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dlarrivee

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Discussion starter · #1 ·
When framing a roof without using trusses, instead using a ridge beam, supported by king posts on gable ends how do I figure out how long to make the rafters?

I will need to choose a pitch, but just for example a 6" on 12" pitch, and the top caps are 10' outside-to-outside.

So, the king post will be at 5', and will reach 60" high, do I subtract the height of the ridge beam from the 60"?

Once I have my ridge beam up, I would like to use rafters with birds mouths cut out of them.

Lets say the ridge beam is a 2x6 and will have 12" overhang on each gable end. So I would need to subract 3/4" from each rafter for the width of the beam.

Pythagorean Theorem says that a^2 + b^2 = c^2

b being rise and a being run I come up with this

60"^2 + 60"^2 = 7200"^2

Square root of 7200" is about 84 7/8", correct?

So I'm assuming my rafter should be 84 7/8" minus 3/4" for the width of the ridge beam plus whatever overhang I'd like?

I'd also like to figure out where to make my birds mouth?
 
#2 ·
Before I add how I would do this, I just have to ask why you are going about this in this way? Some may differ, but roof pitch is purely a "close" to, but not neccessarily exact (like you think someone would actually crawl up there and measure to the 1/8 inch?) Is there a specific dimension of roof you are shooting for, or a reduction in framing?
 
Discussion starter · #3 ·
I'd just like to learn how to do things the correct way, framing of a house should be exact in my opinion, it's the skeleton. If the bones in your body were only built "close to but not exact" you may end up with problems down the road right?
 
#4 ·
dlarrivee said:
When framing a roof without using trusses, instead using a ridge beam, supported by king posts on gable ends how do I figure out how long to make the rafters?

I will need to choose a pitch, but just for example a 6" on 12" pitch, and the top caps are 10' outside-to-outside.

So, the king post will be at 5', and will reach 60" high, do I subtract the height of the ridge beam from the 60"?

Once I have my ridge beam up, I would like to use rafters with birds mouths cut out of them.

Lets say the ridge beam is a 2x6 and will have 12" overhang on each gable end. So I would need to subract 3/4" from each rafter for the width of the beam.

Pythagorean Theorem says that a^2 + b^2 = c^2

b being rise and a being run I come up with this

60"^2 + 60"^2 = 7200"^2

Square root of 7200" is about 84 7/8", correct?

So I'm assuming my rafter should be 84 7/8" minus 3/4" for the width of the ridge beam plus whatever overhang I'd like?

I'd also like to figure out where to make my birds mouth?
Click to expand...


I assume by "Top Caps" your talking about the top plate outside to outside measurement, which is the span of the roof.

1/2 the span of the roof is 5'. So your rafter run is 5' minus 1/2 the thickness of the hip.

Your ridge is a 2x6.

5' - 3/4" = 4' 11-1/4" (Rafter Run)


4' 11-1/4" [Run]

6{Inch] [Pitch]

[Diag] = 5' 6-1/4" (Rafter Length)

[Rise] = 2' 5-5/8" (Not the real rise, doesn't include H.A.P cut)

H.A.P. cut is (Height Above Plate) after birdsmouth is made. You need to know that foir and exact measurement.

Tell me what size rafter you arte using and I can tell you what the exact measurement to the top of the ridge or bottom of the ridge.

It doesn't matter any3way because you don't need to know the height of the ridge and set the ridge first. I've never done that once in my life for a gable roof. I do it for roofs with big structural beams.

You set the front and back rafters in and nail them on top of the plate on both ends of the building with the top of the plumbcuts butting into each other, then you slide the ridge in between them and nail the rafters to the ridge. That sets your ridge height.

If you want too know the math to get the exact ridge height, I can show you that. I use a Construction Master Calculator.

The easiest way to get the exact ridge height is to figure from the inside of the plate deducting 1/2 the thickness of the ridge and then adding the height of the plunbcut at the ridge. That will give you an exact ridge height on paper, not always in the field. You don’t need to know the H.A.P. cut and is less steps to figure.
 
