Before you choose your new pre-hung door the size of the opening is important to know. Getting the right size door for the opening helps to make your installation smooth and will eliminate some very big problems. Measure the opening to your door frame. Be sure to allow for about a 1 1/2 inch larger opening than the standard door sizes. Typically a pre-hung door is measured in 30 inch and 36 inch sizes with additional sizes available in non-standard widths.  The doors are also a standard 80 inches tall. Presuming the door is a 36 inch wide door, the dimensions of your opening should be 37 1/2 inch wide by 81 1/2 inch tall. Reasons for Door Opening Size When a pre-hung door is being installed into an opening there have to be considerations for leveling the door, and ensuring that it has a obstruction free swing. If the pre-hung door is butted up into an opening the swing will be hindered by the door. It may scrape the floor or it may get stuck as it is closing. An opening that is the same size as the frame on the pre-hung door can alter the shape of the frame and make the installation more difficult.

Ideally, door openings have square corners, plumb sides and level tops and bottoms, but construction errors, settling or other factors can cause floors to sag and slip out of level. A serious unlevel condition in a floor may indicate a serious structural problem and should be addressed by a specialist, but warping of boards, wear or other conditions also can make a floor slightly unlevel. That requires require some adjustment in installing either an interior or an exterior door. Modern prehung doors are installed basically the same, regardless of location. Remove any old door and molding, such as a casing or bottom threshold, inside the opening. Strip the frame down to the side 2-by-4-inch studs, top header plate and floor or subfloor. Run a scraper over the rough frame to get rid of any splinters or nail fragments. Put a 2-foot level across the bottom of the door opening to assess the degree and direction of any sag. Measure the side jambs of the new prehung door casing and the height of the rough opening with a tape measure;

some doors come with extra-long jambs, which may have to be trimmed with a circular saw so the casing can fit into the frame. Trim the jambs so the casing top fits under the rough frame top. Set the prehung door into the frame to make sure the casing fits. Lift the door out of the frame and knock out the door's hinge pins, using a nail set or punch, to free the door from the casing. Pull the door out of the casing and set it aside. Set the casing back into the rough frame as level as possible, with the jamb edges on both sides of the wall even with the wall surface, and the top almost to the rough frame's top. Push tapered wooden shims under the bottoms of the side jambs, one shim from each side, to lift the casing top to the rough door frame and hold it in place. Install shims by hand, with the tapers pointing toward each other, and tap them with a hammer until they are solid. Alternate sides so the casing does not tilt, either side to side or across the wall faces. Lay a 2-foot level on top of the casing and adjust shims until it shows level.

Put the level vertically on the jambs to check the plumb and install shims between the 2-by-4 sides and the casing, starting at the widest gap, to adjust the jambs. prehung interior door installationWork on the hinge-side jamb first. garage panel doors adelaidePut shims in as many places as needed, typically at least two spots, and drive them in from opposing sides gradually to adjust the jamb. sliding garage door torontoAdjust the latch jamb with shims once the hinge jamb is plumb. Check the top and bottom corners with a framing square when both jambs are plumb. Put the door back in the casing with the hinge pins and test it to make sure it opens and closes easily. Check the gap at the bottom of the door when it is closed to see what leveling, if any, is needed on the floor.

Remove the door and make any changes needed so it opens and closes. Adjust a side-to-side unlevel floor by putting wood or asphalt shingle shims under the subfloor to level it. Make a similar adjustment if the unlevel condition is preventing the door from swinging open fully; you may have to raise the jambs to provide clearance, then shim the floor under the door. Secure the casing to the rough frame once the door is set and opens properly with 3-inch screws driven with a screw gun through the shims. Make sure the facing edges of the casing line up with the walls. Trim shims on both sides with a drywall saw or by scoring them with a knife and snapping them off. Install a threshold on an exterior door to seal the bottom against air and water. Things You Will Need Scraper 2-foot level Prehung door, interior or exterior Tape measure Circular saw Nail set or punch Hammer Tapered wooden shims Framing square 3-inch screws Screw gun Drywall saw or utility knife Tip References Renovation Experts: Exterior Door InstallationLowe's: Install an Exterior DoorThis Old House: How to Install a Prehung DoorFinish Carpentry Help: Exterior DoorEZ Hang Door

: Raise the Door Off the SubfloorMidwest Manufacturing: Prehung Exterior Door Installation Suggest a CorrectionIf you want to replace an old door, a prehung interior door is an easy replacement option. Since you don't have to hang the door in a frame, you save a significant amount of time and labor. The installation process varies somewhat depending on the type of home interior doors you choose, but it tends to follow a few general steps. Once you have materials, installation of prehung interior doors takes about an hour or two. Remove the current door, if you haven't done so already. Measure that door or the doorway opening before you get your new home interior door. That way, the door fits properly right away, and you don't have to trim it. If you do end up having to trim the new door, you can do so with a saw. Use wood shims to position the door properly in the doorway and to hold it in place once you do. Make sure it's centered, plumb to the door jamb, and at the right height.

It may help to have someone else there to hold the door for you while you add the shims, although you can also do it yourself. Once the door is stable, use a level to check that the entire door is at an even height. Use a hammer and nails to secure the door on the hinge side. Use steady, controlled swings to avoid damaging the door or the doorway. Pound the nails through the door jamb, the wood shims, and the door. After you nail in that side of the door, check that it's level again. Try opening and closing it to see that it follows a consistent path. Adjust the shims to get the door in the correct position, nail in all the sides of the jamb, and trim the shims so they don't stick out past the doorway. Nail the door casing molding around the entire door frame. Molding improves the appearance of prehung interior doors. Once you put on the molding, the installation process itself is complete. Use a nail set to sink all the visible nails into the wood. Cover the holes over the nails with wood putty.