First, let’s define the parts properly. Some people get this wrong and it’s sheer ignorance or laziness. If someone can’t be bothered to get this correct, why would you want to do business with them? Hardware Everywhere knows the difference.
A drywall screw (head shown above) is hardened, fully threaded when under 1-1/4” inch in length, and threaded approximately ⅔ of the screw length when over; it also has an almost straight shank compared to the particle board and decking screws. The threads are rolled and rise well up above the shank of the screw and all have a bugle head or modified truss wafer head. If doesn't have a one of those two heads, it's not a drywall screw no matter what anyone tells you. It may be used in drywall but that doesn't make it a drywall screw. Saying something over and over doesn't make it true. (This definition is also true for drywall self-drilling screws.) Some drywall screws may have a type-17 point, but that doesn’t make them a deck screw; it’s a drywall screw with a type-17 point. Drywall screws are never deck screws and we’ll discuss why shortly. Drywall itself is a soft panel made of gypsum (also called calcium sulfate dihydrate), which is extruded between thick sheets of facer and backer paper. The plaster is mixed with fiber or other additives and then hardened into final form in various thicknesses.
FLAT HEAD WITH COUNTERSINKING NIBS
A decking screw (head shown above) is similar to a particle board screw – which makes sense if you think about it – but has superior corrosion resistance, either stainless or 1000-hour fluoropolymer coated. Decking screws are always flat head1 with both nibs and type-17. If it's flat and has no nibs, then it's not a deck screw. If it's got nibs but no type-17, it still isn't a deck screw. The nibs2 are located on the bearing surface under the head, on the angled sides. Decking screws have asymmetrical threads for easier installation in all types of woods. Stainless deck screws are often waxed for additional lubricity. A deck is made of solid wood or treated lumber. Particle board is rarely used for decks because it is much weaker and isn’t water-resistant.
Why the two shouldn't be confused!
A drywall is screw is designed for use in drywall to attach a sheet of it to an interior stud (usually metal, sometimes wood). They are not designed for use as wood screws. The heads are designed to sit in the soft drywall and be flush on top (not in) with the expectation of additional finishing on the final product. They are not designed to be exposed nor do they properly countersink into wood because countersinks work only with standard flat countersunk (flat) heads.3 If you walk around on a deck in your bare feet and someone has used drywall screws, you’ll notice right away because they don’t sit into the wood and jut out slightly because of how the head is designed. Your feet will know immediately. Nothing is wrong with the screw: the head is designed to sit in drywall and not wood.
In a deck screw, nibs are an integral part of the head design and are to aid in cutting the top material to allow a flush, countersink finish without the need to pre-countersink the hole first. They also have a type-17 auger point which is used to help keep the wood from splitting. A type-17 point serves no use in a drywall screw. Deck screws cannot have bugle heads – bugle heads are drywall screws. The nibs cause the head of the screw to go into the wood and be flush with the top of the wood. If you walk over the surface in your bare feet, you will not feel the screws on the soles of your feet.