Discussion starter · #5 ·
4' 11-1/4" [Run]

6{Inch] [Pitch]

[Diag] = 5' 6-1/4" (Rafter Length)

I understand the run and 6" pitch but how did you calculate the 5' 6 1/4"? And does this include the overhang?

This whole project is hypothetical I just wanted to learn how to calculate a simple roof, so let's say the rafters are 2x4s.

Oh and I understand that I don't really need to set the ridge beam up first too.
 
#6 ·
dlarrivee said:
4' 11-1/4" [Run]

6{Inch] [Pitch]

[Diag] = 5' 6-1/4" (Rafter Length)

I understand the run and 6" pitch but how did you calculate the 5' 6 1/4"? And does this include the overhang?

This whole project is hypothetical I just wanted to learn how to calculate a simple roof, so let's say the rafters are 2x4s.

Oh and I understand that I don't really need to set the ridge beam up first too.
Click to expand...



2’ 5-5/8”² = 6.094727 SQ/FT + 4’ 11-1/4”² = 24.37891 SQ/FT = 30.47363 SQ/FT

Square Root of 30.47363 SQ/FT = 5’ 6-1/4” (Rafter Length to plate)

If you want to include the overhang, you just add 13.414641” or 13 7/16”.

A 6/12 pitch travels per foot on a diagonal 13.41641”. Your framing square will tell you 13.41” and you multiply that by the run of the rafter.

Your example of a 5’ run without ½ thickness deduction for ridge would be.

13.41641 x 5 = 67.08204” or 67-1/16”

Subtract ½ the thickness of the ridge ¾”

.75/12 = .0625”

13.41641 x .0625 = .838526” or 13/16”

67 1/16” – 13/16” = 66-1/4” or 5’ 6-1/4”
 
Discussion starter · #7 ·
Probably easier to just find the common rafter length, subtract half the ridge thickness and use a a rafter square to do the overhang and birdsmouth?

pitch 6/12
span 10'
run 5'
rise = 6" * 5' = 30"

CRL squared = run squared + rise squared
CRL squared = 60^2 + 30^2
(4500)squared = 3600 + 900

square root of 4500" = 67"

Then just use a square set at 6" and 12", mark ridge cut, mark ridge cut taking into account ridge beam width, mark CRL that will give me the vertical line of the birdsmouth cut.

Then I just take 2/3 of the ridge cut which is 2-5/8" for the HAP line.

Add on my overhang using square and I'm done?

That sounds alot simpler than trying to calculate the entire length with all the compensations taken into account then cutting.

Anything wrong wtih my calculations?
 
#8 ·
dlarrivee said:
Probably easier to just find the common rafter length, subtract half the ridge thickness and use a a rafter square to do the overhang and birdsmouth?

pitch 6/12
span 10'
run 5'
rise = 6" * 5' = 30"

CRL squared = run squared + rise squared
CRL squared = 60^2 + 30^2
(4500)squared = 3600 + 900

square root of 4500" = 67"

Then just use a square set at 6" and 12", mark ridge cut, mark ridge cut taking into account ridge beam width, mark CRL that will give me the vertical line of the birdsmouth cut.

Then I just take 2/3 of the ridge cut which is 2-5/8" for the HAP line.

Add on my overhang using square and I'm done?

That sounds alot simpler than trying to calculate the entire length with all the compensations taken into account then cutting.

Anything wrong wtih my calculations?
Click to expand...
Yes, your 1/16" off:laughing:

I just figured you wanted to know the math. It’s a lot easier just deducting 1/2 the thickness of the ridge with your framing square if your going to figure out rafters the way you do.

If your run was 5' 8-1/2", then you just subtract 1/2 the thickness of the ridge in your head and mark 7-3/4" plumb after the 67-1/16"

I use the Construction Master Calculator. It's the fastest and easiest thing to use. Before this I always used the numbers on the framing square. Stepping off is a waste of time and room for to many era's.
 
Discussion starter · #9 ·
You're correct! I was off 1/16" because I rounded off the 0.08 I believe.

I was mostly interested in just figuring out how to frame a roof, without pre-fab trusses, but also the most simple way possible.

I think I've got it figured.

This calculator converts decimals to fractions /8 / 16?

