Replies

1. 
   Guest_ | Jun 23, 1999 04:34am | #1

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   Can't help you with the truss design but...

   You say "Six and a half foot span with 6/12 roof and 4/12 cathedral ceiling". So, what are you building? - a luxury dog house with a fridge so you and Rover can share a beer when things aren't too congenial in the "big" house?

   Sorry, just couldn't resist!

   maybe this will help: http://www.trussnet.com/

2. 
   Larry_T. | Jun 23, 1999 07:31am | #2

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   Hey, Matt G.
   Don't you usually measure rafter span from point of bearing (wall) to ridge board? Therefore six foot six span equals thirteen foot room. Well anyway, that is what it is. So, now you see why I am asking for help ! !

3. 
   Guest_ | Jun 23, 1999 07:52am | #3

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   I thought the span was from one bearing wall to the other - usually the 2 exterior walls - but I could be wrong???

1. 
   Guest_ | Jun 23, 1999 07:58am | #4

   *Site cut the rafters using a structural ridge beam. Have an engineer size the beam and rafters for you.Dave

1. 
   Guest_ | Jun 23, 1999 09:57am | #5

   *The span of a normal flat ceiling joist is from center of wall to center of wall. The span of a roof rafter is the horizontal distance from center of wall to the ridge typically, unless a purlin and struts are used for instance.For a small span, the rafters and the ceiling joists form a simple truss, a single triangle. As long as that triangle isn't too "flat", and it is nailed well, you can use the rafter and joist tables in the code or a good handbook. Some of the values for Douglas Fir changed several years ago for the worse.In your case, you'll probably need a carrier beam or ridge beam of some kind to support the ceiling joists. If the room isn't too long that is.Making a true scissors truss out of the joists and rafters would be too complicated for me to engineer. You'd also have to work around the problem of "kickout" of the supporting walls walls. The tendency of the walls to be forced away from each other as the truss deflects some. Manufacturers often won't warrenty their designs unless the walls supporting a scissors truss are braced by another ceiling beyond or some other engineered system. I doubt if you will want to add a couple steel rods across the room at plate height to counteract this. Its called a scissors truss of course because it is shaped something like a pair of scissors, and inverted vee, pointed at the ends.A rule of thumb for scissors trusses is that the interior slope shouldn't be more than 1/2 the slope of the exterior. Otherwise there isn't enough depth in the truss to provide adequate strength economically.Gary Wheeler, AIA

1. 
   Guest_ | Jun 24, 1999 05:25am | #6

   * Gary Wheeler AIA,

   Span is measured form the outside of the rafter plate( top plate) to the outside of the opposite rafter plate (top plate). This is usually the "short" distance across a roof. The run is half this measurement. Roofs.

   Joseph Fusco View Image

1. 
   Guest_ | Jun 24, 1999 06:03am | #7

   *Ha!I knew Gary was just trying to confuse me with that center of the wall business. Very impressive piece of work Joe. Thanks, JonC

2. 
   Guest_ | Jun 24, 1999 06:47am | #8

   *That's correct, but the "design span" per UBC is measured from the half-bearing points, or as Gary noted earlier, to the center of the walls. This is the span used in calculations for beam stress. The span tables for joists though, show the span to be from the inside dimension. There are three definitions of "span"!

3. 
   Guest_ | Jun 24, 1999 03:25pm | #10

   *Darrell,

   You are correct, I guess it was incumbent on Gary to qualify his reply.

   Joseph FuscoView Image

4. 
   Larry_T | Jun 24, 1999 03:25pm | #9

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   Does any one know of a www site that has truss rafter design information ? Six and a half foot span with 6/12 roof and 4/12 cathedral ceiling.
   Sounds simple, any one care to help ? ?