Shimming, Leveling, and Alignment

Why this matters

Equipment that sits unlevel, unaligned, or wobbling does not just look sloppy. It drains poorly, vibrates, wears bearings and seals early, stresses connections until they leak or loosen, and fails warranty inspections. Getting something level and aligned is a basic craft that shows up in nearly every trade, from setting a unit on a pad to hanging a fixture to coupling two shafts. Done right at install, it prevents a long tail of nuisance failures. This is the procedure.

Step 1: Know what "right" means for this install

Level is not always the goal. Decide the target before you start.

• Level (truly horizontal) is right for most mounted equipment, fixtures, and anything where a bubble must read center.
• Deliberate slope is right when something must drain or flow. Many drain lines and drainage trays are set to a specific fall, not level. Setting them dead level is a defect.
• Plumb (truly vertical) is the goal for posts, stacks, and vertical runs.
• Alignment (two parts sharing a centerline or staying parallel/square) is the goal for coupled shafts, mating flanges, doors and panels, and anything where two pieces must meet true.

Check the manufacturer requirement or the system design. Then pick your reference: the finished floor, an existing structure, or a string line, whichever the job demands.

Step 2: Find the true reference and check your tool

• Confirm your level itself is accurate. Set it on a surface, read it, then flip it end-for-end on the same spot. If the bubble reads the same both ways, the tool is true. If it shifts, the tool is off and every reading inherits the error.
• Choose a reference long enough to span the work. A short level on a long object misses an overall slope. Use a longer level, a straightedge, or a string and line level across the full span.

Step 3: Measure the error before correcting it

Set the equipment in place and read where it is off, and by how much, in both directions. Most things need leveling on two axes (front-to-back and side-to-side). Note which corner or edge is low. Measuring the gap before shimming tells you roughly how much shim you need and keeps you from chasing the level back and forth.

Step 4: Shim the low points

A shim is a thin, load-bearing spacer that fills a gap to bring a surface up to the reference.

• Shim under the low point(s), not the high ones. Raise the equipment to meet level; you cannot lower a high corner without cutting the base.
• Use full-contact, load-rated shims that support the load across the foot, not a thin sliver that point-loads and crushes. The shim must carry weight without compressing over time.
• Tapered shims let you dial in a precise height; drive a pair from opposite sides to make a parallel-faced spacer of the exact thickness.
• Stack minimally and stably. A tall stack of thin shims can walk or settle. Prefer fewer, thicker shims, and where vibration is present, secure the stack so it cannot creep out.
• Re-check after each shim. Add, read, adjust. Work one axis toward true, then the other, then confirm both, because correcting one often nudges the other.

Step 5: Align mating parts

For two parts that must meet true (flanges, couplings, doors, panels):

• Bring the faces parallel and the centerlines coincident before final fastening. A misaligned coupling or flange forces the bolts and gasket to take up the gap, which they do by leaking, loosening, or wearing.
• Check both offset (centerlines not in line) and angularity (faces not parallel). Correct both.
• Snug fasteners gradually in a cross pattern so the parts pull together evenly rather than cocking to one side as you tighten.

Step 6: Lock it and re-verify under load

• Tighten the mounting hardware in stages, re-checking level/alignment as you go, because the act of tightening can pull the work off true.
• After everything is fastened, check one more time. Then, where you can, check again under load or after a short run, since weight and operation can settle shims or shift alignment. A unit that was level on a fresh pad can move as it loads.
• If anything drifted, back off, correct, and re-torque. Do not leave a connection forcing the gap.

Common mistakes

• Setting a drain or tray level when it needed fall, so it never drains.
• Shimming the high corner instead of the low one.
• Thin point-contact shims that crush and let the unit settle back out of level.
• Tightening fasteners that are fighting a misalignment, then wondering why the joint leaks or the bearing fails.
• Trusting a level you never checked for accuracy.

References

• Manufacturer installation documentation for required level, slope, or alignment tolerances.
• Trade-standard practice for shimming and equipment setting.
• See related: Measure Twice: Field Measurement Discipline.
• See related: Tightening a Fitting the Right Way.