A brittle mineral core, held together by paper
Drywall is a panel of set gypsum plaster faced on both sides with heavy paper. That is the whole product, and the paper is the part people underrate: gypsum has almost no tensile strength, so the paper is carrying the panel in tension and letting something this thin and this brittle be lifted, carried and screwed to a wall.
Which is why damage to the paper is damage to the board rather than a cosmetic problem, and why a sheet that has been soaked and dried never comes back: the bond between paper and core has gone.
It is called dry because everything before it was wet
The wall it replaced was lath and plaster: thin timber strips nailed across the studs, then wet plaster pushed through the gaps in coats so it hooked over the back. It worked well and it needed a skilled plasterer, several days a room, and then weeks of drying before anything could be decorated.
A board of gypsum that has already set does the same job in an afternoon with no drying time. The trade called it dry walling by contrast with the wet trades, and the name transferred to the material.
The reason a paper-covered board is a fire product
This looks like it should not work. A panel faced in paper is what you would nominate as the worst possible fire material, and gypsum board is what fire-rated walls are made of. The reason is chemistry rather than the paper.
Gypsum is calcium sulfate dihydrate. Roughly a fifth of its weight is water, and that water is not dampness, it is chemically locked into the crystal. Heat the board and that water has to be driven off as steam before the material itself can get any hotter, so for as long as the steam is coming out the board is held near the boiling point of water.
Once the water has gone the core is left as a weak powder and starts to fall apart, and that is the point at which the wall stops protecting anything. Type X board adds glass fibre to the core so the panel holds together mechanically for longer after calcination, which is what turns a delay into a rating.
Worth saying plainly: a rating belongs to the assembly that was tested, the boards, the studs, the insulation, the fixings and their spacing together. Fitting fire board to something else does not transfer the rating to it.
Which board goes where
- Regular. The default for walls and ceilings in dry rooms. Half inch in the US, 12.5mm in Britain.
- Moisture resistant. Kitchens, bathrooms, utility rooms. Handles humidity and splashing. Not a tanking system and not for inside a shower.
- Fire rated, Type X. Garage ceilings, party walls, protected escape routes, anywhere a rating is specified. Usually 5/8 inch or 15mm.
- Impact or abuse resistant. Corridors, schools, anywhere trolleys happen.
- Paperless, glass-fibre faced. Where mould is the risk rather than fire, because there is no paper to feed it.
- Thin boards. Quarter and three eighths, for curves and for overboarding an existing surface.
Colours follow a rough convention, green for moisture resistant and pink or red printing for fire rated, and it is a convention rather than a rule. Manufacturers differ and the printed face is what tells you, so read it instead of trusting the colour.
For quantities and the finish, the drywall calculator covers sheets, screws, compound and the six finish levels, including why the paint you have chosen decides which level you need. British readers want the plasterboard calculator, which is the same job in metric with skimming rather than taping assumed. And before any of it goes up, the stud wall explainer covers what is behind the board and why nothing heavy can hang on it.
Common questions
What is drywall actually made of?
A core of gypsum plaster pressed between two sheets of heavy paper. Gypsum is a naturally occurring mineral, calcium sulfate, and a great deal of what is used now is a by-product recovered from power station flue gas rather than quarried. The paper is not packaging: it is doing structural work, holding the brittle core together in tension so a large thin panel can be handled at all. Tear the paper off one face and what is left breaks under its own weight.
Why is it called drywall?
Because the thing it replaced was wet. A lath and plaster wall was built by nailing thin strips of timber up and troweling wet plaster onto them in coats, which needed a skilled plasterer, took days per room and then had to dry out before anything else could happen. Panels of pre-formed gypsum go up in an afternoon with no drying time and no plasterer, so the trade called it dry walling and the name stuck to the board.
What is the difference between drywall, plasterboard and gypsum board?
They are the same product under different names. Gypsum board is the technical term used in standards. Drywall is the everyday American and Canadian word, plasterboard is the British, Irish and Australian one, and wallboard turns up in older writing. There is a real difference in how the two markets FINISH it, with British boards usually skimmed with plaster over the whole face and American ones taped and filled at the joints only, but the panel arriving on site is the same thing.
What does Type X mean?
It is fire-rated board, made with glass fibre and other additives mixed into the core so that it holds together for longer once the gypsum has given up its water and started to crumble. Ordinary board already resists fire for a while because of that water; Type X is engineered to keep the panel physically intact through it, which is what an hour rating actually depends on. It is normally 5/8 inch, and where a wall or a ceiling carries a fire rating the assembly it was tested as is what matters, not the board alone.
Can I use ordinary drywall in a bathroom?
For the room generally yes, and not where it gets wet. Gypsum is not waterproof and the paper facing is food for mould, so a board that gets repeatedly soaked will soften, sag and grow things. Moisture-resistant board handles humidity and splashes. Behind tiles in a shower, neither of those is the right answer: that wants a cement backer board or a proper tanked system, because the failure there is slow, hidden and expensive.
Why will it not hold a shelf?
Because there is almost nothing there. The panel is a thin brittle core held by paper and it is spanning between studs with a void behind it, so any load applied to the face is trying to tear a hole through half an inch of plaster. The fixings sold for it work by spreading the load or gripping the back face, and they are rated for pictures and light fittings. Anything with real weight, or anything with a lever arm like a television bracket, has to reach the timber or steel behind.