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Approving Hip Rafter Sizes

jar546

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Educational:You are given a set of prints for a hip roof. The hip roof is typical construction. The specifications are as follows:Building length= 28'Building width= 28'No measurable ridge, center point meeting of hip rafters.Roof Pitch 12/12Dead Load 10psfGSL 30psf#2 SPF Lumber usedWhat size would the 4 equally sized hip rafters have to be prescriptively?

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If I were building this to minimum prescriptive standards I would have to use (3)2x12's as a beam to make that span for each hip rafter.
 
GBrackins said:
I'd use (3)2x12's per Table 3-28 of the 2012 Wood Frame Construction Manual for a 14'x14' area
And prescriptively, that is exactly how I got my answer. Looks like we have some consistency in answers.

Anyone disagree or got a different answer?
 
All answers reference hip "beam". The table in the WFCM is for hip "beam". How do you believe this meshes with IRC R802.3 and the last sentence:

"Where the roof pitch is less than three units vertical in 12 units horizontal, structural members that support rafter and ceiling joists, such as ridge beams, HIPS and valleys, shall designed as beams."

That sort of says when the slope is more than 3:12 that the hips would NOT need to be designed as beams.

For the sake of discussion. I have a 1958 home. It is full hip and 26 ft wide. It is 4:12 pitch. I have a single 2x8 hip board with 2x6 rafters w/purlins. It's still standing, and has under 3 ft of snow.

I don't have a better answer, as the prescriptive methods in the IRC are horrible. However, I'm not sure I would push (3) 2x12's unless it was a vaulted ceiling with no rafter ties.
 
We are assuming a clear span. With consideration for R802.3 and Table R802.5.1(3), (2006 IRC), based on a common rafter length of a little under 20', it is plausable that 2x10 common rafters attached to 2x12 hip rafter can be used prescriptively where common rafters are 12" O.C.

If the building has interior walls or beams centrally located suitable for bracing off of, a number of prescriptive configurations may be achieved using 2x6 common rafters attached to 2x8 hip rafters.
 
Glenn said:
All answers reference hip "beam". The table in the WFCM is for hip "beam". How do you believe this meshes with IRC R802.3 and the last sentence:"Where the roof pitch is less than three units vertical in 12 units horizontal, structural members that support rafter and ceiling joists, such as ridge beams, HIPS and valleys, shall designed as beams."

That sort of says when the slope is more than 3:12 that the hips would NOT need to be designed as beams.

For the sake of discussion. I have a 1958 home. It is full hip and 26 ft wide. It is 4:12 pitch. I have a single 2x8 hip board with 2x6 rafters w/purlins. It's still standing, and has under 3 ft of snow.

I don't have a better answer, as the prescriptive methods in the IRC are horrible. However, I'm not sure I would push (3) 2x12's unless it was a vaulted ceiling with no rafter ties.
This is interesting (I love starting controversy to get things going). 2 of us get the same answer and you point out that the pitch of the roof does not make us go to the WFCM. Well, I agree with you but we still don't have a 2nd opinion on what size hip we would need to make this prescriptively compliant.

Wow, what's going on here? Such a simple, everyday life question for building code compliance.
 
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Jobsaver said:
We are assuming a clear span. With consideration for R802.3 and Table R802.5.1(3), (2006 IRC), based on a common rafter length of a little under 20', it is plausable that 2x10 common rafters attached to 2x12 hip rafter can be used prescriptively where common rafters are 12" O.C.If the building has interior walls or beams centrally located suitable for bracing off of, a number of prescriptive configurations may be achieved using 2x6 common rafters attached to 2x8 hip rafters.
Wait a minute. If you use 2x10 jack rafters to be compliant with the tables at 10# dead and 30# snow, then I agree that the jack rafters must be a minimum of 2x10. But, if they are cut at a 45 degree angle for the 12/12 pitch, the cut ends are now almost 13.5" so the use of a 2x12 cannot be compliant with R802.3

So per the IRC this cannot be prescriptively done unless we use the WFCM?
 
From a framers perspective, you've designed a pyramid. About the strongest shape there is.

