Construction skills are paramount to the DIY tool kit. Part of the fun of tackling projects on your own is learning along the way. While trial and error is one way of doing this, hitting up fellow DIYers familiar with the task is another. This is the reason the Do it Yourself forum has become a popular way to garner information from fellow projecteers. For this particular discussion, a forum member asked, “Can anyone tell me what the rough opening size should be for a 32" door?”
“Frame” of Reference - 32" Doors are Standard at Places like Home Depot
First off, it’s important to understand what we’re talking about here. A rough opening is the opening framed in around where a door will eventually go. It requires planning to have this opening the correct width and height to fit the door chosen for the space. In this case, the DIYer planned to use a fairly standard 32” wide by 80” high door.
Why is the Opening Wider Than the Door?
The door, however, isn’t mounted directly to the construction beams. Instead, it is mounted to a frame, which is then mounted to the construction beams. The frame around the prehung door is typically a thickness of ¾” on both sides, resulting in a total additional width of 1 ½”.
In theory, that means your opening should be one and one-half inch wider than the door. However, DIY hinges on reality, not theory and anyone who has installed a door knows there needs to be some wiggle room to make adjustments. Few openings are truly plumb and square, so knowledgeable forum members recommend an additional ½” gap to accommodate for shims.
So What’s the Answer?
There is a bit of variation in answers, but the standard and most-widely-accepted answer is to allow 2” around the door for the opening. That means a 32x80 inch door needs a 34x82 inch opening.
Is it the Same for Exterior Doors?
One forum member mentioned those numbers might be different for exterior doors and another member was quick to explain why saying, “Interior doors add 2" height and width with 3/4" jambs. [This] allows space for shims and out of level headers. Exterior doors have 5/4" jambs. 2 1/4" trim covers gaps.” In other words, the thicker jambs used for most exterior doors means you’ll need a wider opening to accommodate it.
Is there Any Wiggle Room?
It’s called a rough opening for a reason. While lack of precision in construction can result in big problems down the road, there’s room for a bit of play here. Some members allow up to three inches of a gap, while others pointed out that a gap that large will require a plethora of shims and additional insulation to fill the gap.
With this in mind, it’s better to aim close to the finished opening of no more than two inches. Some also leave an even smaller gap above the door, ranging from one inch to 2 ½ inches. Remember that although the door will only have one jamb along the top instead of one on each side, you will need a gap underneath the door so it can move freely.
Other Helpful Hints
Forum members are looking out for each other so they often provide helpful hints in addition to useful answers. In this case, one response reminded, “Don't forget to put a header, two 2x4's with a 1/2 spacer between them, above the door head.” This is the proper construction technique for framing a door.
Another member provided a helpful tidbit saying, “I make sure my hinge side framing is perfectly plumb, and slam the door frame to it. All the shimming will be on the latch side.”
The great thing about forums is that other members have had the same question at some point in their journey. Maybe they’ve found the answer in a book or called a friend. More common is ‘do it once to learn and do it again correctly technique’.
DIY is all about learning, so knowing your rough opening needs to be wider than your door is a good start. If in doubt after reading this, one forum member had a last bit of knowledge to impart: “All the doors at Home Depot I have seen have the rough-in dimensions on the package,” so knowing what to look for might be enough.