How to measure roof rafters including the pitch and bird’s mouth (seat) cuts (captioned)


Now that we know that our rafter will be 15 ft 2 inches long (from an earlier video) and we know our pitch or slope will be 5/12, Now we know how long a rafter we need. Typically you add 2 feet to the length to be sure you have enough board for the end of the rafter, called the rafter tail. We need a board, 15′ 2″ + 2′ is 17′ 2″… that means we’ll need an 18 ft board for the rafters for this house (off screen) Wouldn’t that be 17’2″? Yes that’s true. The math is that way, but when you buy boards from Home Depot, Lowes or elsewhere they come in multiples of 2. You can get an 8 ft board, 10, 12, 14, 16, 18 ft boards. You don’t find a 17 ft or other odd numbered length board. So even though we need 17′ 2″ we have to buy an 18 ft board. I buy the boards that I need then the first thing I do with this rafter is crown it. A board does one of 3 things: It will rise and fall or if it has a dip, turn it over and it becomes a crown or it has a bow to it so it goes from right to left or left to right. Bows are easier to deal with, you can take them out pretty easily. Or it could have a twist in the board. The optimum is to find straight boards, as straight as you can. They don’t always come that way but you always want to try to get those If you don’t get those boards then you have to work with them. A crown always goes up. Imagine that when you put weight on it, it has a tendency to flatten out. If you have a dip in the board and install it that way, then that make the dip worse. So always put the crown up on all of your boards. First thing I’ll do with this board is crown it. You want to make sure you are reading through the strength of the board. I’ll turn it up this way and you look down one edge, down through here… and see if it goes up, it if goes down or if it has a bow to it or even a twist. You can read all that. This one has a pretty handsome crown in it and I’ll keep the crown up. The next step is to start at the very ridge. (off camera) I can see the crown over on that side, that curve would be the crown. Yes, you can see it even better this way. When I install this rafter I want to be sure this part of the board is up. Once I’ve established that, I make a mark on this board… …that says that’s my crown. It goes up. That way I never get confused, and anyone else can ask where the crown is; there it is. It’s very well marked. This will be my first rafter that I will cut. I will cut this rafter completely then make a duplicate of it. Once I establish that this rafter and its sister will meet my requirements for the roof, then I’ll use this one rafter to cut all of the other rafters. So this is my one pattern or template. It doesn’t matter if you work from right to left or left to right; there are advantages and disadvantages for both of them. I’m right handed so it’s easier to work right to left. I also work on the bottom side of my rafter. That way I always keep the top side of my rafter away from me. I’ll start at the right, I’ll make an arbitrary mark at the far right side. It’s almost arbitrary. If I make a mark down here I have a lot of waste. I want to mark it down near the end as much as I can You can see here there are some checks and cracks in this board so when I put my pitch cut, my top cut on it I’m moving away from these cracks and checks. Remembering what we talked about about our speed square, we’ll put this on the top side Remember, it goes this way or that way. I’ll be pivoting and I want an angle. Since this is the top of my roof, I want the angle coming back towards me to the inside of this board. This is not the proper angle, so I flip it over. This is the proper angle. I’ll start on my pivot point and then pivot to 5 on my scale. On the speed square, if you look at the scale, this is the Common scale, this is the scale we’ll be using. The Hip/Valley scale is different, we’ll have another conversation about that later. Here’s my pivot point and I’m going to pivot until I see 5. Be sure you are on the Common scale The Hip/Valley scale 5 is not the same location as on the Common scale so be sure that you pivot on the Common scale and then always mark on the inside. I’ll put this on my beginning mark and pivot to 5 on the Common scale not degrees or on the Hip/Valley but on the Common scale and then I’m going to mark my inch side. This angle here is called the Pitch cut or Top cut. The rafter itself will sit like this, so that’s the way the rafter will go (in the house). Now, from this point here I’ll measure 15′ 2″ I don’t have a long enough board so I’ll pretend that I measured 15′ 2″ and make a mark. I pretended I came down 15′ 2″ and made a mark. There’s my top cut. Now I come down 15′ 2″ from the top of the rafter and now I’ll do exactly the same thing I’ll put my pivot point on my mark I’ll pivot until I see 5 on the Common scale and I’ll mark the Inch side. If I were to run 90 degrees perpendicular from this mark this would be 14 ft to the center of the building (half the span – see prior video). This is the way the rafters will go. Now I have 2 more things to do to this rafter One is called the rafter tail itself. Typically there’s about 24 inches that roofers or architects like to have as overhang on the roof. I’m going to make this overhang 16 inches It could be anything that the architect wants and it will be on the plans. I always measure from the bottom of my rafter over the length of the rafter tail – 16 inches. I have several different options on what this cut is. I have one cut that architects like, called a square cut. That means from my 16-inch overhang I’m just cutting it square. Another cut is a pitch cut. It’s the same 5/12 line that is parallel with the end of the building (and the