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-   -   Rafter Thrust / Cathedral Ceiling (http://www.diychatroom.com/f14/rafter-thrust-cathedral-ceiling-911/)

sseeley 08-14-2005 12:40 PM

Rafter Thrust / Cathedral Ceiling
 
I am building an attached 16x14 foot addition with a gable roof and a
palladium (half moon) window on the gable end. The attached end is the
14' length so my rafter span is likewise 7' (correction from 14' in orginal post
:o ). My rafters are 2x12, 24 oc attached to a 2x12 ridge beam. Remainer is
2x6 construction. The roof ties directly to the house wall (no new valleys or
jack rafters, etc). I am hoping you got the general discription.

Some more detailed (important) dimensions include (approx from subfloor):

- Top of ridge = 13'-8''
- Top of wall plate = 8'-1''
- roof pitch = 8-12.

So since I have this nice Palladium window, I did not install ceiling
joists, but instead installed some 2x6 collar ties so that the bottom of the
collar tie measures 10'- 9'' from subfloor, which places my collar tie pretty
much at 50% of the roof height (measured as top of ridge minus top of wall
plate).

I have a beautiful cathedral look and feel on the inside, with a nice flat raised
ceiling for which I plan to finish with wide pine and a ceiling fan.. The ceiling
height would be about 6 include above my palladium window.

So the problem is that my town inspector failed the rough framing inspection
claiming that there is not enough support for outward rafter thrust. He said
the collar ties are too high and suggested that I install a *single* 14' beam
from the top of one wall to the top of the other. Obviously, this ruins my
cathedral look and feel not to mention bisects through the middle of my
palladium window. Another suggestion he made was to install scissor
trusses to define a new peaked ceiling that was slightly above the top of
my palladium window. This would really be some make-shift trusses since I
already have my rafters and roofing installed. This seems way overkill.

I offered to lower my collar ties a few inches (5 or 6 inches) but he said
it would not be low enough.. I also questioned the placement of the
beam suggestion; what code governs the placement of a single beam along
my 16' side walls. It seems I have plenty of structure to hold the walls up
straight; One side I have my house (which I tie in nicely with additional
stud blocks behind the original house sheathing) and the other side (16 feet
away) I have my gable end wall. A single beam reduces this "wall span" to
8 feet.. So hypothetically, if my addition was actually 14x8 feet, I would not
have an issue?

My question is three-fold;

1) Is what I have really *not* structurally sound?
2) Where can I find information of this in the form of span tables,
roof pitch, etc..? Is there some code that governs when 50% collar ties
are not good enough, or how many properly spaced beams (like he
suggests) are needed..
3) Can I get some advise on what I should do here?

Thanks in advance.. And thanks for reading my long post..

Steve

Neil_K 08-14-2005 10:05 PM

A couple comments. First, I'm not a pro, but hopefully they will be along soon.

2x12s for the rafters? thats pretty stout. Most of the pro's go for 2x8 16"oc. I don't know the code, but could see with that length where the chance for the middle to fail. Are you up north and get alot of snow?

As for the trusses, engineered trusses are usually capable of greater loads than hand built and they are designed for strength and thrust. It would definitely be overkill to sister them, but I would speculate that the alternative would be to rip the rafters down and put the trusses up and use the 2x12's for something else.

I wonder if theres some other alternative that could be less obtrusive but still make the building sound.

Hopefully, I'm biding time until an expert comes along.

Neil

K2eoj 08-15-2005 11:02 AM

I agree with your inspector. The easiest fix that might be acceptable would be to put some exposed beams at wall height or a little above, maybe on 4 ft. centers. (wall to wall horizontal) This would or could tie your walls together and taking the stess of the collar tie, and maybe not look too bad.> 2) I have had made welded steel beam supports that bolt into the outside wall over and around your half moon essentially making a header over the half moon. Then you could use your ridge beam and maybe set it under what you have existing. This would require an engineer and I think most inspectors would accept a letter from the engineer. > 3) Basically what you have is a tail bearing truss. A certain amout of tail bearing is allowed and with your 2x12 members you might be close. The biggest stress on your roof right now is where the collar tie meets the rafter. An engineer could possibly design a plywood gusset with about 150 nails in each gusset at the collar tie-rafter connection to make what you have work. This design could possibly require doubling or trippling the 2x12's. I have done allot with old roofs and making tail bearing trusses out of the old roofs. An engineer is required to do these fixes and preferably one that is very familiar with truss design. In my area there is an expert truss engineer that does this type of work for $50 to $100 dollars and when the building dept sees his name on the letter they look now further. I would think most engineers would charge more than $100. Hope that helps.

sseeley 08-15-2005 07:40 PM

Thanks for the reply Neil:

[QUOTE=Neil_K] 2x12s for the rafters? thats pretty stout. Most of the pro's go for 2x8 16"oc. I don't know the code, but could see with that length where the chance for the middle to fail. Are you up north and get alot of snow?

