Snow load on an unheated roof
A house roof sheds some snow because heat leaks up through the attic. A detached garage does not, so the full winter's accumulation can sit on it until March. Older garages around Springfield were often framed with undersized rafters and no collar ties, and after a heavy season the ridge sags and the door stops closing square. I have replaced more than one garage roof where the rafters had simply bowed under years of snow.
When I frame a new garage, the roof gets sized for the load this region actually sees, with proper ties and a ridge that will still be straight in twenty years. When I repair an old one, I add the ties and sister the rafters before the new shingles go on.
The door header
The header over a garage door carries the roof above it across a nine or sixteen foot opening, and it is the piece that fails first when a garage was built cheaply. The tell is a door that binds at the top corners or a visible dip in the trim above the opening. I replace sagging headers with engineered lumber sized for the span, and I do it with the roof temporarily supported so nothing moves while the old one comes out.
On attached garages, I also check how the garage roof ties into the house wall, since that wall-to-roof joint is a classic leak point and a classic ice dam spot.
Building new
A new detached garage starts with a slab poured on compacted gravel, with the frost wall or footings below frost depth. Then framing, sheathing, roof, siding, and trim, in that order, with the roof dried in as early as possible. I match the house's siding, trim, and roof color so it looks like it was always there, and I orient the door and the driveway apron to keep plowed snow from piling against the door.
Detached garages with a room above have come up more often lately as families look for a home office or space for a relative. Those are additions in all but name and get framed like one: proper floor system, stairs that meet code, egress, and insulation planned from the start.
Repairs that make sense
Not every tired garage needs replacing. A new roof, new siding on the weather side, a rebuilt header, and a fresh sill plate where the old one has rotted at the slab will bring most of them back for another few decades. What I look for first is the sill: if the bottom of the wall framing has been sitting in wet for years, the whole wall may be held up by the siding, and that changes the plan.
I also fix the small things while I am there: the man door that will not latch, the window that has been painted shut, the trim that the lawn mower has been chewing on.
Garages in the city versus the towns
In Springfield proper, garages tend to be small, old, and close to the lot line, which means working within the zoning setbacks and often rebuilding on the original footprint. Out in Ludlow, Wilbraham, and Agawam there is room for a two-bay or three-bay building, and the question becomes site drainage and how to keep the driveway from turning into an ice rink in January.