Loft trusses

[Rhinoguy]

Hello all.
I've got a question about what I call an attic truss. I've framed the walls of my 16 by 24 cabin to the cap plate stage and intended to make a truss for my roof out of 2X8 with a 12:12 pitch. My plan however is to make the joist portion to include the 16 inch overhang and have the bottom end of the rafter start from that point. This would essentially give me a loft space of a cabin 18' 8" wide. I've done this many times before with trusses on outbuildings with spans up to 36 feet but this would be my first loft space and I'm not sure how the 'truss' will behave without typical webbing. If I put a vertical web that would sit directly over the wall plate, will that transfer the weight to my walls? I also wonder if I need any other bracing besides the collar ties I plan to install at 6' 6" height on each truss.
Any and all advice greatly appreciated 

[MountainDon]

I think I understand you to mean something like the following drawing. (disregard the dimensioning).



As far as transferring of forces, weight, etc., that is entering the territory of an engineer's knowledge.  The roof weight would be, in my amateur opinion, be passing through the outboard ends of the rafter ties / ceiling joists and through them down through the walls. The downward and outward forces from the rafters wouldbe acting on the walls through the ceiling joists / rafter ties. There would be some bending moments on those members. Standard tables don't cover such things. A vertical "web" positioned above the actual wall framing would not likely be supporting any/much weight.

I believe it is a good idea for a method to build for a loft; it does enter into the realm of needing an engineer if one is interested in (a) being certain the structure is safe/strong and (b) passing any building inspections.

These would not be termed trusses, but are simply a variation on a rafter, IMO. A collar tie serves to tie the tops of the rafters together to prevent separation under conditions of high wind. They really do little to distribute any of the roof loads.

Hopefully someone else will have something concrete to add to my comments.  G/L and 

[Don_P]

A conventionally framed roof with rafters and ties is the simplest form of truss. An attic truss from a manufacturer has a web member that directs the load to the wall tops as you described.

It is probably outside of prescriptive code but will work fine if you size and connect the members well. The cantilever tables in the floor chapter will give the max cantilever without accounting for that web member and I think you'll make it there.

In old work it was common to run the ceiling joists out to form the level soffit. A board was laid flat across those extended ends to form the "raising plate" and then the rafters were cut with just a level cut and nailed over the cj's to the raising plate.

I have modified this during repairs, removed the raising plate and sandwiched plywood gussets across the joint, nailed well from each side to each member, truss plates.

When I do something like this I sketch it out and go sit down with the building official. It is then his call if he needs to have an engineer's stamp. I could see this going either way.

Now to the vertical web member over the wall. Load goes to stiffness. If the rafters deflect the stiff point is that post. If the joists deflect in the cantilever, the load goes to the post. That web member is the stiff point.

[Rhinoguy]

Thanks for the advice gents. I'll post some pics when I'm done