It's time to go see an architect who will make up some drawings and then refer you to a structural engineer.

Or, have some decently scaled drawings anc contact an engineer directly.

It's possible that you could install a "collar tie" of sorts, and essentially turn it tinto a truss by tying the collar tie to the existing rafters with webbing and plates.

Essentially, you'd be building a truss or sorts on site.

I don;t have sufficient info from you to run some numbers, and they'd be a wag and not acceptable to your building office at permit pulling time.

Best bet?

Safest bet?

See an engineer.

Probably run you from a few hundred to several hundred dollars.

Best, Mongo

It is unlikely an AHJ will allow changes to rafters to use collar ties without a structural stamp. In some places architects can do it, in many a PE is requried.
Switching to collar ties changes a lot of forces on the roof and the supporting walls.

Thanks for the replies. I think I have decided to scrap this plan for now, in favor of simply exposing the existing rafters in an attractive way (hopefully...)

It is well within the ability of the AHJ and a simple look to the tables in the Code Book to give thumbs up or thumbs down on this issue.

The answer here is simple, and it is thumbs down.

You cannot change the roof configuration without also changing the rafters. Here's why:

Presuming #2 SPF 2x6 rafters, a snow load of 30psf with a dead load of 10 psf...Your existing rafters are already too small for the spans. Maximum span for #2 spf 16" OC is only 11'11". Raising the cross lateral (not 'collar tie') from the top of the wall plates 1/3 the way up toward the ridge further reduces the allowable span of the rafters by 33% or to a maximum rafter span length of about 8'.

Only way you can raise your ceiling by 1/3 is to demolish the roof system and reframe using 2x10s, 2x12s or engineered lumber for new rafters.

Since we do not know the OPs location, the use of a 30 PSF snow load may be excessive. See Figure R301.1(5) of the IRC for the snow load map of the lower 48 states. 30 PSF (and higher) is only used in the northern quarter of the US.

30psf is a good default load when considering snow or wind loads for most of the US.

And in this case, the snow map is completely irrelevant.

Because even if we take the lowest load table in the IRC, a 20psf live load with a 10 lb dead load and use #2 spf 2x6 rafters at 16" oc, the maximum span length is still only 14' 4"...still too small if the span is 1/2 of the 30' width of the structure. ( See Table R802.5.1(1) )

And even if the actual span of the structure is 12' 6" (1/2 of the 25' dimension), although these current rafters [i]can[/i] carry the load at that distance when the cross laterals on top of the bearing walls, they [i]no longer can[/i] once the cross laterals are raised 1/3 the way up above the top plate of the bearing walls.

Maximum span for 2x6 rafters 16" on center with a 20psf live load and 10psf dead load drops immediately from 14' 4" for #2 spf lumber to a maximum span of 9' 7 1/4"....and 2x6s can no longer be used for this roof system. (See Footnote "a" for all rafter tables)

Another thing that has not been mentioned in regards to creating a cathedral ceiling when raising the cross laterals is that one can no longer use the existing ridge board....the ridge board must be replaced with a center bearing engineered [i]girder[/i] at the ridge.

Section R802.3.1 of the 2000 through 2003 IRC states:

[b]"Where ceiling joists or rafter ties are not provided at the top plate, the ridge formed by these rafters shall also be supported by a [i]girder[/i] designed in accordance with accepted engineering practice."[/b]

Similar demands are made in the 2006 IRC.

So the original poster's design question is an elementary one to assess:

Even at the lowest possible live loading conditions, raising the joists above the top plates by 1/3 the distance to the ridge creates all kinds of structural deficiencies, and without increasing the size of the rafters, installing a ridge grider, and other major structural modifications....

.....it simply cannot be done.

30 SPF is way to high for the vast majority of the United States.
It is very far from a " good default load".

The minimum loading in the IRC (or any IRC loading) has nothing to do with how local calculations can be performed by a PE.
The IRC tables provide a 'turnkey' design that an AHJ is typically required to accept as is.
A detailed design by a PE does not have to followthe general guidlines used by the IRC if they can show a specific design meets the local requirements.

Still doesn't matter.

The prescriptive tables in the code and elsewhere are all established by PEs, and standard engineering design will prove no differently. PEs will design to the exact same loading requirements. The laws of physics do not change between the Code and engineering practice.

The bottom line remains, that the original poster cannot raise his cross laterals for this roof without replacing the rafters and replacing the ridge board with a girder.

Anyone arguing to the contrary simply does not understand engineering nor proper framing design.

