Measuring & buying
Drywall thickness: 1/4, 3/8, 1/2, 5/8 and type X
Half-inch board covers most walls in most homes. The exceptions are ceilings on wide framing, fire separations, curves and sound walls, and each has a rule.
The four thicknesses
Gypsum board is made in four common thicknesses, and each has a job:
| Thickness | Weight | Typical use |
|---|---|---|
| 1/4 in (6.4 mm) | 1.2 lb per sq ft | Curved walls in two layers; overlays on old plaster or popcorn ceilings |
| 3/8 in (9.5 mm) | 1.4 lb per sq ft | Overlays and repairs; walls on 16 in framing in older homes |
| 1/2 in (12.7 mm) | 1.6 lb per sq ft (lightweight 1.25) | Walls and most ceilings in homes |
| 5/8 in (15.9 mm), usually type X | 2.2 lb per sq ft (lightweight 1.75) | Fire-rated assemblies, garage ceilings under rooms, ceilings on 24 in framing, sound walls |
The weights are typical manufacturer figures; the sheet weight calculator turns them into the weight of your order, and the sheet sizes and weights table lists every size.
What the code says
The International Residential Code, Table R702.3.5, sets the maximum framing spacing for each thickness. The rules that matter in a house:
- 1/2 in on walls: studs up to 24 in apart.
- 1/2 in on ceilings, sheets across the joists: joists up to 24 in apart. The code allows it, but regular 1/2 in board can sag between joists at 24 in, especially under a water-based texture or heavy insulation.
- 1/2 in on ceilings, sheets parallel to the joists: joists up to 16 in only.
- 3/8 in: walls and ceilings on framing up to 16 in apart, ceilings across the joists only.
- 5/8 in: walls and ceilings on framing up to 24 in.
Sag-resistant 1/2 in ceiling board was made for the 24 in case: it is as light as regular 1/2 in and rated by its makers for ceilings on 24 in framing, including under water-based texture.
Three ceilings, three answers
Ceilings on joists 24 in apart, run through the thickness selector
- Sheets across the joists: 5/8 in regular board, or 1/2 in sag-resistant ceiling board. Regular 1/2 in is allowed by the code but sags more.
- Sheets parallel to the joists: 5/8 in. Regular 1/2 in is not allowed at 24 in parallel; turn the sheets.
- Across the joists, under a sprayed texture: 5/8 in or sag-resistant 1/2 in. Regular 1/2 in is not acceptable under a water-based texture on 24 in joists.
The thickness selector gives these answers with the reason for each.
Type X and fire
Type X is a 5/8 in board with glass fibres in the core, so it holds together longer in a fire. It is the board in almost every fire-rated assembly in homes and light commercial buildings: the one-hour wall of the Gypsum Association’s file WP 3660 is a single layer of 5/8 in type X each side of the studs. In houses, type X is required by the IRC in one common place: the ceiling of an attached garage with habitable rooms above it (Table R302.6). The wall between the garage and the house needs only 1/2 in board, though many builders use type X there too.
A rating belongs to a whole tested assembly, not to the board alone. The fire-rated drywall calculator counts the sheets for the common assemblies and names their source, and the guide to fire-rated assemblies explains how a rating works. The garage rules are in the garage fire separation guide.
Weight, and why lightweight board exists
A 4 × 8 sheet of 1/2 in board weighs about 51 lb; the same sheet in 5/8 in type X about 70 lb, and a 4 × 12 type X sheet over 100 lb. Lightweight 1/2 in board, about 40 lb a sheet, was developed for ceilings and for hangers lifting hundreds of sheets a day; it meets the same standard as regular board.
17 sheets of 5/8 in type X
- 70.4 lb per sheet, 1,197 lb in all
- One trip in a pickup with a 1,500 lb payload, 19 sheets a load
- Two people to carry each sheet, a lift for ceilings
How to get that weight into the house is in the guide to drywall weight and handling.
Thin board for curves and overlays
Two layers of 1/4 in board bend around curves that a single 1/2 in sheet cannot follow, and together they make the same thickness. Flexible 1/4 in board bends tighter still. The curved wall calculator gives the bending radii and counts both layers.
Thin board is also the usual way to cover a cracked plaster wall or a popcorn ceiling that tests positive for asbestos: screwed through to the framing, it encloses the old surface without disturbing it.
Thicker for sound
Mass blocks sound, and 5/8 in board has about 40% more mass than 1/2 in. Sound-rated wall assemblies are almost all built with 5/8 in board, often in two layers. The soundproofing guide sets out what each layer buys.
Matching thicknesses
When a new wall meets an old one, or a repair patch goes into an existing wall, match the thickness: a 1/2 in patch in a 5/8 in wall sits proud by 1/8 in and needs a wide skim to hide. Measure the old board at an outlet box or a cut edge before buying.
Common mistakes
- 1/2 in ceilings on 24 in trusses, hung parallel. Not allowed; turn the sheets or use 5/8 in.
- Regular 1/2 in under sprayed texture at 24 in. It sags.
- Assuming type X makes a fire rating. Only the tested assembly does.
- Mixing thicknesses on one plane. Shim or match.
- Forgetting the screw length. 5/8 in board takes 1-5/8 in screws, the screw calculator picks them.