In terms of truss construction, what is the most cost effective design for a garage with loft including a shed dormer?
That’s a pretty open ended question with lots of answers dependent on the specific conditions. For the purpose of this answer, as the architect I’ll make the following assumptions:
- The garage is a free standing 2 car garage
- All of the roof structure is to be built using pre-manufactured trusses
- The floor of the loft should be capable of supporting typical floor live loads
- The loft should have normal ceiling height in the dormer area
- The loft space is intended to be heated and thus requires insulation
- The loft floor covers the entire garage, access to the loft is supplied by some sort of hole in the floor
A Free Standing Garage with Loft & Shed Dormers
The illustration shows a 22′ square garage with a full head height 2nd floor. It has the appearance of dormers (while not actually having them). This is the beauty of using equal sized shed dormers on both sides; you can build it like you were building a typical 2 story structure while still preserving the appearance of a dormer.
A Roof built entirely out of trusses
If you really want to build the roof entirely out of trusses then you’d frame it as I’ve shown here. The ends of the garage would have an “attic” style truss. This leaves space for walking around. The dormer area would have a conventional floor built out of I-joists, typical wall framing and common trusses.
In this scenario the attic truss would either need a thicker top chord or a second “ceiling chord” to allow for full depth insulation in the sloping ceiling area. The pitch of the common trusses and height of the walls they sit on would need to be adjusted so that the ridge isn’t higher than what should appear to be the dominant ridge. This is undoubtedly the most efficient way to build this while using trusses.
My Suggestion - Build the steeply sloping section using stick framing
On the other hand, I wouldn’t build it using only trusses. Instead of the attic truss I’d continue the floor system across the entire garage and then build the steeply sloping part with 2×12s or 12″ I-joists. This is probably cheaper than the attic trusses and makes the floor framing simpler. This scheme is dependent on the bottoms of the 2×12s resting on the floor system (or the walls below). If you built short knee walls to raise the 2×12s then you’d need a ridge beam as well.
I wrote an article last month about adding a similar dormer to a 1920s era bungalow. This is a simple and cost efficient system because it takes all of the roof loads out to the exterior walls. Traditional dormers would break up the continuity of the roof structure and would require support at places other than the outside walls. This is no big deal but it’s not the most cost effective solution.
This is a sort of Carriage House
This raises the issue of “Carriage Houses” which are generally detached accessory structures comprised of a garage on the ground floor with a living unit on the second floor.
It’s been legal to build these in the south end of Seattle for a couple of years. The city is now getting ready to approve these in all single family residential zones. I’ve designed several carriage houses as part of larger townhouse developments and I love them. I’m really looking forward to being hired to design one of these.
Feel free to ask follow up questions to this. If you provide me with more information I can probably tailor my answer to fit your particular situation better.