# Rafter Thrust / Cathedral Ceiling



## Neil_K (Oct 30, 2004)

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 (Aug 14, 2005)

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 (Aug 14, 2005)

Thanks for the reply Neil:



Neil_K said:


> 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
> ...


----------



## K2eoj (Aug 14, 2005)

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 (Aug 14, 2005)

hammerslammer said:


> 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)..  

Thanks for the replies.

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

Thanks,
Steve


----------



## K2eoj (Aug 14, 2005)

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 (Oct 30, 2004)

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 (Oct 5, 2005)

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/Architecture/windows.htm
http://ah.bfn.org/a/DCTNRY/p/pallad.html
http://www.hartfordpreservation.org/styles/palladian.html


----------

