It's important to get the rough openings of a doorway right. Doors have standard sizes, but if you're not sure what those are, or your opening is not quite what you imagined, measure the opening to make sure you get it correct.
If you're starting with a rough opening, the measurements for your door include jambs, which are 3/4-inch thick. This includes the horizontal jamb across the top and both vertical sides. Since most doors are manufactured with frames already on them, or more commonly referred to as a prehung door, carpenters routinely make their openings 2 inches wider than the door to allow for the jambs. For example, if your opening measures 32 inches in width, the door size is 30 inches for a prehung door. That allows 3/4 inch on both sides for the jambs, with 1/2 inch of space for the door to operate inside the jambs. If the jambs are already in place, order the door 1/2-inch smaller than the opening.
The Tall and Short
Industry standards have simplified door height; it's almost always 80 inches. This means the rough opening for height should measure 82 1/2 inches for a prehung door. The extra 2 1/2 inches allows for the jamb at the top plus a buffer for floor coverings such as tile, hardwood or carpet. Once you decide what type of floor covering to use, shim the bottoms of the door jamb up to the desired height. For example, if the rough door opening measures 80 1/2 inches, it's perfect for a standard prehung 80-inch-tall door. If the jambs already are in place, then the door height should be at least 1 inch shorter than the opening. It's rare that any door is anything other than 80 inches in height, but if the door is too tall to fit inside the jambs without rubbing the carpet, trimming it down is not out of the question.
If the door opening is slightly wider than the traditional 2 inches, it's fine. Carpenters use shims on both sides and at the top to square the door inside the frame to allow for discrepancies such as out-of-square framing. The shims stay in place and the gap between the jambs and studs is covered with casing. If the framing is under 2 inches smaller than the traditional door sizes, which typically are 24, 26, 28, 30, 32, 34 and sometimes 36 inches, then the rough door opening may need to be enlarged slightly to allow the door to work properly. If it fits too tight, there's no room for adjustment. The only other solution to a rough opening that's too small is to order a custom door. It's more affordable to change the opening size.
Working the Angles
Some door manufacturers cut angles on the exterior edge of the door. This allows the door to fit snug, without rubbing the jamb when it's opened or closed. The angle is not much, typically between 2 and 5 degrees, but it allows the leading edge of the door to clear the jamb on tight-fitting doors. If you measure the door perpendicular across the interior and exterior sides, the measurements are slightly different because of the angle. If you're ordering doors, request the angle cut for a better fit. If your new doors don't have it and fit too tight or rub when you open or close them, use a handheld planer to cut a shallow angle on the exterior side; your door will fit better.
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