– In our first hip roof video, we learned how to lay out a hip roof using a construction calculator and a framing square. And if you missed that video, I'll leave a link below so you can watch it now. Today we'll be answering two important questions that came out of the last video and they are, why do we use a 17 inch run for a hip rafter layout? And why do we add 7/16 of an inch to a jack rafter when using a Construction Pro calculator. First up, why do you use a 17 inch run for a hip or valley rafter? It starts off by understanding that there is a difference between a common rafter layout and a hip rafter layout. And that difference is a five inch run. If you remember, common rafters are installed 90 degrees to the ridge, which means that the run is always 12 inches. However, hip and valley rafters are not installed 90 degrees to the ridge, but are installed at a 45 degree angle. Which means you can't use the same 12 inches because the hip rafter is actually climbing at a slower, more gradual rate than a common rafter. To better explain this, if we look at an overhead shot of our roof, imagine with me for a minute that this is a 12 inch by 12 inch square. Again, this is for illustration purposes only. But if we look at the square, each side would be 12 inches. And the measurement for the common rafters run would be 12 inches and the measurement for the king rafters run would be 12 inches as well. For our hip roof here, which has a six inch pitch or rise for every 12 inches of horizontal run these common rafters rise six inches. However, if we now look at the diagonal measurement of the 12 inch by 12 inch square, that number is actually 16.97 or simply 17 inches. Therefore for this roof for every 17 inches of run on the hip rafter, it rises six inches. So you can see that the 17 inches actually comes from the diagonal measurement of a 12 inch by 12 inch square. With that knowledge, now, you know that if a rafter is coming off the ridge at a 45 degree angle, like a hip or a valley, you must use 17 inches and not 12 inches. Next step. Why do we need to add 7/16 to the jack rafters on our roof? If you use the numbers coming right off of the Construction Pro calculator, your jacks will actually be about 7/16 of an inch short. If you're trying to make them land 16 inches on center. For example, if I pull a tape from this king common rafter, over 16 inches to locate the jack rafter, you can see that it's off and needs to move to the right and actually be longer. Now, if you look at the right side jack, where I added the 7/16, you can see that the 16 inches on center layout is working perfectly. So where does this mysterious 7/16 come from? Well, it has to do with the dual bevel angle we made at the top of the hip rafter. This can be a complicated topic to teach on. And honestly, there are other carpenters that know roof framing better than I do but let me give you my cliff notes version. The simplest way to come up with the 7/16 is to subtract the rafters thickness with 1/2 it's diagonal length. To get the diagonal length, you simply draw a 45 degree line and measure 1/2 that distance. And for an inch and 1/2 rafter like this, the diagonal measurement is two and 1/8. So half of that would be an inch and a 1/16, therefore, an inch and a 1/2 minus an inch, and 1/16 equals 7/16. Which then would be added to the jack rafter lengths coming directly from the calculator. I find this method very simple to understand, but for the fun of it, let me give you one more answer to why the 7/16 is there. If you look at the top of our roof, I've extended the hip rafter line, which is this line here. I've also added this line, which extends the king common rafter. And if you look now really close, you can see that the lines actually create a little triangle, which is really a mini roof. That has a run and it has a pitch. And guess what the run of this little roof is? That's right, 7/16. And the measurement then from this point to this point is an inch and a 1/16, which is exactly 1/2 the diagonal distance we came up with earlier. So hopefully that sheds a little bit more light on where all of these numbers are coming from. Now I'm not quite sure why this calculator can't make those jack rafter adjustments, but I'm sure it's for a good reason. Personally, I find it really easy to just to add the 7/16 to all my jack rafter lengths. But I do understand that some of you like to do it a different way. So in that spirit, let me give you one more work around so that your jack rafters are coming off of the Construction Pro calculator correctly. Because the 7/16 is a run measurement you can add that run to your existing run. But this can only be done after you've already calculated your common rafter and your hip rafters. For example, the run of this roof is 23 and 1/4 inches. So if we add the 7/16, we come up with a alternative run if you will, of 23 and 11/16. This is solely for the purpose of just figuring out the correct jack lengths. So once we work through all of the calculations using the new run, the jack lengths, as they're coming off of the calculator now are more accurate and no longer need further adjustments. As you can imagine, there are many different ways to approach coming up with the right numbers. And honestly, in rough construction, sometimes getting close enough works too, but now at least you have a little bit more knowledge so you can move forward more easily. Well, that's it. I hope those two questions have been clarified for you. If you need further assistance, you can drop me a comment below, or you can find me on Instagram. That platform works really well too for sharing information. Thank you so much for watching today. God bless. (upbeat music)
