How to Measure and Cut Rafters

[Music] on today's video we're going to be showing you how to measure and cut rafters today we're installing yellow pine rafters that are two by twelves on the job we're working on and there's three main things you'll need to know if you're going to be measuring and cutting rafters number one is the pitch of the roof which in this case at the 12:12 which is steep okay so if you don't know your roof pitch I'm going to show you a real quick trick to figure out your roof pitch without any real math so first you'll set a string line that runs from the inside a wall plate on the exterior wall you run that string line up to where you want the heel of the rafter to hit now I've already got a rafter installed here but you would be doing this beforehand to figure the pitch and then you'll just slide your speed square in to the pivot point on that string line and see where the string line runs across your square right there at a 12 12 on the common scale and that's a quick and easy way to figure out the pitch of your roof without doing any math number two is how wide your wall plate is that the bottom of the rafters gonna sit on in most cases it would be a 2x4 or 2x6 and the third thing you'll need to know is a measurement and I'll show you how to do that I like to measure my rafters from short point of the plumb cut to short point of the bird's mouth so the reason I like to measure a short point to short point well there's two reasons actually one is that it's on the same edge of the board the short points are both on the same edge so you're not measuring diagonally across the board so that it's easier the second thing is that sometimes our rafters stick up past the top of the beam so they're just floating in the air there's nothing to measure to up there the reason for that is that sometimes you need an air gap to let air flow out a ridge vent on your roof if you're not doing spray foam which sometimes we don't but if you're a numbers guy you can also just do some simple math to figure out the length of the rafter from the same points you could use the Pythagorean theorem to figure the length of your after you just need to know a the length between the inside of your beam and the inside of your wall plate horizontally and be the height from the top of your wall plate to where you want the heel of the rafter to hit on your beam plug the numbers into the formula shown here and it will give you the rafter length before we get going let's look at a speed square because this is what we're to be using the speed square has a pivot point which is marked here it has a hook that you can hook on the edge of a board and it also has several scales here what we're gonna be using is the common scale and this is for cutting common rafters there's also a hit valley scale for cutting the ends of hip and valley rafters and there's also degrees down here at the bottom and that can be used as well in cutting rafters but today we're going to be mainly looking at the common scale and using this pivot point and also this inch scale across this way okay the first thing we're going to do is mark the top cut of this board we'll just say that's our short point where our heel is and I know this roof is at 12:12 so I'm gonna put my speed square with the pivot point right on that mark now I'm gonna turn it till I see 12 on the common scale right here and that's the 1212 pitch which will make this a plumb cut against the beam that's going to be right here so I'll draw this all the way through 12 12 by the way is also 45 degrees so just hook across the top edge of the board and I did check my crown by the way this is crowned up on the top edge of the rafter so that's the top edge of the board and now we'll hook right here and pull this way to measure to the other short point tiny nail let's pull down and Mark the short point of the berth now okay now I'm down on the other end of the board and I've got this short point marked where the birds mouth short point is going to start right here and just for reference I'm on the same side of the board so our top cut angle is like this way so we're on the same side of the board so I'm going to take my speed square and we're going to go the opposite direction and go ahead and spin around till we see 12 on the common and make a line and now that's your horizontal cut that's going to sit right on top of your wall plate which would be right there two by six and there's several different ways you could do this birds mouse we're gonna look at that if you're using a 2 by 6 plate which we are you would measure along the angle like this using your scale five and a half inches because that's how wide a 2 by 6 is and and then we will go 45 degrees 12 12 down and we'll cut this out and that's your bird's mouth you will sit on the plate right here and this will go down the outside of your wall now if you have plywood on the outside of your wall you're gonna have an extra half inch of thickness here so when you measure this way you're gonna go six inches before you go down and plumb so that's the two ways you could cut your birds mouth now let's look at rafter tails we've slid down the board a bit more now this way and let's look at doing this rafter tail cut at the end so we use a two by six fascia board so the end of this rafter tail needs to be the same width as a two by six so we can come down here and do a plumb mark I didn't measure how far this is but just for reference we can do a plumb cut and then measure down five and a half vertically and that's that's gonna be the two by six fascia board right there that nails on and then we will run this cut out horizontally like so and we'd cut this line in this line and this would give you about a foot overhang now this part of the board down here isn't really necessary so a lot of times what I'll do is actually cut that off as well take your speed square and scribe and you'll get something like that and you can just leave just this part as your rafter tail if you don't need this much hanging down on this particular rafter that I'm cutting right now I don't need any overhang there's gonna be a poor truth that butts in way up here so I don't need this board to hang off any so what I'm gonna do is clip this rafter tail off vertically right at the outside edge of the house so to do that again I'm using this speed square as a this is a 12 12 pitch so I can also just use this as a 45-degree the same as 12 12 I will run this line up here now I've got a vertical line that's flush with the outside of that wall plate and I'll just cut this and this to give me what I need for this rafter a quick note to get these more accurate is to cut on the correct side of the line in this case we're going to be cutting on this side of the line on both cuts if you cut on the other side like on this side it'll take an extra eighth inch out of your measurement because of the width of the saw blade itself the kerf we call it will get taken out of the measurement of the board instead of out of the cutoff so that's a good tip if you want to get these real accurate now we're gonna make this cut this is our plumb cut the top of the rafter I like to pull this guard back as I go so it doesn't you know try to steer the saw where I don't want it to go so here we go alright that's it guys I got this one cut and ready to go remember to size your lumber either get a code book or get an engineer or you can use this app buy the lumber Council that I use I'm gonna flash some screenshots up of that right now here's this app by the American wood Council called span calc it's a free app you can get on your smartphone you can have it with you on the job and it's updated a lot more regularly than the code books are it'll tell you what material you can use how thick of or what species and what spacing you need for your Raptor so that your roof doesn't collapse [Music] you

Channel: Perkins Builder Brothers

Views: 1,147,282

Keywords: carpentry, build, rafter, beam, cut, make

Id: 40Em6uUqOwE

Length: 8min 27sec (507 seconds)

Published: Sun Aug 04 2019