Deck screws are fasteners of various sizes used to fasten decking down to the treated or steel framing.
Decking screws are generally manufactured in 2-1/4" to 3" lengths, and are between #7 and #10 sizes (diameter of the screw).
There are different head types and thread types, depending on the intended use.
Screws for PVC decking usually have either a "trim" style head, or a cap-head screw, which if installed correctly, will have the top of the screw head sitting flush with the top surface of the decking.
Trim Head Screws
Trim head screws have a conical head that will sink into the decking. This style of screw will only work on PVC decking, because the PVC material will compress enough to let the screw head sink in without "mushrooming" out the material around the decking.
If this style of screw is used on composite decking, the screw head will try to compress the composite material, but because of the density of the composite material, the decking will bulge out around the screw, which we call "mushrooming". For that reason, a cap-head style screw is usually recommended for composite material.
Trim head screws, usually a stainless version, are the most common type of screw used to face-fasten Ipe and other hardwood decking. Hardwoods generally require pre-drilling for each screw, which can be made easier with a pre-drill/countersink combination bit, to make sure the screw heads are flush with or just beneath the surface of the decking.
Composite Screws
Cap head screws, or "composite screws" are specifically designed to shear off and compact the material where a composite screw is installed.
Composite screws have a short length of reverse threads near the head, so that when the screw is set into the decking, the reverse threads will shear off the material and push it downwards and away from the decking surface. The head of the screw will also shear off a small amount of the decking surface to make a clean hole the correct size for the screw head.
Screws with Plugs
Many decking brands have sets of screws with matching plugs, and when installed with a specific depth-setting bit, allow for a plug to be inserted into the hole over the screws.
FastenMaster manufactures a version known as "Cortex", and Starborn makes a version they call "Pro Plugs". Simpson Strong Tie has also released a plug and a specific bit that is designed to be used with their DCU screws.
FastenMaster has versions of their Cortex screws available in collated plugs (on a strip) so that they can be lined up and installed more efficiently.
Cortex is also available for Trex fascia, with a unique pre-drilling bit and different sized plug specially made to work with the modern composite-fastening methods used with composite fascia.
