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ridge height calculation

13K views 19 replies 8 participants last post by  creeper 
#1 ·
can someone check my working please? lgth 7500 x width 6050.wall height 3000. roof pitch 32.5.ridge beam 90 wide .made up of 2 x360 x 45 lvls(-15 mm notches both sides reduces wdth to 60)seat cut 90x57
tan 32.5 x 3025=1927 rise.theoretical ridge height 4927
hap 190-57=168
actual ridge height 4927+168-19(from 1/2 ridge)=5076 to top of beam?
i used an online calculator that gave 5095 as the height.did he allow the reduction in height?
using secant 1.1857 x run 3025 i get a rafter lgth 3587 -a 36mm shortening(from a 30mm ridge) giving me 3551 to the seat cut.im not worried about the overhang,i just cut oversize and chalk it to get a straight fascia.but the ridge has to be right as it weighs a ton.its supported by a saddle post made of 4 x 140 x 45 in a balloon framed gable end wall.any help would be gratefully received as i really dont want to blow this thanks
 
#16 · (Edited)
Ridge height = theoretical height + rafters' height above the plate - ridge drop.

I checked your numbers and I found one mistake: 190 minus 57 does not equal 168, but 133. That 190 mm dimension in your rafters implies that your rafters are 11-3/4" wide, right? And your walls are 2x4s because your seat cut is 90 mm for the level cut?

Theoretical height = 3025 mm*tan (32.5d) = 1927 mm

H.A.P. = 190 mm - 57 mm = 133 mm

Ridge drop = 30 mm*tan (32.5d) = 19 mm

Height above the walls = 1927 mm + 133 mm - 19 mm = 2041 mm

I also get 3551 mm for the rafter length from the ridge beam to the plumb cut at the bird's mouth.
 
#3 ·
ridge height calc

yes thats right.measurements are expressed in mm and the pitch is expressed in degrees.its called the metric system.we havent used feet and inches since the 1970s and most calcs are done using a basic school scientific calculator with trig function.
an 8 in 12 pitch has no relevance with the metric system and all the architects and engineers use degrees for roof angles now.
i guess im asking if someone could check my methodology to see if ive missed something.
it would be the same situation as if the pitch was not in the book or you couldnt find your tables or that guy you lent it to never returned it.
so anyway span 6050 roof angle 32.5 degrees.i put all the measurements in the previous post
heres hoping someone can help.thanks
 
#6 ·
Oxbow, you are almost correct about the English system of units, I believe it is still used in Burma. Course they changed their name to Myanmar, and maybe now they use the metric system? Course you can still get a pint of ale in UK, and standard gage railroad track is still 4 foot 8-1/2 inches (width of a Roman road). And I bet you never heard anyone say

Give him an inch and he'll take 1,609.344 meters.

Also consider what a metric version of Whole Lotta Love by Led Zeppelin would sound like.
 
#8 · (Edited)
oxbow said:
yes thats right.measurements are expressed in mm and the pitch is expressed in degrees.its called the metric system.we havent used feet and inches since the 1970s and most calcs are done using a basic school scientific calculator with trig function.
an 8 in 12 pitch has no relevance with the metric system and all the architects and engineers use degrees for roof angles now.
i guess im asking if someone could check my methodology to see if ive missed something.
it would be the same situation as if the pitch was not in the book or you couldnt find your tables or that guy you lent it to never returned it.
so anyway span 6050 roof angle 32.5 degrees.i put all the measurements in the previous post
heres hoping someone can help.thanks
I don't use a table or anything. Just my brain.
Just tell me the span and the desired pitch and I'll set you straight.
I guess I could figure it out in
metrics, but that isn't the way we do it in America, and I'm an American.
I don't have a square in front of me either. That must be about a 5 1/2//12? Why?
Sent from a Samsung Galaxy S2
 
#9 ·
ridge height

titanoman sorry but we're just on different pages.what i have to deal with is a set of plans drawn how they are drawn using this system.its just the way it is and like all carpenters everywhere i have to make it work with how it is not how i'd like it to be
 
#12 · (Edited)
oxbow said:
titanoman,thanks i appreciate your effort.heres hoping someone familiar with degrees and metrics can help
Your welcome. Given a little time, I could convert and, check your figures for you.
But that would take a lot of math, therefore a couple hours.
With my luck, somebody that already knows the metric system would beat me back to you and all my efforts would be in vain.

