Framing - Rafter Principles And Common Rafters

the completed roof frame of the building we are constructing you'll remember will look like this when we finished using a scale model so we can see the tails more clearly we also remember that we have so far learned to layout and cut the common rafter another type of writer which we must cut for the building is the hip jack rafter this is similar to a common rafter but meets the hip rafter instead of going all the way through to a ridge board we can see this if we imagine them we simply have a gable roof changed to a hip roof but it is changed these common rafters become hip Jack's exactly in this matter the major difference is that now they must be cut at an angle to fit the hip rafter in the same way that the hip rafter itself is cut as an angle to fit the ridge board to understand how these rafters are cut let's look at another drawing one of a perfectly flat zero pitch roof the cut here at the end of the jack rafter and here where it meets the ridge board are now 45 degree miter cuts in this flat roof now notice that as the ridge board and rafters rise the angle of cut becomes more and more acute and we have a longer and longer point on the end of the rafter to illustrate the method of laying out the cottony if Jack Raptor let's go back to the flat roof and layer square on the hip Jack Raptor the other leg of it extending along the top plate to the corner of the building looking down on the roof we notice the distance a is the same as distance B in other words the run of the jack rafter is exactly the same as the distance between the standard line of the jack rafter and the corner of the building therefore to get the cart at fifth end of the rafter we flopped the square over and all the line length of the Raptor on one leg of the square and the run of the Raptor on the other leg assuming that the car is not yet been made the stock will extend on through in this manner the cut is then established here although for purposes of illustration we have used line lengths and the run of the Raptor in actual practice we would use the length of one unit of Raptor which we will generally call the bridge measure and unit of run which you'll remember is 12 inches in this perfectly flat roof these two dimensions of the same but as the roof Rises and the slope becomes steeper and steeper the line length of the Raptor becomes longer and longer the counter however is made in exactly the same manner the bridge measure or unit of Raptor on one side of the square the unit of run on the other side of the square will produce the cut notice that the side cut line falls in the bridge measure side of the square now on this model notice another difference between the jack and the common rafter while the run of the common rafter of our building is one has to span the run of the jack rafter is last because of an optical illusion the outside raptors do not appear to be square however they are still that each pair of jack rafters forms a perfect square with the edge of the top plate if the center of the first jack rafter is faced 16 inches in the corner of the building the run of the first jack will be 16 inches now that the other principles we've seen work out and actually laying out of jack right there you'll remember that the count of our roof is 6 and 12 we hold 6 and 12 on the square then on the edge of the wrap to stop and draw the first or line length mark on the unit of rise died since this Jack rapture has a run of 16 inches we must lay out the 16 inches of run on the stock to this point we've laid out one unit or 12 inches of rock [Applause] we now applied the square down and lay out another four inches which gives us the 16 inches necessary [Applause] we then slide the square down and draw the second line through the pork you've established notice that we continue to hold six and twelve as always not 6 and 16 this line indicates the ads of the top plate and in our rafters finally cut it will sit here as you see we must now proceed to layout the projection for the eave we do it in the same manner as we did with the common rafter sliding the square down still holding our usual 6 and 12 and measuring off what amounts to 18 more inches of run to give us 18 inches of projection to match the 18 inch projection of the common rafter tail the bird's mouth is laid out in the same way as we did for the carbon rapture we must be sure that the distance above the bird's mouth from the jack rafter is exactly the same as it is above the one in the common rafter so we go back to the common rafter pattern and measure the distance above the bird's mouth transfer this measurement to the hip jack stock and strike off the level line completing the bird's mouth [Applause] if this measurement is not carefully taken we may get an uneven wavy line and the completed route we're now ready to shorten the record if we would have carried this line length mark up to the center of the stock we would find that it would fall at this point in the center of the hip racket obviously this would not be possible the right there must be short since of a level plane the F Jack Raptor meets the hip rafter that angle of 45 degrees the amount of shortening will be one half the thickness of the hip rafter and taken on a 45 degree line find out what this distance is we draw a 45 degree line across the back of a piece of Raptor stock we divide this distance in half and find out that this rafter must be shortening one and 1/8 inches [Applause] this distance is measured off at right angles to the line length mark transferred to the edge of the rafters stopped and carried to the center of the top of the rafter we're now ready to lay off the sidecut it will be made by holding the bridge measure in this case 13 and 7/16 inches on one leg of the square and even of the run for this rafter twelve inches on the other leg [Applause] we haven't draw the line on the bridge measure side and the new plumb line is struck off with the layout complete the hip