To determine the size of the door you need for a 35.5 x 77" rough opening, you should consider the standard door sizes and the necessary clearances for installation. Standard door sizes are usually slightly smaller than the rough opening to allow for proper fitting and adjustments. Here's what you can do:
Determine the Door Size: Standard interior doors are typically available in various widths, such as 24", 28", 30", 32", and 36". To choose the correct door size, measure the width of your rough opening, which is 35.5". The closest standard door width that will fit your opening is 36".
Account for Height: Next, measure the height of your rough opening, which is 77". Standard interior doors are commonly available in heights of 80" and 84". Since your rough opening is 77", you'll need to trim the door to fit. Consider purchasing an 80" door, leaving 3 inches at the top for trimming.
Check the Door Swing: Determine the direction in which you want the door to swing (inward or outward) and make sure it aligns with the room layout and space restrictions.
Allow for Clearances: Remember to leave some clearance for the door to swing freely without rubbing against the walls or other obstacles. A minimum of 1/2 inch to 1 inch clearance on each side is generally sufficient.
So, for a rough opening of 35.5 x 77", you would need to purchase a standard-sized interior door of 36" x 80". After installation, you can trim the door's height to fit the 77" rough opening, leaving enough clearance at the top.