Our home has 2x4 roof trusses spanning 25 ft. spaced 16 in. O.C. Currently, there is limited attic storage on a plywood subfloor on the bottom chords, accessed via a small hatch in a closet. We would like to install a fold-down attic stair with a rough opening size of 25.5 in. x 54 in., with the long dimension perpendicular to the trusses. This would mean cutting the bottom chords of three trusses to frame the opening. Is this possible? If so, would double framing to the bottom chords of the adjacent trusses in the first and fourth bays be adequate to header off the three cut bottom chords?
NEVER cut any structural member of a truss! Any field modification that involves the cutting, drilling, or relocation of any structural truss member or connector plate shall not be done without the approval of the truss manufacturer or a licensed design professional. The building code (IRC R802.10.4) prohibits altering trusses.
Also, what you are planning on using the access for? The trusses most likely were not designed to carry additional loads on the bottom chord, so if you are planning to use the attic space for storage, we advise you to contact the truss manufacturer to determine the amount of load the trusses were designed to carry.