 4 inches.
And so, as you can see with this dotted line, that yields a much flatter slope or roof pitch. So there you have it. Run, rise, and roof pitch.
Here we have a drawing showing the span, run, and rise of a regular gable roof. Now span is the full width of the building, so from right here all the way over to right here. And run would be half of that, so right here in the center.
So the run would be the horizontal distance covered by the common rafter, that's from here to here. And of course the rafter length would be calculated based on that run length. So Anna, what's the run?
The horizontal distance covered by the rafter. Exactly. So on our model here, our span is eight feet. Half of that will be our run right here.
Okay, do you remember when I said you only need two things to know how to cut a rafter? One, how to make the cuts on the ends. Two, how long to make it. Well, now that you understand the principles of run and rise, we're going to show you how to make those cuts. Okay, Ann and I went ahead and put these rafters up because it's important for you to get...
the perspective of the whole. Very often when people are showing you how to lay out rafters, it's down on a sawhorse, and I think people lose the context of what they're doing. So let me introduce you to the framing square. You use the framing square like this to lay out rafters. Now, of course, you can use a speed square like this, but I prefer the framing square both for accuracy and especially for today because I think it more clearly illustrates those concepts of run and rise.
Okay, I want to introduce you to the iconic carpenter's tool that's useful for many things, but especially for marking rafters. Now, it's just an L-shaped tool with rulers along all of its edges. As you can see, it's got inch marks here, here, and on both sides of the body of the square. Now, a common mistake that people make when marking their rafter is mixing the outside scales and the inside scales. So when we're putting this on the rafter, we can put our 8 mark for rise here and our 12 for run down here.
We want to do it on the outside. Or, if you like, you can do it on the inside, here and here, but never mix the two. Either all on the outside or all on the inside.
So that's how we're going to use it to mark our rafters, and we're going to show you that in just a second. Now, often it's kind of hard to see the small numbers on a framing square, so I've modified this one so you can see these larger numbers. So we've got the square like this. Remember our bottom number here represents run.
So from the corner it runs over here 12 inches or one foot and then it rises vertically eight inches on this side. So now we're going to position this square on the rafter and we're going to align the 12 mark with the top edge of the rafter and the 8 mark with the top edge of the rafter. This square is now in position to mark our cuts, both plumb cut and level cut. We mark on this side to make the plumb or vertical cut and on this side to make the level or horizontal cut. And that's all we need to cut a rafter either a plumb cut or a level cut.
Now we would use that to mark this ridge cut here. We also need to make the seat cut or bird's mouth. Why is it called a bird's mouth? And I have no idea. I've cut tens of thousands of these things, but I've never heard the reason why they call it that.
But the seat cut makes sense because that's where it sits on the wall. Okay, now that you know how to use the framing square to make the cuts on the ends, the next step is to find out how long to make the rafter. So there's three basic ways that you can get that.
You can use trigonometry or the Pythagorean theorem. Anna, do you remember high school, the old A squared plus B squared equals C squared? I'd rather not.
No, I'm just kidding. Well, Pythagoras came up with that 2,500 years ago, and it still works if you don't mind doing the math. They also have special calculators that have those functions built in.
The second way is to use a framing square and you can use it to step off the link and we'll show you how to do that in a minute or you can use the tables that are on the side of the framing square and lastly another way is to use a rafter table book. It gives the rafter links and angles for all different roofs of all different pitches. Now the most comprehensive one is Roof Framer's Bible and we'll put a link in the description below so you'll know where to get one. Okay, with any of these methods, you'll need to know the total run of your common rafter.
So, Anna, what was our run? Four feet. Four feet.
From the outside of the wall to the center of the span, four feet. And this next drawing will show you very clearly how we get those lengths. Okay, this drawing will make it really clear.
Remember, we're using rafter runs to get rafter lengths. So if we're going like our model from the outside of this wall to the center of the ridge for a run of four feet and that will give us a rafter length of 57 and three quarters inch. Now we'll show you how we got that number in a minute but right now I want to show you very clearly how this rafter length is measured. Always right along the top edge of this rafter.
