in today's video we're learning how to calculate the height of a ridge board for a common rafter roof today's maka may look familiar to you it's the same rafters that used in a previous video the only difference is today I added a lot more markings on them and if you missed that video I'll do a quick overview now to get you all caught up and also leave a link up here and at the end of this video so you can watch it what we have here and what we built in the beginner rafter video is a 912 common rafter roof meaning that for every 12 inches of horizontal run the roof rises 9 inches vertically the overall span of the roof is 4 feet and the total run is half that distance or 2 feet we use the unit length method represented by these two triangles to determine our rough rafter length after that we determine the bird's mouth cut length of the overhang and remove half the thickness of the ridge board to arrive here with 2 complete rafters overhangs and a ridge now that y'all caught up I do want to add 3 things that I didn't have time to talk about in the first video that's important to know first make sure to sight down all rafter and ridge material making sure to install with the crown facing up second it's normal to size the ridge board up a size or more from the rafters our rafters are 2 by 6 so you can use a 2 by 8 a 10 or 12 for a ridge in third I want to talk a little bit more about the measuring line the measuring line is a theoretical line and it's really important because it's where all of our initial math is based from and if you remember this is the line that our unit triangles followed to determine our rough rafter length the measuring line starts at the corner of the bird's mouth and continues all the way up to the top of the rafter maintaining an equal distance from the bottom of the rafter lastly it's important to note that this line can either move up or down depending on the size of the seat cut so never assume that the measuring line is always the same distance from the bottom of the rafter moving on during the process of laying out for the rafters we uncovered a few numbers that we can now use to determine the height of the ridge the first measurement that we uncovered was labeled B or the unit rise which in our example is 9 inches the other measurement we found was the total run which was 2 feet or 24 inches but that measurement for the thickness of the ridge so for this time around we need to calculate the total run minus half the thickness of the ridge in order to get our total Ridge height so in this case that's 24 minus 3/4 of an inch again because our ridge board that we're using is an inch and a half thick with a total round of twenty three and a quarter inches and a rise of nine inches it's time to look at our first equation which is f equals B times D divided by 12 so for our mock up the F is the theoretical Ridge height B is the unit run and D is the total run therefore the math looks like this 9 times 23 and 1/4 divided by 12 for a height of 17 and a half as you can see 17 and 1/2 gets us up to the theoretical Ridge height which is at the intersection of the ridge and the measuring line so the next step is to find the height above plate which is the final number we need to solve for our total Ridge height to get the height above plate simply extend the heel line of the birdsmouth up and then measure from the corner of that bird's mouth to the top of the rafter in case you're wondering why they call it height above plate it'll make better sense if you look on the backside of the rafter here you can see I've actually drawn the same extended heel line but this time you can actually see the top of the plate so the height above plate is exactly that it's the distance from the top of the plate to the top of the rafter and this measurement is really important when we start working with hip and Valley rafters well the height above plate of 4 and 1/8 and with a theoretical Ridge height of 17 and a half it's time to look at our last equation which is G equals F plus E for our mock-up the G is the total Ridge height F is the theoretical Ridge height and E is the height of a plate therefore the math looks like this 17 and 1/2 plus 4 and 1/8 equals a total Ridge height of 21 and five-eighths as a summary you need to know four things in order to calculate for the total Ridge height first you need to know the ridge thickness second you need to know the total run – the ridge thickness and third you need to know the unit rise or pitch which is often noted as a ratio like our roof which is a 9 12 pitch 9 being the rise and lastly you need to know the height above plate if you have any questions leave them in a comment section below be sure to check out this video right now if you didn't already watch it thank you for joining me today thank you for subscribing and I'll see everybody next week bye
