How to lay out a common rafter with a carpenter's framing square - Imperial

[Music] in this video we're going to make a rafter that looks like this out of a length of 2x4 about 2 and 1/2 ft long the rafter we're making is from this rafter layout skill development exercise sheet it's a rafter like you would use on a shed or a dog housee it has a total run of 1 and 1/2 ft which means it's for a three foot wide building measured to the outside of the wall framing the slope of the rafter is 9 to 12 which means it has 9 in of rise for every foot of run at the top the plum cut will be adjusted for our 1 and 1/2 inch thick Ridge board and at the bird's mouth the plum cut will be adjusted for half inch thick sheathing applied to the outside of the wall frame the tail of the rafter has Plum and level cuts that are laid out for 1x4 sfet and Fa shut all the specs you need to make your rafter are given on the worksheet below the specifications you'll find the step-by-step procedure the diagram on the worksheet explains all of the new [Music] terminology first let's have a quick look at some of the geometry behind roof framing here's a line 1 and 1/2 ft long representing the total run of our rafter and here's the slope of our 9in rise to 12-in run when we extend that slope to the center line it shows the total rise of our roof the hypotenuse of that triangle gives us our line length which is the starting point for laying out our rafter the line length is the distance from the top outside corner of the top plate of the wall to the center line of the building measured along the slope at the top the rafter will be adjusted for half the thickness of the Ridge board at the bottom an adjustment will be made to add the thickness of the wall sheathing to the plum cut of the birds mouth when you lay out a birds mouth joint you have to watch out for two things number one is that the seat cut has to be fully supported by the top plate number two is that there needs to be enough wood left over after you cut your bird's mouth joint so that the rafter tail is strong enough to support loads without breaking for this rafter we're looking for the height above plate to be 2/3 of the vertical height of the installed rafter measured along the building line remember that the building line is the outside face of the wall framing and the sheathing is on the outside of the building line so this triangle that we've just drawn is the basis for all our roof calculations let's see how it relates to an actual rafter here's what our rafter will actually look like relative to the triangle we've just drawn the base of our theoretical triangle is the top of the top plate or C plate and the vertical leg of the triangle is the center line of the building where it gets confusing is that the measuring line or the hypotenuse of the triangle that we use to get our line length from doesn't correspond with either edge of the rafter the measuring line intersects with the base at the top outside corner of the top plate and it rises to the center line at that 912 roof slope the height of our theoretical triangle is called the total rise of the roof and you can calculate it by multiplying our 1 and a half half fot run by our 9in unit rise to get 13 1/2 in of total rise if you want to know the actual height of the ridge above the top plate though you need to add the height above plate to that total rise you can see that the height above plate is measured along the building line and that the plum cut of the bird's mouth is cut past the building line to allow for the wall sheathing [Music] now I'll run through the procedure on the handout to lay out that rafter using the step off method here's a fence set up on a framing square to that 912 slope the fence is made out of a piece of scrap wood with some slots cut in it and it's adjusted by loosening and tightening the screws that clamp the blade the length along the fence between the 9 and the 12 is our unit line length or length of rafter per Fort run using Pythagoras Theorem we know that that length is 15 in but using the step off method we don't calculate the line length instead we use the square with the fence on it to step off as many lengths per foot of run as there are feet of run the first thing you want to do when you start laying out a rafter is check if there's any crown the board if there is Mark the crown and put that side up next start laying out the line length of your rafter by marking a plum cut representing the center line of the ridge now without moving the fence Mark the other end of the square to get mark one of your unit line lengths and that's where you'll move the square to to mark your next unit line length our total run for this Rapter is only 1 and 1/2 ft so we put a plum line at that 1T Mark and then find the 6 in mark Mark and that's where we'll put the plum line for our building [Music] line now that we've stepped off our line length it's time to make the adjustment for half the thickness of our Ridge board and mark the plum cut line for the ridge half the thickness of the ridge is 3/4 of an inch so we're going to mark that using the square and the fence to mark it perpendicular to the plum line then strike that line down and that's the line will actually cut when it comes time to saw out our rafter next we'll move down to the heel of the rafter and we'll figure out our height above plate 2/3 height above plate our ver vertical height is 4 and 1/2 in 1/3 of that is 1 and 1/2 and there's the seat cut for our Bird's mouth joint now we're going to make an adjustment at the building line to get the plum cut for our Bird's mouth joint which is going to be the thickness of the wall sheathing past the building line so we're using half inch wall sheathing make a perpendicular Mark 1/ half inch past the building line and there's the plum cut for our Bird's mouth the plum cut of our rafter tail allows for the half inch sheathing and then 3 and 1 half inch 1x4 for the SOP so we measure 4 in past the building line out toward the rafter tail make a Mark slide the square down and then make your Plum cut line for the rafter tail now we've got our Plum cut line on the tail next we'll mark our level cut line on the tail our tail cut level line is located 2 and 1 12 in down from the top edge of the rafter measured along the plum cut line once you've marked your level cut on the tail your rafter layout is complete it's time to get a skill saw and a hand saw and cut out your rafter don't overcut your bird's mouth joint with the skill saw finish it off with the Handa to keep it as strong as it can be [Music] [Music] now let's have a look at laying out the same rafter using a little bit of math rather than the step off method calculating the line length tends to be a lot more accurate than stepping it off calculating the line length is done by multi multiplying the total run of our rafter by the length per unit run which in our case is the length of the hypotenuse of a 9 in x 12 in right triangle the total run for this rafter is 1 and 1/2 ft and to find the length per unit run we can look at the rafter tables for the length of common rafters per foot run if we look under the nine we'll see the number 15 which means that a roof with a 912 slope is 15 in long for every foot of run by multiplying 1 and 1/2 ft times our 15 in of length per foot of run we find that our total line length is 222 in now we go straight to marking a plum line Mark representing the center line of our building if we mark it right on the corner we can hook a tape on that corner and pull the line length that we've calculated all the way down to the building line by calculating the line length and then pulling it with the tape from one end you eliminate the chance for compound error that you get with the step off method now that we have our line length marked on our rafter between the center line and the building line all the rest of the steps are exactly the same as what we saw in the earlier example we'll make an adjustment for half the thickness of the Ridge and mark the ridge Plum cut line we'll make the adjustment for the wall sheathing and mark the birs mouth Plum cut line and we'll find 2/3 of the vertical height to Mark our height above plate and Mark our B mouth seat cut line we'll mark the plum cut on the tail the level cut on the tail and then cut out the rafter and see if it fits [Music] [Music] [Music] he [Music] he [Music] he

Channel: Joe Calnan

Views: 58,669

Keywords:

Id: v4nCi8zMWeE

Length: 13min 36sec (816 seconds)

Published: Sun Oct 08 2023