So if the rafter run goes to the outside of the wall we want to come straight plumb up here to this point at the top of the rafter. And along this line is where the rafter length begins. Also on this end, since the rafter run is to the center of the ridge, the rafter length will also be to the center of the ridge, right along this line. Now you'll notice the rafter doesn't actually go to the center of the ridge, it stops here on the corner.
So we would need to make an adjustment to the rafter length number for half the thickness of this ridge. Now in our case, in most cases you'll be using a 2x material say a 2x8 ridge and it's an inch and a half thick so we need to take half of that which is three quarters of an inch as shown here at the bottom of the drawing and make an adjustment to this rafter length we also need to add for the tail we have a 12 inch horizontal tail so we would need to add this rafter length on for the tail so that's one way of doing it but there's an alternative you can instead of making adjustments to this rafter length, you can go down here and first make adjustments to the rafter run. So on this side of the drawing, it's the same rafter run from the center of the ridge to the outside of the wall, but we can go ahead and make this 3 quarter inch deduction from that run number, and then we can add the 12 inch run of the tail, and that will give us a net run of 4 foot 11 and a quarter inch. So if we use that run number to get the rafter length, 71 and a quarter. That is the full rafter length with no further adjustments required.
So it's just an alternative way of doing it. But for our purposes today, we're going to use this method shown on this side of the drawing because I think it more efficiently illustrates some of the various methods we're going to use to get the rafter lengths. Okay, I want to make very clear that our rafter lengths are always measured right here along the top edge of the rafter. And since our run number is beginning here at the outside of the wall, we want to put our level on here and mark a plumb cut straight up here to the top of the rafter. And this point, plumb straight up from the outside of the wall, is called the hap.
And that's an abbreviation for height above plate. So it's the distance between the... level of the top plate here and the top of the rafter.
And this is an important point because it's from there that our rafter lengths are measured. Okay, now we're going to show you the traditional step-off method of getting your rafter lengths with a framing square. So if we begin, remember as we position, we're going to start here at our half at the top of the rafter. And we position the 12 mark here and the 8 mark right here.
And it's going to mark that for us. That's one foot of run. So we can simply step off the next section. So we're going to go to that mark, 12 and 8. Step again, 12 and 8. And the final step. Right here.
Okay. We had four feet of run. So we individually went in increments of one foot of run.
And that put us right up to the top of the rafter. Now if we measure that to our half, it's 57 and three quarter. And you may remember that from the prior drawing as the rafter length from the top of the rafter at the half to the center of the ridge. Now we would have to make allowance for half the thickness of this ridge, so we could just pull over horizontally three quarters of an inch and shorten that rafter or we can take 7-8 inch off of the slope of the rafter, either way. Now that gets the job done, but I'm not saying it's the best way to do it, because there's some inherent problems with the step-off method.
Because you've got a long rafter, and you've got to do this stepping off multiple times, and the edges of the lumber can be barked off. So any inaccuracies of positioning the square will result in a cumulative error that will throw off your rafter. length.
So an alternative way is to use the tables that are on a framing square. Okay, instead of doing the step-off method because there's some inaccuracies involved in that, you can use the charts off of the framing square. So if we flip the square over here, there's a chart here and the top line is common rafter length per foot of run.
So let's look up here closer at that. Common rafter length per foot of run. You're going by the pitch, so you want to go down here to the number 8, and down below it's 14.42 inches per foot of run for an 8-12 pitch roof.
Now, we would simply take that and multiply by our feet of run. Now, in our case here, we had an even number of 4 feet of run, but if you had a fractional run of feet and inches, you would have to convert all that to inches. to get your rafter length. So it adds a little extra complication to it. Now the third way to get rafter lengths is by use of rafter tables.
And the most comprehensive one out there is Roof Framer's Bible. I really like it, but then I'm a little biased. You see I wrote this some years ago.
