I'm building a shed/barn with a 6ft doorway for two 3ft doors. If I assume the doors will be exactly 3 ft each, do I need to add extra space in the framed opening to allow them to swing easily, and if so how much?
You'll add a list of things to get your R.O.:
- The width of both door slabs together
- The width of both side jambs together
- 1" gap +/- for shimming the jambs plumb inside the framing (1/2" for each side)
- ~3/4" for slab gaps (1/4" each left, center, right)
The slab gaps may need to vary from 1/4" depending on your hinge and latch hardware. 3/16" (9/16" total) is more ideal, as it makes for better latch engagement, but requires fairly precise carpentry.
Of course, you can frame your opening and build the doors to fit it instead. Just work the same numbers.
Assuming standard outswing doors and 3/4" jambs, your 36" slabs would need:
1/2" + 3/4" + 1/4" + 36" + 1/4" + 36" + 1/4" + 3/4" + 1/2" = 75-1/4"
If, however, you don't use floating jambs, disregard the jamb thickness and the outer (1/2") gaps. You'd then simply have:
1/4" + 36" + 1/4" + 36" + 1/4" = 72-3/4"
An astragal doesn't usually add to r.o. in a simple shed scenario. It simply mounts to the face of one slab and laps over the opposing slab. There's really no need for it to be mounted to the edge of the slab. If you do want to use an astragal between the slabs for more stable latching, take that width into consideration.
Also, you mention that there's no space needed for your hinges. Even if you're using strap hinges you don't want the slabs tight to the jambs. That would be a recipe for binding and noise.
I would suggest that you purchase a set of prehung exterior shed doors that suit your design and budget. This is the simplest approach since the doors can be pre-drilled for the type of hardware (door handles, latches) that you want. The manufacturer will give you the dimensions for your rough opening which is going to be about 1 1/2 inches wider than your doors plus the jambs - although this dimension varies.
Pick your door before framing. If you search Home Depot's website you will find both 74 in. x 81.625 in. for 36" wide doors and 72 in. x 80 in. for 6' wide rough opening.
I have built lots of barn doors with 2x4 and plywood. The additional width comes down to the thickness of the doors and the type of hinges, drop on pin & strap style usually need close to 3” these are handy because the door can be removed when fully open providing the extra few inches clearance to get equipment in and out. T style are also popular not removable but take up less room still the thickness of the door affects the needed opening , is the door going to be insulated ? A thicker door will be needed,