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Why Use a Plumb Bob on Door Jambs?
A plumb bob gives you a true vertical reference when setting a door frame. Unlike a short spirit level, it is not affected by a slightly uneven floor or by the length of the jamb. That matters because a jamb can read “level” in one small area while still leaning several millimetres from top to bottom.
A jamb that is out of plumb can cause the door to swing open or closed by itself, rub at one corner, leave an uneven reveal, or make the latch miss the strike. The most reliable method is to use the plumb bob to check the hinge jamb and latch jamb over their full height, then use a level to check the head and cross-check the work.
You do not need an expensive tool. A basic carpenter’s plumb bob with a pointed tip and a stable string is sufficient. A 6- to 8-ounce bob is easier to control indoors than a very heavy masonry bob.
Tools and Materials
Gather the door unit or separate jambs, shims, casing nails or screws, a tape measure, pencil, sharp knife, hammer, and a 4-foot level. A pair of clamps can hold the jamb temporarily, but wedges or shims are usually more useful on site.
Use a string that does not stretch noticeably. Plumb-bob cord, mason’s line, or fine braided nylon works well. Avoid a loose cotton thread that twists and catches air. If you regularly install doors, a carpenter tool belt with a hammer holder keeps the bob, pencil, and knife accessible while you work.
| Tool | Best use | Trade-off |
|---|---|---|
| Plumb bob | Checking a jamb over its full height | Needs space to hang and must be allowed to settle |
| 4-foot level | Fast checks and checking the head | Can hide a gradual lean if used only in one position |
| Laser level | Repeated layout and hands-free reference lines | More expensive and less useful in bright light or dusty rooms |
For one or two interior doors, the cheaper plumb bob and a decent level are normally the sensible choice. A laser earns its cost when you are setting many openings or working alone.
Prepare the Opening Before Plumb Checks
Remove loose plaster, drywall lumps, and protruding fasteners from the opening. Check that the rough opening is wide enough for the frame and that the subfloor is reasonably sound. Do not assume the floor is level; it often is not.
Mark the hinge side and latch side clearly. If the floor changes height across the opening, record that difference before trimming the jamb legs. The head should ultimately sit level relative to the finished floor or the intended reference line, not necessarily relative to an unfinished slab.
Fit the jamb temporarily, leaving enough clearance for shims. For a typical interior prehung door, allow roughly 1/2 inch of rough-opening width beyond the outside frame dimension, although the manufacturer’s instructions take priority. Keep the hinge jamb straight rather than forcing it to follow a bowed wall.
Hang and Read the Plumb Bob
Choose a fixed point at the top of the opening, such as the underside of the header or a temporary nail driven into the framing. Hang the bob so the string is close to, but not touching, the jamb face. The string should run nearly the full height of the jamb.
Hold or tape the top of the string in place and let the bob hang freely. Wait for it to stop moving. In a drafty room, close the door or window and lightly steady the bob with one finger, then wait again. Never read the line while the bob is swinging.
Measure the gap between the string and the jamb near the top and again near the bottom. If the gap is the same at both points, the jamb is parallel to the plumb line. If the bottom is 1/8 inch farther from the string than the top, the jamb leans 1/8 inch over its height. That is enough to affect a door, particularly a tall or heavy one.
Keep the string several inches away from the jamb if possible and use a small ruler or pencil mark to compare the gap. Do not rely on your eye. A difference of 1/16 inch is difficult to judge accurately without a reference mark.
Set the Hinge Jamb First
Start with the hinge jamb because it controls the door’s position. Place shims behind each hinge location, plus additional shims near the top and bottom. Drive fasteners through the jamb and shims, but leave them just loose enough to make corrections.
Hang the plumb bob again and adjust the jamb until the string-to-jamb gap is consistent from top to bottom. Check the jamb in both directions: across its face and across its edge. A jamb can be plumb vertically while twisted, which leaves the hinge leaves or door edge out of alignment.
Once correct, fasten the jamb securely. Use pairs of shims where possible so tightening the screw does not pull the jamb sideways. Recheck the plumb line after every major fastener. A screw can move a carefully aligned jamb by 1/16 inch or more.
Set the Head and Latch Jamb
Set the head against the hinge jamb and check it with the level. If the finished floor is uneven, keep the intended door reveal in mind rather than blindly chasing a level reading. A common target is about 1/8 inch clearance at the sides and top for an interior door, but follow the door manufacturer’s requirements.
With the head secured temporarily, hang the plumb bob on the latch side and adjust that jamb until it is parallel with the hinge jamb and truly plumb. Check the diagonal measurements from the top hinge corner to the opposite bottom corner, and then the other diagonal. Equal diagonals are a useful check that the frame is not racked.
Close the door and inspect the reveal. It should remain fairly even from top to bottom. If the reveal narrows at one corner, do not immediately plane the door; first recheck the jambs, shims, and frame twist.
Common Mistakes and Fixes
A bob that is too light moves constantly in a draft, while one that is too heavy can damage finished flooring. A mid-weight bob is the practical compromise. Keep it away from metal studs, electrical cables, and moving tools; the string can snag and give a false reading.
Another mistake is plumbing only the casing. Casing can be crooked, especially on an uneven wall. Reference the actual jamb, and check near the hinge and strike areas rather than only at the ends.
If the wall is badly bowed, do not bend the jamb to match it. Keep the jamb plumb and straight, then use tapered shims or adjust the casing to bridge the gap. Finally, operate the door several times before trimming shims flush. A frame that looked correct while loose may shift when fully fastened.