Pre-built trusses are great when;

1- The lower framing is accurate
2- You can get a crane in there
3- Its cost effective

Where, geographically, are you?

Small town in the midwest. Labor costs are reasonable.

This doesn't seem to be a big enough job to merit renting a crane. Though there is access from the neighbor's driveway.

Trusses are designed to support weight with smaller lumber than site built rafters and are able to be installed much faster.

The only down side is the webbing prevents using the attic area for storage/easy access.

In building many Habitat for Humanity houses, we usually installed trusses by hand---about 5 or 6 volunteers handing the trusses up to the 4 volunteers on ladders to install them(using temporary bracing to hold in place). I've done that on rooms that size(garages)with 5 total people.

If you have the trusses made, the company should be able to make the trusses that will be installed over the existing roof to install just like the open room trusses.

Cost should be lower for the trusses, since the labor necessary for installation is much less.

"The vertical cross-gabled portion is the addition."

It's deceiving, as pictured, or is "C", the addition actually wider than "D" and "E", as it affects the roof tie-in.

"It will be 11' wide and 15' deep"

Aesthetically, the matching and maintaining a uniformity of roof framing, except when addressing specific architectural elements is desirable.

"Is the use of trusses a matter of quantity? Or construction design?"

Quantity.

As a matter of practicality, in a small addition, combined with the roof tie-in you'll not save anything, one vs the other.

"Question #2. What's the math/geometry that tells you whether the roof portions intersect at the peak or lower down?"

The aesthetics are generally the governing factor.

Another aesthetic to consider is matching the eaves and facia.

"I'm assuming the proper thing to do is match the roof pitch and it intersects where it intersects."

And how you address the ridge intersection.

"So, if I've got a 6/12 roof and the addition is 11' wide (outside dim), the peak would be 11 x 6/12 = 5'-6" high? And if it's 4/12 pitch, then 11 x 4/12 = 3'-8" high?"

Actually, at 11' the ridge will be directly above the 5'6" dimension, so with a 6:12 pitch the ridge will be roughly 33" above the ceiling or top plate.

Since, as pictured, the addition is wider than the main dwelling/building, the ridge will be above the main ridge. So you have a couple of choices; you ca stop the new ridge directly above the existing, creating a gable end, you can stop the ridge back from the existing ridge and frame a hip section that conforms with the main roof, or you can continue the ridge past the existing ridge and create the gable there or shorten the extension slightly and create a "flying dutchman."

handymac: I've installed trusses on HFH houses exactly the same way. :-) Fond memories....

snoonyb: sorry for a bit of confusion with my drawing. I just pulled that off the internet as an illustration. It's not to scale for my project, it's just a rough representation. This drawing is the plan view. Main house is ~25'x35', addition is 11'x15'.

Here's a photo of the side of the house. We're tearing out the poorly-built shed of a laundry room and replacing it with the 11x15 room with proper footings, etc.

We plan to have it go either from the edge of the house to the edge of the bump-out with the bay window, or else start just in from the edge of the house, where the existing "room" starts. Which would be better from a roof design perspective? I'd like to have the gutters even/connected between the 2 structures.

So would it be better to do this (random internet pic):

or this:

And I see that I was off on my math by 50%. Duh...it peaks in the center, not at the far end. :-/

Hi,
Even if the common part of the roof is trusses, the valley portion (intersecting part from the wall line to the main ridge) is all going to be stick-framed.
If I were building it, I'd design to stick-frame the whole thing, and that way the soffits will line up irrespective of getting the stud length perfect. Trusses would require you to work backwards from the soffit height when planning the walls; rafters gives you flexibility to match it all up at the roof framing stage.
Casey

The roof won't , or shouldn't, be affected and the ridge will die into the existing well below the existing ridge.

Stepping the addition away from the plane of the consistency of the exterior wall is a marginal aesthetic, but can also affect a future functional change, such as combining the two rooms for another purpose.

I'd let the owner make the choice.

The roof appears to be 4:12 so, 2x6, 24"oc.= 16 common rafters.

11' is also over-spanned for 2x4 ceiling joist.

Easily a 1 day cut and stack for a single carpenter.

I'm the owner, and possibly the carpenter (though we're leaning towards hiring it out to get it closed in quickly), so I think I'll opt for simplicity and maximization of the available space, by making the addition even with the edge of the house.

Thanks for all your guidance.

only down side is the webbing prevents using the attic area for storage/easy access

Storage Trusses