A PE will perform a detailed set of calculations for the exact application.
All the codes have margin built into them based on the worse case for the conditions defined.
In many cases these conditions can be changed to allow for a different answer.
Wood species, wood grade, and actual loading conditions (snow, wind, seismic, structral differences from the assumed design used for the code tables, etc.) for a location can be exactly accounted for. Even a 20 PSF snow load is not used in most of Texas and the south. The fact that the tables in the IRC do not cover this case does not mean you cannot design for it. It means they saved space in the code book for more important things.
Table R502.5(1) covers girder and header spans in exterior load bearing walls.
It only covers #2 wood, assumes the lowest strength wood, and makes assumptions about roof type in load determination. While many (probably most) houses have gable roofs, a hip roof and a header near a corner have different loading parameters.
Te code tables are designed to allow a non-engineer to use them and produce a structure that will meet the minimum code under the most pessimistic assumptions.
An analysis by a PE of the exact structure and loads will need to be stamped off to be acceptable to the AHJ, but changing any of the assumptions inherent in the code tables will change the final design.
The code does not cover everything, including flitch beams, various engineered wood products (I-joists, gluelams in there many flavors, etc.). The manufacturers typically provide the design services to satisfy the AHJ requirement for an engineering review and a PE stamp. Many of the early I-joist long spans had problems. The code allows 1/360 deflection. A 30 foot I-joist with this deflection moves 1 inch at mid-span and maximum load. Structurally fine, but the complaints of bouncy floors poured in. The deflection must be reduced below the code to crate a decent floor. Most quickly changed the Ârules they use.
The codes do not even address 'more than minimum' requirements that can arise from the use of large ceramic tiles, stone, etc. The normal floor deflection limits are not suitable for these materials. Joist must be spaced closer, have additional height, or be stiffened in other ways to provide a stable installation base. Closer is usually not a very good solution.
If you have a standard gable roof house of 1 or 2 stories with no oversize openings in bearing walls the code tables will allow you to build something without paying a PE.
Change from 'standard' and you will need a PE (or architect in some places) to seal off the plans.
I have a PE and perform a lot of these design reviews.
A recent one was an owner who wanted 16 inch sidelights on the front door. This requires a larger header, but there was not enough height to use a plain wood header (it was heading towards 2x14 size). A 2x10 flitch beam with #8 steel neatly solved the problem for a pretty minimal cost. The AHJ will inspect to the drawing for bolt placement and jack stud requirements (3 per side), along with checking that the installation is in accordance with the drawing.

Brickeye still just doesn't get it. (shaking head)

All his irrelevant opining about spans and species and loads add up to nothing since the simple fact is he still skirts the main structural issue: the proposed changes by the original poster won't work given the OPs existing criteria.

The OP's proposed alterations to the roof will not meet minimum code nor any accepted engineering practice.

It's that simple.

If Brickeye needs a PE to explain it to him because he just has no elementary understanding about roof systems or framing structure, then by all means Brickeye should hire one to confirm what I have rightly stated so it can be explained to him in simple language he can understand.

Until then, Brickeye is only fooling himself and those who likewise have no clue what they are talking about.

But his claims won't change the loads nor engineering facts...and in this case, Brickeye simply doesn't understand whereof he speaks.

I've wasted enough time in this thread.

The Op's proposed changes won't work without major structural alterations. PERIOD.

If Brickeye and others need to hire a PE to confirm it, then spend your money and be satisfied.....
---------------------------------------

Reminds me of a joke:

"A man in a hot air balloon realized he was lost. He reduced altitude and spotted a man below. He descended a bit more and shouted, "Excuse me, can you help me? I promised a friend I would meet him half an hour ago, but I don't know where I am."
The man below replied, "You are in a hot air balloon hovering approximately 30 feet above the ground. You are between 40 and 42 degrees north latitude and between 58 and 60 degrees west longitude."
"You must be an engineer," said the balloonist.
"I am," replied the man, "but how did you know?"
"Well," answered the balloonist, "everything you told me is technically correct, but I have no idea what to make of your information, and the fact is I am still lost."
The man below responded, "You must be a contractor."
"I am," replied the balloonist, "how did you know?"
"Well," said the man, "you don't know where you are or where you are going. You made a promise which you have no idea how to keep, and you expect me to solve your problem. The fact is you are exactly in the same position you were in before we met, but now, somehow, it's my fault."

My firsy reply to this issue:

"It is unlikely an AHJ will allow changes to rafters to use collar ties without a structural stamp. In some places architects can do it, in many a PE is requried.
Switching to collar ties changes a lot of forces on the roof and the supporting walls."

Manhatten42 has made a whole series of assumptions for which there is no supporting data including a 30 PSF snow load.

"Presuming #2 SPF 2x6 rafters, a snow load of 30psf with a dead load of 10 psf..."

He does not understand the first thing about structures and how they are engineered.
There are MANY solutions to almost every engineering problem. The building codes only cover the most common ones, and are subject to detailed engineering analysis of a specific application.
If you want to rely strictly on the present code, you are welcome to tear down all the structures built under earlier versions that no longer met the code, and be sure to tear down anything that was more carefully engineered than the broad brush taken by the code.

This is a homeowner based forum.
The AHJs do not give building advice in most locations, are not required to be PEs in most places, and cannot be held responsible for any advice they do offer.
In most parts of the country they check for code compliance. If anything out of the ordinary appears on the plans they expect a PE seal, including changes to exiting structures that likely were fully code compliant when built.

It is unlikely that a homeowner has enough knowledge to report all the factors that can affect a roof span in a few short sentences. All it takes is a few extra supports that an inexperienced person would not even realize an important to the structure. I have seen enough of them removed by folks trying to get more space and suffering the consequences to realize they do not understand why they are there.

There is simply not enough detail given to claim
"Your existing rafters are already too small for the spans."

"If Brickeye needs a PE to explain it to him"

I AM a PE.
I understand exactly how to perform the calculations for roof designs, including truss designs.

You appear to be a book hack who knows how to read and possibly apply the broad brush of the building code, but are likely incapable of actually performing all the calculations required to determine if a design is acceptable. This is what an AHJ office in most paces does.
The code does NOT cover all cases. These cases require a PE stamp in most jurisdictions to assure the AHJ that a competent professional has reviewed the plans, performed the required engineering analysis, and will stand behind the design.