Jack Raptor is cut it looks like this and will fit the hip rafter here on the roof model this is the hip Jack Raptor which we have just finished notice that the next one alongside it runs exactly twice as far therefore it will obviously be twice as long the foreshortening of course and the third one similarly will be three times as long now let's move up and look at the valley jack Raptors the same principles of layout may be applied to them as to the hip Jack the runnable Valley Jack rafter may be determined by the distance between the center of it where it meets the ridge board and the point where the two ridge board standard lines intersect since that distance as you can see is the same as the valley Jack run the time cut of the valley Jack is produced in the same way as the sidecut of the hip check the major differences between hip and Valley Jack's are first that the valley Jack meets the ridge boy and so has a plumb cut on that end and of course must be shortened in the same way that we shortened the common rafter second the valley Jack naturally does not have a tail but meets the valley Raptor here it has a side cut and must be shortened the same amount that we shortened the hip jack next the crippled Raptors rating between the hip Rathdrum the valley Raptors [Applause] since the hip and Valley run parallel all cripples not to layer the same length any them will fit here [Applause] and here and also here as we look down on the crippled rafter at this point notice the perfect square it forms with the two edges of the top plate and the hip Jack Raptor the run of the crippled Raptor then is the same as the distance between the point where the wing meets the main structure and the outside corner of the building this must be so since this distance if they flew the Rif distance being two sides of the square form now in particular building this measurement the run of the is four feet the has stepped off just as the common and jack rafters were we must put five got the bull bands Oliver and both bands will also be shortened the amount of shortening for each end one and one eighth inches is identical with the shortening of the hip jack right for convenience we add both measurements together and take two and one-quarter inches off of one end the tide cuts are made by holding the bridge measure on one side of the square and the unit of run on the other and drawing the line as usual on the bridge measure sign when we're finished the will look like this and will fit between half and Valley rappers as you see here there are two other Raptors which we must learn to cut in order to complete the roof one is a valley rafter the other is the hip practice these two Raptors have to support other Raptors so we cut them out of heavier stock in laying out these Raptors the 12 inch unit of run which we've always used with other Raptors will no longer apply the reason is this since hip Raptors and of course Valley Raptors also move them toward the center of the building on a diagonal line it's obvious that they have to travel farther than a common Raptor to get to the same place in other words the hip rafter them has a longer total run from the common rafter the total rise though naturally remains the same as that of the common Raptor since both rise to the same height above the walls of the building translating these toggles to unit this means that we will still hold the same unit of rise on our Builders square but a new unit of run now we'll see what this new unit of run is - Jack Raptors placed here in our zero pitch roof will each traveled 12 inches to meet the center line of the hip rafter notice that the hip rafter now traveled across the diagonal of a square the diagonal of this square measuring 12 inches on each side is sixteen point ninety seven inches long or for all practical purposes seventeen inches so we've established a new unit of run seventeen inches which is used only with the hip and Bali Raptors this plan is what we hold on the steel square six inches as the unit applies and 17 inches as the unit of run [Applause] using these measurements we draw the first plumb line which we will call the line length mark it corresponds to the center of the building whether hip rafter meets the ridge boy in the same way as we've always done we lay off a total of 12 units one for each unit of common rafter run after we've laid out all 12 units we must lay out the projection producing the tale of the hip rafter to see how to do this let's look at this plan view of the hip rafter and to Jack practice each Jack rafter having the same projection as the carbon rafters in the building 18 inches this 18 inch projection we can represent by this line 18 inches long and by this line also 18 inches long when we close the square of which these two lines are two sides we see that the tale of the hip rafter runs from corner to corner of it forming its diagonal so to find out the run of the tale of a hip rafter we lay off the two sides of the 18 inch square on any flat surface and measure across the diagonal with a rule we find that the tale of the hip rafter runs 25 and 7/16 inches which we lay out on the stock this distance am 25 and 7/16 inches becomes the projection of the hip rafter and produces its tail we must now shorten the hip rafter and to do this we follow the same procedure we've used before 1/2 the diagonal thickness of the ridge board is 1 and 1/8 inches which we lay off from the side of the rafter and right angles to the plumb line transfer to the edge of the raft of stock and carry to the center of the top of the rafter the next step is to lay off the sidecut in the finished Raptor it will look like this and will fit against the ridge board like this the sidecut at the top end is produced in the same way as the sidecut emerge a crafter but remember that this cut is based on the bridge measure of the hip rafter and the unit of run of the hip rafters or in our case an 18-inch bridge measure and a 17 inch unit of rod since the tongue of the