Don't you have someplace you can chalk out what you have?
If it looks good on the floor, it'll look good in the air.
Also, I think you said you had a gable pocket to stick the ridge in?
So...the wall hasn't been built yet?
Which means you will need to chalk it out anyways for your plate and stud lengths, etc anyways?
Or are you trying to make a material list? It's all still on paper?
If the gable's already built...well, obviously there's nothing to check...?
Just curious.

Sent from a Samsung Galaxy S2
 
#13 ·
ridge height

yep, both the gables are up and i think i need to lift the beam because i miscalculated.i can simply put blocks in the pockets so all is not lost.i just want to feel confident that my rafters and birdsmouths fit.im pretty much doing this job by myself. theres no ceiling joists so ive got a mobile scaffold under the beam but i hate lifting timber up and down more than i have to.i found a site called blocklayer.com.au with a calculator that looks pretty good and agrees with my numbers.(he also does feet and inches)
im just going to go for it.wish me luck
 
#14 ·
oxbow said:
yep, both the gables are up and i think i need to lift the beam because i miscalculated.i can simply put blocks in the pockets so all is not lost.i just want to feel confident that my rafters and birdsmouths fit.im pretty much doing this job by myself. theres no ceiling joists so ive got a mobile scaffold under the beam but i hate lifting timber up and down more than i have to.i found a site called blocklayer.com.au with a calculator that looks pretty good and agrees with my numbers.(he also does feet and inches)
im just going to go for it.wish me luck
Good luck and be careful.
 
#20 ·
Just FYI we still mainly use Feet and inches in Canada too. Some metric though.:furious:
This is where I disagree with you. With the exception of perhaps your trade, of which I know nothing about, Canadians mostly use the metric system. School kids are taught kilometres and litres. They have no idea about miles and quarts
Food labels
Road signs and speed limits
Temperatures
Gas fill ups
 
#17 ·
ridge height

thanks abracaboom,you are right .the 190 is a rafter size that has a plumb cut of 225.minus 57 is the 168 hap.my mistake.i knew what i wanted to say but got confused.
the 90mm seat cut gives full seating on a 90 x 45 plate
HAP = 225-57 = 168
Height above walls would be
1927 + 168 - 19 = 2076
what do you think? go for it?
 
#18 ·
thanks abracaboom,you are right .the 190 is a rafter size that has a plumb cut of 225.minus 57 is the 168 hap.my mistake.i knew what i wanted to say but got confused.
the 90mm seat cut gives full seating on a 90 x 45 plate
HAP = 225-57 = 168
Height above walls would be
1927 + 168 - 19 = 2076
what do you think? go for it?

If your gables are up, it seems to me like your dimensions are set. If you are hesitant, you might want to cut only two rafters and set them against the gables to see if they match at the height you calculated.
 
#19 ·
I know what you mean about not wanting to lift lumber up and down any more than you have to. My wife and I built our log home by ourselves. Our roof is 12/12 pitch (45 degrees to you) so the ridge beam is way up there. Took a lot of scaffolding but we got it done.

I'm not a carpenter, or any other building trade, just an engineer and experienced DIYer. For my rafters, I drew everything up with AutoCAD and pulled needed dimensions and angles from there. Worked like a charm. I can't read a rafter square worth a hoot.

Talking about the metric thing, my wife called the other day and said she needed air in one of her car's tires but couldn't find a pump with enough pressure. She said the sticker on the door jamb said she needed 200! I told her that was kPa, which is around 32 psig in our world.
 
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