How to install a prehung door unit into an existing rough opening. First lets become familiar with the parts of the unit. The DOOR is the slab of wood, metal or plastic usually flat or with decorative molding. It attaches to the JAMB by way of HINGES to a jamb LEG. There are 2 LEGS one for the hinges and one for the STRIKER PLATE. The jamb is made up of a jamb HEAD, 2 LEGS and sometimes a SILL. The unit which comes preassembled installs as one piece into the wall rough opening. There are 2 ways you can install the unit into the opening, 1 with casing (the TRIM) attached to the hinge / striker plate side of the jamb or 2 with out the trim.
For ease and clarity our sample prehung unit will be a 2868 hollow core interior unit, like the ones you see at the big box home supply houses. These are by far the lightest, and easiest to install and very popular. 2868 = door unit is a 2′-6″ X 6′-8″ , 3068 = is a 3′ -0″ X 6′-8″ got the idea?
I personally like to attach casing to the jamb on one side and install. Start by setting a tri-square to 3/16″ and on inside of jamb edges mark full length of legs and head both sides.
Step 1 check floor and head of rough opening for level.
Step 2 check left and right vertical framing in rough opening for plumb.
Step 3 if floor and head are perfect level then measure OAH (over all height) then subtract 1/4″-1/2″ from prehung unit legs OAH and cut. Now the unit should slip into opening with gap at top.
Step 4 attach casing to hinge/striker plate side, and head of jamb using 4P finish nails, make sure to hold casing 3/16″ off from the inside of jamb all around.
Step 5 place unit in wall rough opening till casing is against drywall. Have some one hold the unit from falling out and step back. At this point your looking for an even reveal (the 1/8″ or so between the door slab and the jamb).
Step 6 read step 19 before continuing. Adjust unit in rough opening so reveal is equal and uniform across the head and down the striker plate side jamb leg using your thumb or palm by pushing against the jamb edge where casing is attached. If you can achieve this, good because you can now attach unit to opening. Sub-note what you want is the hinge side plumb and the reveal along the top even. Attaching casing to wall at the top of hinge side about 6″ down will allow you to adjust striker side and head with out unit falling out on you. Attach by nailing 6P finish nail but do not set the nail you may have to remove it.
Step 7 with hinge side plum (that’s level in the vertical) attach bottom casing to wall same as you did at the top, check for plumb and casing flush against wall full length.
Step 8 with palm of your hand push up on jamb head at striker side until the reveal is even across the top from left to right tack casing to wall at top.
Step 9 with door in closed position check reveal at top left jamb leg and make it the same at the bottom tack to wall. The next step is very important.
Step 10 using your thumb push against door slab at the top corner on the striker side until the door slab is flush with the jamb not the casing. look at the bottom, is the door slab flush with the jamb or is it inside or outside of the jamb? If flush good add nail to bottom casing, if not go the next step.
Step 11 jamb not flush at bottom. You should have three 6P nails in the door casing 1 at top and bottom hinge side and 1 at top striker side. If bottom door at jamb is in move top of jamb out until slab is flush top to bottom. If bottom door at jamb is out move bottom of jamb in if you can until slab is flush top and bottom or move top of hinge side out until slab is flush top and bottom. All the while keep mindful of the reveal on both sides and top.
Step 12 If you wondering why shims have not been mention it is because these door units do not need to be shimmed. They will work fine hanging on the casing because they are that lite.
Step 13 if reveal is close to equal and door slab is flush then add 6P nails spaced 12″ apart down the hinge side casing, down the striker side casing (adjusting reveal as you work your way down by pushing or pulling casing) then the head.
Step 14 step back and look at the door. Make sure all is correct. Open door make sure door swings properly and does not want to open or close on its own.
Step 15 trim the other side to the jamb first with 4P nails, hold on the line 3/16″ in you made earlier.
Step 16 before nailing casing to wall (same procedure as outlined above) start at top install 6P nail at each side, the jamb head will hold the jamb square to opening.
Step 17 hing side at bottom start nail with in 6″ from floor but don’t drive in until you make sure the jamb is square to the opening. Finish nailing hinge side top to bottom.
Step 18 measure between jambs at top and ideally when tacked at bottom will be the same between jambs, follow steps in step 17, then nail head casing.
Step 19 if all looks and operates correctly then finish nailing 6P nails flush with casing both sides.
Step 20 use latex caulk at casing to jamb and casing to wall all the way around both sides. When dry the light-weight hollow core prehung unit will be locked in place ready for paint. Your finished.
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