2x8 rafters, 16" o.c

Single 2x12 hips

(the above assumes standard ceiling joists, and a point to bear a center post)
 
On the 4 sided pyramid, which way do the ceiling joist or rafter ties run, N to S or E to W? Also in the above example, what size brace would be required?
 
Unless I'm mistaken the total load on the hip is 3,920 pounds. Here's how I got that number... the hip area is 14' x 14' x 40lbs= 7,840 lbs. divided by 2 =3,920 lbs the span is about 20 ft ...looks like 3 2x12's Not sure where to look for this prescriptively without doing the math.
 
STB said:
On the 4 sided pyramid, which way do the ceiling joist or rafter ties run, N to S or E to W? Also in the above example, what size brace would be required?
Ceiling joist perpendicular to a center bearing wall if there is one, otherwise doesn't matter. Roof is symmetrical, add look-outs [stub joist/cripples] and a stiff-back [strong-back], and you're all tied in.

Post/brace = 2-2x4
 
rleibowitz said:
Unless I'm mistaken the total load on the hip is 3,920 pounds. Here's how I got that number... the hip area is 14' x 14' x 40lbs= 7,840 lbs. divided by 2 =3,920 lbs the span is about 20 ft ...looks like 3 2x12's Not sure where to look for this prescriptively without doing the math.
I'm no engineer, but it seems many are looking at the problem two-dimensionally.

In three dimensions, there is much less load on the hip, the jacks actually help hold it up. On a valley rafter, it's the opposite, increased load. That's when the kite/diamond come into play.

How about some SE input??
 
Partially correct. The comment regarding the brace size and the ceiling joist location is a non-prescriptive response. Most building codes require ceiling joist and/or rafter ties to provide resistance to the thrust reaction at the plate connection. This in turn "defies the gravity loads" that are on the roof system, that are increased when subject to snow loads. So per prescriptive requirements of a building code, the rafters must be tied at the plate connection (or close to it) with the ceiling joists and the perpendicular rafters must be tied with rafter ties located above the ceiling joists to form an effective and code compliant connection to resist the rafter thrust. As far as not being an engineer, you sort of just became one (not legally) because your explanation above cannot be found in any prescriptive building code.

"Ceiling joist perpendicular to a center bearing wall if there is one, otherwise doesn't matter. Roof is symmetrical, add look-outs [stub joist/cripples] and a stiff-back [strong-back], and you're all tied in.

Post/brace = 2-2x4 "

Please provide the code section that allows what you refer to as joist/cripples that will typically have a drywall ceiling attached to them to resist the rafters thrust (not sure if drywall can provide resistance), the size, type,quanity, spacing and connection details for your "stiff-back", and what table was used to determine that an approximately 14' Post/brace is an adequate brace.
 
STB said:
Please provide the code section that allows what you refer to as joist/cripples that will typically have a drywall ceiling attached to them to resist the rafters thrust (not sure if drywall can provide resistance), the size, type,quanity, spacing and connection details for your "stiff-back", and what table was used to determine that an approximately 14' Post/brace is an adequate brace.
Let me first admit, I'm playing both sides of the net.

As a Code Official, I completely agree with your statements above. If it's not in The Code, It can't be allowed (prescriptively).

As a Carpenter, I would say wait a sec.

28' square building

12/12 pitch

4 common rafters at 19' 9-9/16 (no ridge needed, drop 3/4" from 2 side commons to sandwich the first 2 mains)

Commons set at 13'-11-1/4" from corner

4 equal hips at 24' 3"

4 left, 4 right of each jack;

17-10

16-0

14-1

12-3

10-4

8-5

6-7

4-8

2-9

((approx.))

Stiff-back at the commons perpendicular to the joist.

Done.

Carpenter: "Now, Mr. Code Official, you want me to put a stiff-back on EVERY rafter?! Even the last (shortest) 4? There's no outward thrust there! The jacks meet at the hips, 1/2 of each pair is tied to the cj. Where's it gonna go?! ..and a triple 2x12 hip!!!!?"

There needs to be some common sense in the Code interpretation. The Code can't outline every situation, nor should we expect an engineered solution every time.

The stick frame methods I learned, were passed down for generations. The homes my great-grandfather built are still standing (they're a bit drafty, and they don't have fire sprinklers...but I digress)

The Code guys (myself included) like to slam the "we've always done it that way" response. But some things work regardless of what the Code says.