[sseeley] I am in New Hampshire (not tons of snow).. I based my decision
partly on some plans and some rafter tables. According to a rafter span
table, my 2x12 @ 24 oc can span 20 feet ( I have 14 feet).. I also used 3/4
ply to avoid sagging between rafters... You are not the first to comment on
this decision. It make me wonder. They were part of some plans I got though.

As for the trusses, engineered trusses are usually capable of greater loads than hand built and they are designed for strength and thrust. It would definitely be overkill to sister them, but I would speculate that the alternative would be to rip the rafters down and put the trusses up and use the 2x12's for something else.

[sseeley] Yikes! My Inspector said that a couple 2x4 members would do the trick; from top plate to opposite rafter at some angle less steep than the
rafter, Then block them up to the roof (ridge or rafter).. I wanted to avoid
this.

K2eoj 08-15-2005 07:59 PM

oops. I didn't read your post carefully and thought you had a much bigger addition. I'll still suggest and engineer, plywood gussets, and a nail gun. Sounds like you have plenty of lumber for those spans. The problem is there is so much tension at the rafter-collar tie connection that 5 or 6 nails is not enough to hold it. The gusset allows you to get 40 or 50 nails into each member. > Another ridge beam under what you have existing would be a solution also. I was thinking your half round was too close to the ridge with no room for a wood header. If you can get the header in, this solution would require just the just the basic span charts for the header and the new ridge. > Probably what you have would hold for a couple of hundred years but we always have to go with the building dept.

sseeley 08-15-2005 10:08 PM

Quote:

Originally Posted by hammerslammer
oops. I didn't read your post carefully and thought you had a much bigger addition. I'll still suggest and engineer, plywood gussets, and a nail gun. Sounds like you have plenty of lumber for those spans. The problem is there is so much tension at the rafter-collar tie connection that 5 or 6 nails is not enough to hold it. The gusset allows you to get 40 or 50 nails into each member. > Another ridge beam under what you have existing would be a solution also. I was thinking your half round was too close to the ridge with no room for a wood header. If you can get the header in, this solution would require just the just the basic span charts for the header and the new ridge. > Probably what you have would hold for a couple of hundred years but we always have to go with the building dept.


[sseeley] Actually, I think you read it carefully, but I (being a newbie at this
whole construction thing) got my terminology wrong, which made my addition
sound bigger than it is.. My error is that I said my *rafter span* is 14'... But
my addition width is 14', therefore my *rafter span* is only 7' (from wall to
ridge).. :o

Thanks for the replies.

Do you still think that my 50% 2x6 collar tie is not good enough?

Thanks,
Steve

K2eoj 08-16-2005 12:46 AM

I really think your fine. We built huge roofs back in the 70's with collar tie connections like yours. I will say that on a large roof when the temporary bracing is released and the collar ties go under tension the walls will push out, in one particular case I remember the 4 ft. walls pushed out about an inch to 1.5 in. I'll also say that 30 years later the walls probably didn't move out much more. In your case your ties are probably already under tension if you've taken down any temp braces. Do you see any bowing out in the center of the wall Or has the center of the wall pushed out of plumb? I'm still for a few gussets. The collar tie ends can crack and split over the years and usually there is some scrap plywood or osb around and a guy can scab a 3 or 4 ft 2x to the tie or rafter to make them even and shoot a piece of plywood on. The center of the wall of course has the most stress so if you did a couple of gussets there I would probably give it my 1000 yr guarantee. That is my opinion but it really doesn't matter what I think because you have your inspector to deal with. It seems to me that in this case he is seeing a real issue and not something he made up.. As far as I know there are no charts that deal with this type of roof and you would have to go to the engineering part of whatever code book they are using in your area, plug in the numbers and come up with the answer. Sounds like you built a great little addition and you'll get by this little glitch soon.

Neil_K 08-16-2005 08:48 AM

I like hammer's idea about the gussets. I'd check with the inspector to see if thats do-able. With 24" spacing, that leaves you plenty of room to swing a hammer (or shoot a nail). I'd verify the span is equal at the ends and the middle between the top plates first to see if it has pushed out.

mighty anvil 10-06-2005 12:19 PM

There is nothing wrong with the concept of your framing; it just deviates from the simple platform framing system prescribed in the building code and therefore the inspector needs an engineer's stamp in order to accept it as equivalent. An engineer will determine if the outward force of the rafter bottoms can be resisted by the rafters and collar ties and tell you if the collar ties need to be moved. He will also tell you how to connect these elements in order to utilize their full strength. Since the spans are so small, I have no doubt that the system can be made to work with little or no modification. Good luck with it.

A "half-moon palladium" window? I'm sure Andrea Palladio is turning over in his grave quite frequently these days.
http://www.primaryresearch.org/PRTHB...re/windows.htm
http://ah.bfn.org/a/DCTNRY/p/pallad.html
http://www.hartfordpreservation.org/...palladian.html


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