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Height left under a grid
A suspended ceiling needs room above the tiles to tilt them into the grid. Set the drop to the gap your system needs below the lowest joist or pipe. Drywall fastened directly to the joists only loses its own thickness, typically ½ in.
| Slab to joists | Drop 3 in | Drop 4 in | Drop 5 in |
|---|---|---|---|
| 7′ 2″ | 6′ 10.2″ | 6′ 9.2″ | 6′ 8.2″ |
| 7′ 4″ | 7′ 0.2″ | 6′ 11.2″ | 6′ 10.2″ |
| 7′ 6″ | 7′ 2.2″ | 7′ 1.2″ | 7′ 0.2″ |
| 7′ 8″ | 7′ 4.2″ | 7′ 3.2″ | 7′ 2.2″ |
| 7′ 10″ | 7′ 6.2″ | 7′ 5.2″ | 7′ 4.2″ |
| 8′ 0″ | 7′ 8.2″ | 7′ 7.2″ | 7′ 6.2″ |
Includes ¾ in of new flooring.
If the result is under 7 ft, consider drywall on the joists with soffits only around ducts, or a ceiling system that mounts directly to the joists. Lowering the floor is the expensive alternative; see the dig-out cost.
Worked example
A suspended ceiling over the example's 712 sq ft needs 196 2 × 2 tiles or 98 2 × 4 tiles with 10% waste, and about 116 ft of wall angle around the room. With joists at 7′ 8″, a 4 in drop and ¾ in of new flooring leave 7′ 3.2″, which still clears 7 ft. A 3 in drop leaves 7′ 4.2″.
Where basement inspections usually fail
- Bedroom windows that are too small or too high, especially sliders and double-hungs measured by the frame instead of the opening.
- Window wells under 36 in deep from the wall, or deeper than 44 in with no ladder.
- Ceilings that dip under 6 ft 4 in at a duct or beam once drywall and flooring go in.
- Stairs with a short top or bottom riser after new flooring changes the floor levels.
- Missing fire blocking at the top of furred walls and around soffits.
Many cities publish one-page handouts on these items, like the St. Paul, Boise and Des Moines sheets cited on this site. Ask for your city's version before you frame.
Before you call for an inspection
- Measure every egress opening with the sash open and write the numbers on the frame.
- Measure ceiling height at the lowest duct or beam after the ceiling material is up.
- Check riser heights top and bottom after flooring.
- Leave insulation and vapor details visible until the insulation inspection if your town does one.
- Have the permit card and approved plans on site.
Common questions
How much height does a drop ceiling take?
Usually 3–4 in below the lowest joist or pipe, plus the tile itself.
Related calculators
Sources
- https://www.cityofboise.org/media/3924/434-irc-code-section-r305-ceiling-height.pdf
- https://www.mckissock.com/blog/appraisal/understanding-gross-living-area/