Wednesday, April 18, 2018

A Garage Attic Design

I've seen a couple posts about garage attics lately and that prompted me to document mine a little better. Having done it, frankly, I think it is a mistake to build any garage that has a good pitched roof without an attic (unless you are doing a cathedral type ceiling for your car lift or something).

We moved from a larger home to a much smaller one in the city, and the new home is a 1926 Craftsman, with very limited closet space. We knew we were going to build a garage and a very usable attic became a priority. I'm mostly a motorcycle guy so the garage didn't need high ceilings for a lift and I kept 8' ceilings in there. But the city also limits the heights of garages to 15' so my attic space was destined to be cramped.

I decided it was very important to have a great garage stairway, and I documented that a while back here:

I found getting the trusses I wanted a bit of an exercise. The truss company was run by a fairly ornery codger and communicating with him was tough. He was happy to design the trusses for free as part of the truss order, but I wasn't sure he was really optimizing the truss to my needs. But I found he had the trusses stamped by an outside engineer, and I just contacted that guy and he was very happy to help.

Given that I wanted to maximize my headroom, we used a triangle at the apex of the truss, instead of the typical cross piece that would have reduced headroom.

Also, my trusses are "strangely" spaced, as I wanted a larger gap for a wider stairway, and the building itself wasn't designed in an increment of 2' for the trusses to be evenly spaced.

My design was checked for 200# per truss for the stairs, plus 40 PSF total load in the 9’ wide attic area. Note also that my truss is worse case design at 28.5” on-center, with the trusses at 1’-4” and 2’ on center not carrying as much load.
The wide spacing was to accommodate the stairway, and that is only in one spot. Most of the trusses are on 24" centers, so that effectively makes things more robust than the calcs show.

These are still pretty basic trusses made of 2x6, 21' long, 6/12 pitch. 13 trusses (9 attic plus 2 end trusses) cost me $1870, delivered, here in expensive San Diego.

Here the attic truss design. Note the triangle at the peak, and the maximized attic area. It's not quite tall enough for a short person like me to stand it, but you can move around up there.

Here are the end trusses. A wider space open in the center is for the attic fan on one end. On the other end we have a fancy vent, so the verticals were spaced to match that design.

Here are two shots during construction.

And on this shot shows the fan end vent from the inside. Also note the 3/4" plywood floor.

A garage attic. I highly recommend it.

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