Mayhem
Bronze Member
If I have a building 30' wide with a 7/12 pitch gable roof, what would my rafter length be? /forums/images/graemlins/confused.gif
Math Guru Needed
- Thread starter Mayhem
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Gatorboy
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I believe with no overhang, it would be about 17' 4"
Half the width is 15', so over 15' the rise would be (7"x15) = 105" or 8.75'
15^2 + 8.75^2 = 17.366^2
Half the width is 15', so over 15' the rise would be (7"x15) = 105" or 8.75'
15^2 + 8.75^2 = 17.366^2
Bird
Rest in Peace
17.365554 feet, so yep, that .365554 of a foot is just a hair over 4"; better add a hair over a third of an inch. /forums/images/graemlins/grin.gif
Dave, that formula is one I'll probably never forget because a fellow who worked for me asked the same question in 1969 and I couldn't remember the formula (the square of the hypotenuse is equal to the square of the other two sides of a right triangle). So I looked in the Dallas yellow pages and found two people listed under "mathemeticians" and called one of them. He couldn't remember either, but about 30 minutes later, he called back with the answer. Of course we had a good laugh over it, but I'll bet neither of will ever forget again.
Dave, that formula is one I'll probably never forget because a fellow who worked for me asked the same question in 1969 and I couldn't remember the formula (the square of the hypotenuse is equal to the square of the other two sides of a right triangle). So I looked in the Dallas yellow pages and found two people listed under "mathemeticians" and called one of them. He couldn't remember either, but about 30 minutes later, he called back with the answer. Of course we had a good laugh over it, but I'll bet neither of will ever forget again.
EddieWalker
Epic Contributor
Do you need the pitch that steep because of snow loads? I'd lower it to 6:12 just to make the math easier if I was doing it myself.
I'd also bring the walls in a couple feet. so I could use 16 foot long boards. It really sucks to have to scab together two pieces when your only short by a foot or so.
I made my workshope plans based on how far I wanted to span the space with standard sized material.
Have you run the price differenc on a 28 foot wide building wiht a 6:12 pitch? I haven't checked, but I think it should work.
Eddie
Eddie
I'd also bring the walls in a couple feet. so I could use 16 foot long boards. It really sucks to have to scab together two pieces when your only short by a foot or so.
I made my workshope plans based on how far I wanted to span the space with standard sized material.
Have you run the price differenc on a 28 foot wide building wiht a 6:12 pitch? I haven't checked, but I think it should work.
Eddie
Eddie
Charlie_in_TX
Gold Member
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16ft is only a magic number if you are not building trusses.
EddieWalker
Epic Contributor
True, you can scab together just about any length you want, and the truss manufacturers do this all the time. If you're building your own truss, then I prefer to use a solid piece of wood for the top chord.
If his length required for the top chord is over 17 feet, then he'll have to increase the length of readily available lumber. This will create a weak point in the design that has to be compensated for. This is not overly dificult, nor will it result in a weaker structure, but it does add cost to the design.
If his length is 16 ft or under, then he can avoid that and build a simpler, cheaper and easier truss.
My suggestion is to save him money with a small modification to his plan. He can easily maintain his square footage by increasing his length of the building. Besides, 28 feet wide as compared to 30 feet really isn't a significant diference in my opinion.
Just my thoughts,
Eddie
If his length required for the top chord is over 17 feet, then he'll have to increase the length of readily available lumber. This will create a weak point in the design that has to be compensated for. This is not overly dificult, nor will it result in a weaker structure, but it does add cost to the design.
If his length is 16 ft or under, then he can avoid that and build a simpler, cheaper and easier truss.
My suggestion is to save him money with a small modification to his plan. He can easily maintain his square footage by increasing his length of the building. Besides, 28 feet wide as compared to 30 feet really isn't a significant diference in my opinion.
Just my thoughts,
Eddie
Farmwithjunk
Super Member
OR... Buy 18 foot lumber. It's a bit more costly, but it's available. I'm doing a material take-off for a job I'm estimating right now. (Actually, I'm playing on the internet, but don't tell my boss!) The job requires some 26 foot 2"X10"'s. Those babies ain't gonna be cheap.
bugstruck
Platinum Member
The others have it right @ 17'-4 3/8". Deduct half your ridge thickness (not height) if you have one. 3/4" deduction if you have a 1 1/2" ridge (close enough on a 7/12). Make the 7/12 plumb cut @ the ridge using the short leg of the framing square or a speed square. Pull the tape 208 3/8" and make another 7/12 plumb mark parallel to the first, but don't cut it there unless you have zero overhang, which would be near unimaginable. Then measure up about 1 1-2" from the bottom of the rafter on that second line and make a mark. Flip the square around and use the long leg, or you can just square off the second line you just made, at the mark you just made, and draw a line going back to the ridge. That creates the horizontal portion of the birds mouth cut that sits on the beam or wall. Remove the materials below that line (small triangle). You should extend the Overhang on the rafter as desired and make the same plumb cut you did for the other two. Rafter is now done, unless you have a secondary overhang cut to accommodate the fascia height. Similar to a birds mouth cut
If you have a ridge beam that needs to be set first..... do the following items. Many struggle here and it's easier than the rafter for certain.