Also, if my roof had gable ends, I'd need to add framing to support the sheathing, do I basically just measure 16" centers from the center of the span and miter 2x4s to fit under it?
 
#10 · (Edited)
Geez,,......reading that made my head hurt....1/16"......I admire you for wanting to learn the absolutely correct way to calculate the r/r and tech side of the roof, and I hope my guys never want to know...just let me tell them without an explanation.
 
#11 ·
dlarrivee said:
You're correct! I was off 1/16" because I rounded off the 0.08 I believe.

I was mostly interested in just figuring out how to frame a roof, without pre-fab trusses, but also the most simple way possible.

I think I've got it figured.

This calculator converts decimals to fractions /8 / 16?

Also, if my roof had gable ends, I'd need to add framing to support the sheathing, do I basically just measure 16" centers from the center of the span and miter 2x4s to fit under it?
Click to expand...
If you want the fastest way and most simple way to figure a rafter are with a Construction Master Calculator. No need for A² + B² = C². You need to figure out the rise to use that.

CM calculator, you just punch in the Pitch and the Run and hit Diagonal for your rafter length, Rise for your Rise and Hip/Valley button for your hip or valley length.

Example for your roof if you had a hip roof or valley using a 10’ span, 5’ run and a 2x ridge.

4 [Feet] 11[Inch] ¼ [Inch] [Run]

6 [Inch] [Pitch]

[Diag] = 5’ 6-1/4” (Common Rafter length)

[Hip/Val] = 7’ 4-7/8” (Hip and Valley Length)

[Hip/Val] = 19.47° (Hip Plumbcut Angle or 6/17 with the Framing Square)

[Rise] = 2’ 5-5/8” Rise without Plumbcut Height)


As for your gable ends, I just stack the gable studs over the walls studs underneath with the saw set at the 6/12 pitch angle which is 26.57° or 26.5°.
 
Discussion starter · #12 · (Edited)
Wow. That calculator sure makes things alot easier!

Now for hip/valley jack studs, I have heard the term "dropping the hip" does this apply?

Also, is [Hip/Val] = 19.47° (Hip Plumbcut Angle or 6/17 with the Framing Square) the angle I would miter the ends of the rafters where they meet the hip?

I suppose a miter saw is prefered for framing, over a table saw w/ miter gauge?

Oh and you can add / subtract [feet] [inches] [fraction of inches]?
That would have helped me a great deal the other day, and I'm not even a carpenter!
 
#13 · (Edited)
dlarrivee said:
Wow. That calculator sure makes things alot easier!

Now for hip/valley jack studs, I have heard the term "dropping the hip" does this apply?

Also, is [Hip/Val] = 19.47° (Hip Plumbcut Angle or 6/17 with the Framing Square) the angle I would miter the ends of the rafters where they meet the hip?

I suppose a miter saw is prefered for framing, over a table saw w/ miter gauge?

Oh and you can add / subtract [feet] [inches] [fraction of inches]?
That would have helped me a great deal the other day, and I'm not even a carpenter!
Click to expand...

Dropping the hip is a term used for the H.A.P.cut (Height Above Plate) where you have to mark the same H.A.P. cut at the plate line as the common rafter H.A.P.cut.

Your hip runs a 45°. When you mark the H.A.P. cut at the measured line, you have to come in towards the plumbcut of the hip at the ridge 1/2 the thickness of the hip and measure down from the top the same H.A.P. cut as the common rafter H.A.P. cut. That will plane the top of the hip and the common at the plate line.

You can also figure the Hip Drop amount using math, but it’s just easier marking it when you layout. This is something that’s not on the calculator, it’s just something that you have to know and do when laying out.

The 19.47° or 6/17 is the plumbcuts you make at the top of the hip and when you make the birdsmouth plumbcut at the plate line. The saw setting for the jack rafters hit the hip is always set at 45° because the hip is running at 45°. If the hip wasn’t running at 45° and there were two different pitches, the saw setting would be different.

Anytime you have a hip with the same pitch that runs at 45°, no matter what the pitch is the saw setting for the jack rafters is always 45°. The plumbcut angle is the same as the commons and the hip plumbcuts at the top and bottom are always whatever the pitch is on 17.

