Currently designing a carriage house that includes a hand framed 10/12 gabled roof line. Hand framing was choosen to maximize the interior space and for authenticity of matching the vintage house on the property.
The plan reviewer at the permit office sees the ridge board as a self supporting header, in this case 38′ long. That is a LVL that is 28″ deep… and weighs about 650 lbs…
Granted its been a while since ive hand framed like this, but I recall that a 1x 10 was used knowing that the rafters with collar ties and knee walls formed a stable triangle.
So what am i missing here?
Replies
Do you have ceiling joists in this system?
if not, and the rafter tie is located more than a third of the way above top of wall to ridge, then the ridge beam is most certainly necessary.
To add to what Cal said...
you must not have ceiling joists...
RE >> Granted its been a while since ive hand framed like this, but I recall that a 1x 10 was used knowing that the rafters with collar ties and knee walls formed a stable triangle. << The 1x10 was a ridge board. The permitting office is saying you need a ridge beam. Generally, knee walls don't help - but it depends on the building configuration How about a quick sketch or something of a cross section of the bldg? Or, look at the attached - anything look familiar? :-) What are the aprox overall dimensions of the bldg?
I'd definately try and get away from a 28" LVL..
Assuming your design intensions, if you want the collar ties to be high (for high ceiling effect), using wider stock (2x10 or 2x12) for roof rafters may get you closer to what you want without using the ridge beam.
Also, there was article in FHB about strengthening the perimeter sill plates (extra layers of 2x stock (2x6?) as not to use a beam or collar ties. This may apply to you.
You may also replace collar ties with cable system. Any case, you should consider the material and labor costs. Ridge beam that big sounds like you may need a crane. Hope it helps.
Heavier rafters won't do much to prevent wall spread (in fact, the added weight might make it worse). You may be able to use thrust beams along the sills to avoid the need for a heavy ridge beam or lower rafter ties, but that takes some engineering.
RE: "Heavier rafters won't do much"
Dan:
Upsizing rafters depending on the ceiling joist height is SOP up to a certain point (ceiling joist height) and rafter tables often provide adjustment factors.
For example, the IRC makes provisions for this. If you look at the random copy of the IRC rafter tables I Googled up in the link below, and then look at the 10th page in the document (marked with page # 722) it is explained there in note 'a'.
http://www.mass.gov/Eeops/docs/dps/780%20CMR/780058.pdf
Of course reading books or stuff off the internet doesn't really teach people how to build - there has to be field experience to go with it.
If you work through a few examples you'll see that in order to move the rafter tie up to the 2/3rds point (1/3 from the top), which is roughly how one would define a "collar tie", you have to more than double the depth of the rafter (because you size for a rafter that's twice as long). Ie, if you could get away with a 2x6 with standard joists, you now need at least a 2x12. If you needed a 2x8 before (more likely), you now need a least a 2x16.
Right...
and that is why I said "up to a certain point". Further the example table only allows for raising the ceiling 1/3rd of the way up so I'm not seeing that moving the ceiling joists up 2/3s of the way up is relevant.
Really though, this is all academic until the OP gets back with the needed info.
In the mean time I see that in the OP he used the words "collar ties" but did not use the words "ceiling joists" but at the same time his use of the words "knee wall" would indicate ceiling joists which confuses me. I think part of the deal here is that we are all having different interpetations of the vague post.
Agreed, but k1c said "if you
Agreed, but k1c said "if you want the collar ties to be high". When you say "collar tie" you're already talking about something elevated, and I would interpret "high" to mean well above the halfway point.
He may have meant pony walls instead of knees walls. If he uses kneewalls for roof support, then he has to account for that load in the floor joist system.
He could balloon frame the walls for some help, or use some simpson wall braces for the ponywalls to resist thrust, but like you say, it's all academic, and he is in this over his head, but at least has the sense to ask for help instead of doing like some, and fightiung both city hall and gravity.
You need a ridge BEAM designed to carry ahlf the load requirement of the roof if you have no rafter ties.
Collar ties will not help foir normal situations. For the kind of alternate design you want, there are other ways of doing it, but you will need an engineer, not just a bunch of us hand framers.
This could involve cable ties, diaphraghm sandwiches, midpoint posts, horizontal soffit beams to resist thrust, or a combination of all the above. Only your engineer knows for sure.
Keep in mind that the plan reviewer likely is neither an enginner nor an experienced framer. He could be just offering an idea that he's picked out of the code book. Not saying it won't work, but he may not be in a position to come up with some of the viable options that others in this thread have mentioned. An engineer experienced in roof design would be worth the cost.
BTW, one key to understanding this is to know the difference between a ridge board and a ridge beam. The 1 x 10 you mentioned from past experience is a ridge board and not meant to carry the roof load.
"he may not be in a position
"he may not be in a position to come up with some of the viable options"
No - a revier's job is not to design the structure, but to approve or disapprove is all.
Exactly...
I've had some exchanges with plans reviewers that showed me that they really didn't understand some of the basics of home building. Other times I have had to explain parts of the drawings to them. Standard stuff - like how to read a truss layout sheet (from the truss mgf)... Or perhaps they were having trouble visualizing the 3d view of a complicated roof drawn in 2d. Not that these people aren't sharp people - some of them can recite code in their sleep - but that isn't the same as actually designing and building something - they are just there to check other people's work. In some smaller areas the plan checker may also be the BI, and you would expect that the BI might have some more practical knowledge - but the same applies - not a designer.
Sounds like in the case of the OP the plan review person is doing their job: Running up the red flag.... I would expect the recommendation of the ridge beam sizing is more of a "courtesy service".
Another Waste?
Looks like this thread becomes another one of those deals where someone stops by and asks a Q with some incomplete info and then doesn't extend the courtesy to answer a few Qs aimed at trying to clarify the situation. Like Jeff Buck said "It's the internet - it is supposed to be a waste of time". I'm kinda thinking about not answering Qs for first timers though... Or maybe give 'em once sentence.... until I can see they are “in it”.
Kinda smacks of a HO who doesn't know what the **** he is doing and is too cheap to pay someone who does.
Maybe my imagination but I seem to see that happening a lot on this "New Breaktime". Real sense of community….
certainly is a slap in the face.
But that's the way it goes.
You're right on replying with just a simple response-if they acknowledge-then the thread most likely will continue.
or not.
He probably can't remember what user ID he used.
That's why I messaged him directly.
Or maybe it's a waste for people who don't know how to type?
Once time I made a mistake ... but then I corrected it.
Well, Matt ... why do you waste your time telling the rest of us it is a waste? Don't bother replying if you don't like the information provided to enable you to provide better feedback ... or simply have the patience to ask for the additional information you might need to provide that.
Why waste our time?
Your positive and productive participation is preferable to prolonging the participants pugilistic pondering of possiblities and perverse pontentiality postualtion under portents of prejudice, pardner.
You're missing as others pointed out ... that "collar ties" don't form a stable structure ... as by definition they are too high up to qualify as a 'rafter tie' ... which is much lower and does as you say ... forms a stable structural triangle where it keeps the walls from being pushed out. The ridge board simply becomes a method of tying the roof rafters togther and spacing them out, then (not really required).
Also if you indeed need a ridge beam ... you'll need to support it on either end w/ appropriate structure all the way to the ground. If that means a point load in the middle of e.g. a large garage door, that beam will have to transfer that load as well. Collar ties vs. rafter ties becomes an important distinction.
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