1. measure the remaining length of the second line you made on that rafter from the birds mouth cut up to the top edge of the rafter. Follow the line!! Lets say for illustration it measures 5". Will vary depending on exact placement of the birds mouth and rafter size.
2 . measure up from the top of the wall or beam that the rafters bear on and measure up 110" to the top of the ridge. How did I get there? 7" rise over half the span 15' is 105" plus the 5" plumb mark left on the rafter second cut. Remember the rafter is sitting up above the wall bearing point by that amount, so you need to move the ridge up by the same amount to maintain the true 7/12 pitch or slope.
I've skipped a couple of refined dimensions for positioning the ridge and technically just drove it about 7/16" too high for reasons I'll wear us out with if I continue. So assuming it's a 1 1/2" thick ridge , drop it 7/16" to 109 9/16". Just remember to follow 1 and 2 above, and then drop 7/16". That nails the ridge height for a 7/12 pitch.
You need 20' rafters with the overhang. You can order them, just make sure they are sized right for you snow or wind loads.
Also, if you can mock the roof up on the slab, it's allot easier and certain to work. Don't always have that option though.
HTH
If you have a ridge beam that needs to be set first..... do the following items. Many struggle here and it's easier than the rafter for certain.
1. measure the remaining length of the second line you made on that rafter from the birds mouth cut up to the top edge of the rafter. Follow the line!! Lets say for illustration it measures 5". Will vary depending on exact placement of the birds mouth and rafter size.
2 . measure up from the top of the wall or beam that the rafters bear on and measure up 110" to the top of the ridge. How did I get there? 7" rise over half the span 15' is 105" plus the 5" plumb mark left on the rafter second cut. Remember the rafter is sitting up above the wall bearing point by that amount, so you need to move the ridge up by the same amount to maintain the true 7/12 pitch or slope.
I've skipped a couple of refined dimensions for positioning the ridge and technically just drove it about 7/16" too high for reasons I'll wear us out with if I continue. So assuming it's a 1 1/2" thick ridge , drop it 7/16" to 109 9/16". Just remember to follow 1 and 2 above, and then drop 7/16". That nails the ridge height for a 7/12 pitch.
You need 20' rafters with the overhang. You can order them, just make sure they are sized right for you snow or wind loads.
Also, if you can mock the roof up on the slab, it's allot easier and certain to work. Don't always have that option though.
HTH
rox
Veteran Member
bugstruck,
I hope the heck you hang around TBN for a long time.
I had absolutly NO IDEA what you wre talking about, but you sure as heck convinced me that you know the subject matter.
You obviously are a pro, and we need those around TBN.
Not to lay on the flattery too much but, da*n you sure do seem pretty smart....
I hope the heck you hang around TBN for a long time.
I had absolutly NO IDEA what you wre talking about, but you sure as heck convinced me that you know the subject matter.
You obviously are a pro, and we need those around TBN.
Not to lay on the flattery too much but, da*n you sure do seem pretty smart....
bugstruck
Platinum Member
Rox,
Appreciate the kind comments, but it's really just a case of "you can't beat a man at his own game". Makes me look better than I actually am. Funny how the things we did in our 20's stick better than what we did years later. UT-O, that's a sign of old age and I'm a little young for that /forums/images/graemlins/grin.gif. Knew that post would go past allot of people for good reason. It's one of those "if I have to know it" I'll read it 5 times and figure it out" , or ask another question.....things. Not something the average reader is going to stay with. So I'm not suprised with your "No Idea" remark. I have the same sentiments when these guys go over my head with some of their posts. Just say HUH? /forums/images/graemlins/tongue.gif /forums/images/graemlins/smile.gif There are a some real brainiacs up here but I ain't one of them. /forums/images/graemlins/grin.gif
Appreciate the kind comments, but it's really just a case of "you can't beat a man at his own game". Makes me look better than I actually am. Funny how the things we did in our 20's stick better than what we did years later. UT-O, that's a sign of old age and I'm a little young for that /forums/images/graemlins/grin.gif. Knew that post would go past allot of people for good reason. It's one of those "if I have to know it" I'll read it 5 times and figure it out" , or ask another question.....things. Not something the average reader is going to stay with. So I'm not suprised with your "No Idea" remark. I have the same sentiments when these guys go over my head with some of their posts. Just say HUH? /forums/images/graemlins/tongue.gif /forums/images/graemlins/smile.gif There are a some real brainiacs up here but I ain't one of them. /forums/images/graemlins/grin.gif