Your 6/12 pitch common rafter, the hip is 6/17. When you make your cheek cuts for your hip at the top where the ridge is, it’s always 45°. When you make the cheek cuts at the fascia line of the hip it’s always 45°

I’ve never used a miter saw for framing before, only with the exterior trim. Every rafter is cut with a circular saw.

The CM calculator does everything you want in feet and inches.

Here's a drawing I made showing the proper way to mark the Hip for the "Hip Drop " amount.
 

Attachments

Discussion starter · #14 · (Edited)
Thanks alot Jim for all your help and expertise, please excuse my ignorance for I am not a carpenter.

I found this page which in combination with the information you provided has helped me to understand this subject alot better.

eteknic.com/HipRafter.html
(I'm not allowed to post links yet?)

I am still a little unsure though...

On that page they show the HAP being adjusted for the drop aswell as the length of the hip rafter being shortened?
Your explaination only mentions taking off half the hip thickness?

Is there any real benefit of a hip roof over a gable end other than esthetics anyways? It seems like alot of work just to reduce the room in your attic.

I think I may just purchase one of these calculators, it seems they can make everything about roof framing more simple.

Thanks again!
 
#15 ·
dlarrivee said:
I am still a little unsure though...

On that page they show the HAP being adjusted for the drop aswell as the length of the hip rafter being shortened?
Your explaination only mentions taking off half the hip thickness?
Click to expand...
The Hip runs at 45°. Your hip length goes from the ridge to the outside corner of the top plate. The hip is 1-1/2” wide so half of the hip will sit off the corner on each end of the top plate ¾” each side.

Now the ¾” will have a ¾” run to the top plate. That is the same ¾” run I was talking about coming in from the hip length measurement to mark you HAP cut.

That is because where the outside of the hip hits the plate its ¾” in running at 45°. The height from the top plate and up should be the same as the common rafter HAP cut because that’s where the Common Rafter HAP cut hits.

If you took your hip and spun it around perpendicular to the top plate like the common rafter is, the Hips HAP cut would be in the same spot.

The hip running at 45° creating that little ¾” run to the plate also has a little rise, that rise amount is the “Hip Drop “ amount.

I’ve added more to this picture and drew in a plan view looking down so that you can see the Hip how it sits on the outside corner and creates a small triangle showing you the ¾” run to the plate line.

I hope this helps.
 

Attachments

Discussion starter · #16 ·
Sorry I think I understand...

The birdsmouth needs to be longer because it's coming into the top plate at an angle of 45degrees not 90degrees. And that is what makes the hip drop. Correct?

I suppose there is probably 2 or 3 different methods for this.

I still don't see the point of a hip roof over a gable end, but they seem alot more popular these days...
 
#17 ·
I'd say most of the time a hip roof is chosen over a gabel for appearance reasons only but there are some other advantages. A hip roof resists high wind better, provides better weather protection (the lower soffit height protects a greater % of siding from the elements), more window shading and requires less siding. Of course, there are also advantages to a gabel roof.
 
#19 ·
dlarrivee said:
Yeah I suppose with a hip roof there's less vertical surface area for wind to run into.

Protecting from the elements on all sides...

I still prefer a gable.
Click to expand...
It doesn't matter what roof you choose. It's all a matter of what roof one likes for curb appeal, more space in the attic with a gable roof. Gable roof you have more siding and less fascia and soffit work. Hip roof you have less siding and more fascia and soffit work and less room in the attic.

Sometimes a house is designed with a gable because the second floor has a cathedral ceiling with a circle top window 2-3' above the top plate, so now you can put the widow in.

If you have a hip roof you can do that also but you would have to have a dormer roof with walls above the hip roof so that you can put the window in. Some people don't like dormers, so they go with a gable roof.

There are many different scenarios you have for choosing a style roof. It's nothing structural.
 
Discussion starter · #20 ·
Good to know.

Thanks again for all your help Joe, I think I should be able to get by on my own w/ that calculator and a square now.

You said you usually just use a circular saw for framing?

Do you use a miter at all?

Do the sheathers usually use a table saw to cut OSB down to size?
 
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