top cut). The way I do that is I put my pivot on the starting point… …I pivot to 5/12 on the Common scale and I mark it. I have marks here for the end of my rafter. The architect wanted 16 inches here for the rafter tail, measured from the bottom. If I were to measure that 16 inches from the top, that would work out magically if I had a plumb cut – see the 16 inches at the top and bottom are parallel lines. But if the architect wanted a square cut and I measured the 16 inches from the top …if I measure from the top and have a square cut, then my overhang is only 12 1/2 inches. So that’s why I always measure my rafter tails from the bottom. So I can accommodate either a square cut or a plumb cut. Here comes the tricky part: This is the advantage of this particular speed square. On this particular scale you can see it has all of the roof pitches up to 20/12 and it givs the number of degrees, the angle of that particular roof pitch. It looks like on a 4/12, it’s 18 1/2 degrees. Ours is a 5/12 so that makes it 22.5 degrees. When I get on a 5/12 roof pitch or slope, then I can read the number of degrees across here … …and it is 22.5 degrees. I’m on 5/12 and that translates to 22.5 degrees. That’s why all these numbers here, even if it’s a 15/12 pitch, that’s 51 1/2 degrees. All of the degrees are right here on this particular speed square. Not all of them have it, that’s why I like this one. The way this comes in handy is I have to cut a bird’s beak or bird’s mouth (seat cut) I have to create another cut right here so it fits on top of this wall. I want to be sure I have a 90 degree right here so when I’m cutting in this bird’s mouth I want to have a 90 degree angle here and this distance needs to be 3.5 inches The reason for that… (drawing wall studs with top plates) …these are 2 examples of if I have this cut less than
3 1/2″ – if it’s 3 inches then my seat (bird’s mouth) cut will come up that way and that’s what my rafter will look like. It will come up short. If it’s more than 3 1/2 inches (off camera) and that’s because dimension lumber is 3 1/2 inches wide? Yes, that wall is 3 1/2 inches wide… …unless it’s a 2×6 wall then it would be 5 1/2 inches wide. That’s why I use the 3 1/2 inches. If it’s more than 3 1/2 inches for this particular cut, then our rafter will look like this. you can see that where it’s a bit short and here it’s a bit long. My sheetrock comes down like this and my other sheetrock comes up like this it’s a difficult joint, and technically the ceiling height is not where the rest of the height is. so I like this cut to be 3 1/2 inches How do I do that? If you think back to geometry, there’s 180 degrees in a triangle. I take away 90, that leaves 90 degrees for these 2 angles. If I know that my 5/12 pitch is 22.5 degrees, that means that if this is 22.5 this (other angle) is the compliment – what’s left over from 90. …which is 67.5 degrees. I have pivoted to 22.5 degrees on the speed square right now I will flatten this out and turn it over this way and I want to pivot to the 67.5 degrees. Here’s 67.5 degrees, and this is 3.5 inches so I’m going to slide this so I make sure that I have a good strong pivot point and contact I have my 67.5 degrees here and I’m on 3.5 inches. Those are the 3 things I have to have. This here goes away, this is my seat cut. Now this (too short) won’t happen and this (too long) won’t happen. My wall comes in right at this point. That’s almost it. The last thing I need to do is make a duplicate of this after I cut it all out. Now I have 2 of them. I take those 2, put them up on the roof and look at my cuts If my angles are good and my angles are good on top of the wall, and the top cut or pitch cut is good, then I’m ready. Then I come down here (top or pitch cut) Since I have a ridge rafter…
(off camera) what’s a ridge rafter? When the rafters come together like this (at the top) there’s one rafter that runs parallel with the foundation and runs perpendicular to all the common rafters These rafters set against that ridge rafter all the way down the line and that’s what keeps everything stable. That ridge rafter is typically 1.5 inches thick. To accommodate that distance I have to take half of that distance out of this one, my template, and then half that distance out of its sister, which means I’m taking 3/4 inch out of here and 3/4 inch out of here (the other rafter). I’ll measure 3/4 inch… …get the speed square … …and create a parallel line. Now I’m holding my 5/12 pitch — good contact – and I slide it to 3/4 inch mark This (first line) is not my cut line; this one is right here (the 3/4 inch line) I did that on both of them (both rafters) and then I can mark all my rafters with the one template. I make sure to mark TEMPLATE or a similar note. Make sure this is marked and not installed until you are finished.

13 thoughts on “How to measure roof rafters including the pitch and bird’s mouth (seat) cuts (captioned)

  1. Great video.
    Please don't stop.
    Trick: If you want to record a better audio so you can do it with your smartphone because cellphones are made to record only sounds next to the person and it will minimize the external sounds.
    On YouTube editor you can delete the camera audio and use only the cellphone audio.
    Look it up on YouTube How to use smartphone to record videos audio and put it on video for YouTube
    Thank you so much for the videos.

  2. thank you very much ,you are really very a great teacher.
    how I wish that I could join your class and be one of your students .
    thanks again I really learned a lot from you videos .

  3. great video. I watched several video but I couldn't understand how to calculate bird mouth cut. you did a great job of explaining this. Thx

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