Another approach to installing prehung doors

When I see a fellow carpenter install a prehung door the old-fashioned way with layers of opposing shims, I shake my head. Why hasn’t my new and improved way caught on? It’s faster, stronger, and less filling.

Other installers place the door jamb in the opening and start pushing in and pulling out tapered shims, then checking the hinge-side jamb for plumb. The procedure goes in a little, out a little until the jamb looks right. Then the carpenter drives the fasteners home through the hinge jamb. This sometimes compresses wooden shims, and if the installer doesn’t overcompensate a pinch, the fasteners can knock the jamb out of plumb.

My installation method minimizes tapered shims and begins the night before I plan to install the door. I start by making a pile of square shims of various thicknesses. I make them with 1/8-in. and 3/16-in. hardboard, and 3/8-in. and 1/2-in. plywood. They are typically 4-1/2 in. or 6-1/2 in. square.

Then, on my level, I use an erasable felt-tip pen to mark the centers of the hinges of the doors to be installed (get a 6-ft. 8-in. level; you’ll thank me later). Next, I mark which end of the level is up.

Then I mark the hinge locations on the hinge-side trimmer and install a top and bottom shim of the same  thickness (usually 1/8-in. hardboard) at the hinge locations. Now I check the two shims for plumb and add shims as necessary until perfectly plumb. Then I screw (nails are passé) the hinge-side jamb leg to these shims, and I’m nearly done.

If there is a center (third) hinge, I shim that location after the top and bottom are perfectly plumb. I use a thinner shim so that the level touches just the top and bottom shims. After I have the jamb screwed to the trimmer through the top and bottom shims, I tweak the center hinge as necessary the old-fashioned way, with tapered shims.

I leave the door on its hinges during this operation because it’s more efficient. To hold up the latch side of the door until the jamb is positioned properly, I use a nifty tool originally designed for lifting cabinets during installation. FastCap’s Jack of All Trades jack (available at Amazon.com for about $60 per pair) makes this operation a breeze. I take the plastic jaws off the jack’s mast and use the angled flange at the bottom to lift the door. This method for adjusting the height of the door is much easier than using a pile of shims and 2x scraps.

I use the traditional shim method to finish the installation of the latch side of the jamb.