square is too short to hold either of these figures we simply divide them both in half and hold nine and eight and a half to produce the angle the line as usual is drawn on the bridge measure side to produce the side cut mark we are now ready to lay out the tail cut up the hip rafter you'll remember that it looks like this the gutters made in much the same manner as the topside cut was made we hold the bridge measure and the unit of run and draw a line on the bridge measure side reversing the square and repeating the process gives us the other half of the combination side cut [Applause] a new plum line for the tail car is drawn on both sides of the stock and it's ready for cutting cut and in place it projects like this that the Eve's line of the roof we are now ready to lay off the bird's mouth to see why we lay it out as we do let's go back to the roof model and use the hip flap there purposely left unsecured and cut especially for demonstration purposes you'll note that when we've laid out the line length of the hip rafter we've reached the corner of the building [Applause] with the birds mark cut square at this point as has been the case with all the other factors it fits against the building like this as you can see with these demonstration pieces the flat surface of the bird's mouth touching only the sharp edge of the top plate corner we want to make it fit more tightly to do this we first cut off the corner of the top plate like this the amount we cut off measured straight in or the level line [Applause] we'll always be equal 1/2 the thickness of the Raptor stock now are continuing our demonstration suppose we lay the square cut birdsmouth of the hip rafter in the place CR we stand see what's wrong that's right the plumb cut has to be moved in with this done we have a good fit against the cutoff corner of our top plate now let's check the fit of this rafter along its length here's the fit at the top end and it's not good notice that the long point of the rafters fixed up above the ridge board if we're to hold the center of our rafters flush with the top and here's how it fits the jack rafters so good again with the jack rafters in the proper position as we see here the roof sheathing would have to be bad up at the end obviously something else must be done this something else is known as lowering the hip rafter so that the top end is flush with the ridge board as it should be and of course cutting away this section of the richboys in lowering the hip we must also lower the rafter at the bottom where it meets the plate on this demonstration hip rafter here is the plumb line of the rafter before it was moved this distance measured nominal of plumb line is equal to that of a common rafter you'll remember that we moved it one half the thickness of the Raptor stock here's the plumb line after it smooth [Applause] if we hold the same distance on the new plum line as we did on the old plum line we automatically get the amount necessary to lower the wraptor with the birds mouth cut on the new line it moves in the proper position with the raft of property dropped they have jacks now fit tightly to the hip rafter and no longer project above it laying off the hip rafter birds mouth in accordance with these principles it's a very simple matter first we move the plum line of the bird's mouth in toward the building we move at half the thickness of the rafters stock so that it will fit properly after the corner of the top plate has been cut off next we check the common rafter to determine the distance to the top of the wraps of stock above the bird's mouth this we find is 4 and 1/2 inches we measure that distance down the new plumb line and strike it off to form the feet of the bird's mouth don't hear that if we had measured down the original plumb mark representing the actual building line the seat cut would lie here so by using this new plumb line to measure on we've raised the seed cut this much and the rafter has been lowered the same amount so that the tail of the hip will look the same as the tail of the common and jack rafters it's reduced in which the needle width is equal to the plumb line of the common rafter when cut it may be used as a pattern for the other hip rafter which of course that the top end will have an exactly opposite side cut so that the two hips will fit to the ridge board like this the valley rafters are laid out in very nearly the same way at the top after shortening the rafter in the same manner that we did with the hip rafter we lay off the other side of the combination side cut and since the tail must look like this we produce this type of combination side cut at the tail cut mark the line length of the valley rafter runs from the center of the building to the inside corner so if the birds mouth were cut off square here at the line length mark it would sit at this point so here again as you can see we've had to move the plumb line notice though that this time we've moved it in the opposite direction from that of the hip fracture move to the way from the building [Applause] this done the birds mark this is absurd in this matter when the valley Raptor is in position jack and crippled Raptors will be a little above the edge of it we don't raise the Raptor however the roof sheathing can easily be nailed to its center the way it stands to summarize the layout of the hip and valley Raptors here are the major points to remember while the side cut at the top of the hip raptor looks like this the valley Raptor has a combination side cut which looks like this the tail cut of the hip rafter is in the form of a point while the tail cut of a valley raptor is a notch [Applause] the birds mouth of a hip rafter is moved toward the building and the seat cot is raised causing the rafter to the lower but the birds mouth of a valley rafter moves away from the building and no lowering or raising of the Raptor is necessary [Applause] but all of the rafters have been put up our roof is completely framed and ready for the shingle you