Simple Jack Rafters Length Calculation Method Used By My Grandfather - House Framing Secrets

[@juanrojas7505]

This is the first time I find someone explaining with math the magic of the jack rafters without just extracting data from a construction calculator. This is freedom. No gadgets. Outstanding. Respect from chile.

[@justinowen8749]

How did you get the number for the rise is what I am not sure of like I asked before pitch x itself or pitch times the run

[@justinowen8749]

So how did you get the number 16 ? If it were a 5/12 would the # be 20 or 25

[@justinowen8749]

And then do you divide the longest jack rafter by the number of jack rafters or is it always divided by 2 3 and 4 for 1’ oc 2’ oc and 16” oc

[@justinowen8749]

Where does the 4x 4 come from ? 4 inch pitch x itself or 4inch pitch x 4’ run

[@Jay-tk7ib]

Something to remember, once you know your common length difference, you can simply subtract that amount from the length of your common rafters to get the length of the longest Jack rafter. If you're starting from the bottom and going up, the common length difference will be the length of the shortest jack rafter.

[@clayjones553]

good comment 👍

[@juanrojas7505]

What kind of sorcery is that

[@Jay-tk7ib]

@@juanrojas7505 I wouldn't know about sorcery, we were discussing carpentry.

[@vivalaleta]

Mr. Van, I have a question about architecture that is mostly about appearance. The house we are buying is a small farm house with a lean-to addition on the front. We want a smallish mudroom/wood storage room built on the front of the lean-to but can't figure a way to make it look right. Any ideas?

[@gregvancom]

Check out our home addition design videos and if you can't find something then email me some pictures.

[@vivalaleta]

@@gregvancom Thanks. I will.

[@gozo165]

Love your videos. I would like to clarify the length of the rafter, which is 4' 2 5/8" = 50.625" in the example shows 50.596" is that an error or I missed something. I know is not a big deal but since I am new I like to make sure. Thank you again and hope is not an offence.

[@gregvancom]

I probably said about 50 and 5/8, but no biggie and no offense, that why I put the numbers so viewers can break it down to /16 or /32 of an inch if they want.

[@andygrossauer8192]

How do much do you adjust the rafter length to account for the birdsmouth?

[@gregvancom]

The rafter length is usually not effected by the size of birdsmouth.

[@justinowen8749]

So if it were a 5/12 would it be 5x 5 or 5x 4

[@arsitekpintar]

Great

[@fasteddie8782]

The best thing would be to figure out before you lay your wall so too so you have something underneath them but you got to remember that's to the center of where a 40 will be nice to use a 48-inch hypotenuse so you could use one sheet of plywood and just measure it down to nothing and have a fit perfect no waste at all and the other cut will fit in the other side but I wonder on the difference in lengths of Jack wrap this is that one foot four and 7/8 the same as the first line under a four pitch on a framing square just wondering what good video man I liked it keep up the good work

[@Jay-tk7ib]

Yes, that is why the 16 7/8 is stamped on the 3rd line of your framing square, under the 4 inch mark. As it says, "DIFF. IN LENGTH OF JACKS 16 INCH CENTERS. If you were using 24 inch on center spacing, you would use the 4th line instead of the 3rd.

[@gregvancom]

Thanks for sharing:)

[@gregvancom]

Some framing square measurement can create problems on longer spans.

[@Jay-tk7ib]

@@gregvancom It’s good to know how to do things mathematically, as well as using your square.

[@Jay-tk7ib]

@@gregvancom The numbers on the framing square that FAST EDDY was referring to is giving the common length difference in Jack rafters. The math is already calculated for you. You are correct, when you're calculating incrementally you could get a bit off by using the step-off method, etc. if you're not careful.

[@pcatful]

I think you don't talk about why dividing it into equal units works. I don't see you explain that these sloped triangles are similar triangles and so the sides will have a direct proportional relationship.

[@Jay-tk7ib]

Why not just measure out from the center point of your King Common rafter to the corner and calculate to see how many jack rafters you'll need (counting the corner as one, though a rafter may not actually go there). Keep it simple.

[@gregvancom]

I would disagree, but if you didn't get it, then hopefully other read your comment.

[@gregvancom]

You might have made it to simple. You might have problems with on center spacing unless half the rafter span is in 4 foot increments or sections that can be divided into rafter on center spacing.

[@Jay-tk7ib]

@@gregvancom The way I’m talking about works.

[@Jay-tk7ib]

@@gregvancom The "keep it simple" comment was meant for Peter B. You actually made it simple in the video.

[@jasonhayes5036]

5/8 to decimal is .625

[@Outrunninaround.]

Rafter spacing divided the cosine of the rafter angle. 📐 this is the difference in length jack rafters decrease by size as they go down. 16/ cos18.43 degrees = 16.86” then subtract the 1 1/16. This is where your 45 degree bevel intersects.