Rant over. Happy Independence Day. Independence from tyranny. Thinner Codes are better Codes.

mj
 
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R802.3 Framing details.

Rafters shall be framed to ridge board or to each other with a gusset plate as a tie. Ridge board shall be at least 1-inch (25 mm) nominal thickness and not less in depth than the cut end of the rafter. At all valleys and hips there shall be a valley or hip rafter not less than 2-inch (51 mm) nominal thickness and not less in depth than the cut end of the rafter. Hip and valley rafters shall be supported at the ridge by a brace to a bearing partition or be designed to carry and distribute the specific load at that point. Where the roof pitch is less than three units vertical in 12 units horizontal (25-percent slope), structural members that support rafters and ceiling joists, such as ridge beams, hips and valleys, shall be designed as beams.

I believe you would have to go to a SP SS or #1 for the rafters to get them down to a 2x8 because anything bigger on a 12/12 roof would create an end cut larger than 12" and would not meet 802.3. I don't think you can easily find a 2x17

Then again I am pretty rusty with the charts since I haven't seen a framed hip roof since 1996.

Anything over a 3/12 pitch is acting as a ridge board, correct? Don't the rafters have to sit on top of the hip in order for the hip ridge to considered a beam?
 
mtlogcabin said:
I believe you would have to go to a SP SS or #1 for the rafters to get them down to a 2x8 because anything bigger on a 12/12 roof would create an end cut larger than 12" and would not meet 802.3. I don't think you can easily find a 2x17Then again I am pretty rusty with the charts since I haven't seen a framed hip roof since 1996.

Anything over a 3/12 pitch is acting as a ridge board, correct? Don't the rafters have to sit on top of the hip in order for the hip ridge to considered a beam?
2x8 spf #2, 16" o.c. DL 10psf, GSL 30psf Table 802.5.1.(3) max span 15' 1"

R802.5 Allowable rafter spans. Spans for rafters shall be in accordance with Tables R802.5.1(1) through R802.5.1(8). The span of each rafter shall be measured along the horizontal projection of the rafter.



12/12 cheek cut on a 2x8 (45º) is +/- 10-5/8" - 2x12 = 11" + It works



mj

 
So to prescriptively meet the code without engineering and according to the IRC, in order to make this roof code compliant for a #10 dead and 30# gsl, we would need: (no purlins in this design)

2x8 rafters and a 2x12 hip. Correct? :)
 
jar546 said:
So to prescriptively meet the code without engineering and according to the IRC, in order to make this roof code compliant for a #10 dead and 30# gsl, we would need: (no purlins in this design)2x8 rafters and a 2x12 hip. Correct? :)
That's my story, and I'm sticking to it!
 
"Carpenter: "Now, Mr. Code Official, you want me to put a stiff-back on EVERY rafter?! Even the last (shortest) 4? There's no outward thrust there! The jacks meet at the hips, 1/2 of each pair is tied to the cj. Where's it gonna go?! ..and a triple 2x12 hip!!!!?"
Part of the language you have used is a typical everyday response, "YOU WANT ME TO PUT", no, the code requires it. My typical response would be "You couldn't build the house the way I want it"!

If you tie the rafters to ceiling joists (both directions) and install a properly sized post/brace, 3 2x12's wouldn't be needed. And furthermore, if there is no thrust or downward weight, why would you need the post/brace that you originally suggested?

If the thrust in both directions is resisted by CJ and Rafter ties and the hip or valley is braced to bearing below, then the rafters are not treated as load carrying members. I have been around the block with this for many years and every organization that is affiliated with the codes has agreed with the above statement. I have recently contacted the AF&PA who is referenced in the roof code sections and that is their stand also. So understanding that you as well as I have a framing and a code official background, as a code Official, how can you allow non-prescriptive design and sign off on inspections and CO's which states that it is in conformance with the adopted codes?

There are things that I know as a framer that can be done that are not addressed by code, but without my 16+ years as a plan reviewer and inspector, but as a everyday framer, I cannot provide the proper paperwork for the file of a non-prescriptive design. I think the point of this post was to show that although we may think that something that is not prescriptive can or may work, without the required documentation, how are we as BCO's compliant with the codes.
 
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