But all the answers are all pre-calculated. Every length, every cut, every miter, all the information you need to frame any roof of practically any configuration and any pitch. But for the standard roof we're using today is an 812, we want to flip over to page 48. Here on page 48, and this is for the 812 pitch roof, we come over here and we have run in feet. Now we know our run is four feet, so we go down here to that, and we come across, this is pre-calculated for us, common rafter length 57 and three-quarter. You may recognize that from our earlier drawing as well.
Our rise is 32, our hip valley rafter length 75. Now, if we have a fractional run that includes feet and inches, we just simply go to the opposite page here, and we have run in inches, and we'll go down this column, say, to here, 11 and a quarter. Now, coming across, we'll give the rafter length for those inches of run of 13 and a half inches, and we'll simply add that to the length from our chart for feet of run. Now, there's a lot of other stuff in here as well.
A factor chart, which I find... very useful. You can convert your rafter run into inches times 1.202.
It'll give you your common rafter length. It also has all the information you'd need for say a hip roof, so you'd have your jack deductions, your hip pitch, hip drop, your sheathing cuts for your roof sheathing, anything and everything you'd need already pre-calculated. Okay, let's lay out our common rafter. We want to put our 8 right here on the side of the board and our 12 on the top edge of the rafter here. We need to get that lined up just right.
The square is now in position to mark our Plum cut. So we mark along here. We now have our plum cut mark. This will go right up against our ridge board.
Okay, we're now ready to mark our common rafter length, and we've already made the deduction for half the thickness of the ridge. So we're going to hook on the end of our common here and pull down 56 and 7 eighths right here, and that will be our half right at the top of our raft. All right, now we need to mark a plum cut right down from our hat. So to do that, we're going to put the 8 right here at the top edge of the rafter and the 12 here at the top edge of the rafter, just like that.
And we're ready to mark our plum cut. So now that is straight down from our hat, a vertical plum cut. Okay, now we need to mark the seat cut or horizontal cut of the bird's mouth. Now since we're using a two by four wall, you generally want to position that. where it's here at the corner.
That way you get full bearing of the rafter on the top of the wall. Now, since we're using a 2x4 wall plus half inch sheathing, we want to use four inches for our level cut. So to do that, we simply take our square and we align it with this plumb cut and we slide it up that plumb cut until the four inches is aligned with the bottom of the rafter and we mark the seat cut. Okay, now we need to add our overhang onto our common rafter. So if we position our square right along this plumb line, 8 at the top of the rafter, 12 at the top of the rafter, and we come across 12 inches on the horizontal, this will be the horizontal run of our rafter tail, and we mark 12 inches.
Now we need to mark a plumb cut for the end of the rafter tail. So again, 8 at the top of the rafter, 12 at the top of the rafter, and we can mark our plumb cut. Okay, if we had a house with sloped soffits, we'd be done.
We could just cut these marks and we'd be finished with our common rafter. But since a lot of times you want to have a 1x6 fascia with a flat soffit, we need to bob this tail off. In order to do that, we'd simply flip our square over here, align it with our plumb line, and slide it up to 4 inches to the top of the rafter, which is generally what you want for a 1x6 fascia, and mark your level cut.
Okay, you see that wasn't so hard. We've completed our common rafter layout. We just cut our marks and we're done. Okay, now we've got our common rafter cut and we can finish framing our roof.
See, that wasn't too hard. Now this is a standard gable roof, but any roof can be fairly easy if you break it down into its component parts. You remember the drawing we showed you before? And you can see if you put the principles of run and You can build any roof.
Here are three different roof configurations. This one's a simple shed roof, but you look at these you think, wow these are hard. This is a non-centered ridge and here's a gambrel.
I could never do those, but that's not true. If you just break them into their component parts and you solve for the run number of each roof section, you can easily get your rafter lengths. So just divide and conquer.
Take each section one at a time and solve for your run numbers and your rafter lengths. You can easily frame any roof regardless of how complex. It's really very easy.
So guys roof framing is a lot of fun. It is fun. It's a super feeling to know you framed the roof over your head. And Anna thank you so much for your help today. And we'll see you guys on the next video where we show you how to build a hip roof.