Wood Screw Heads - Flat vs Oval vs Round
I often get customers asking what head type to choose when ordering some of our silicon bronze wood screws. The answer is always, "Well... That depends!" It seems simple enough that a flat head's would screw would be flat, and oval head wood screw's head be oval, and a round head wood screw's head would be... You get the idea. But there is a little bit more to it. Different use cases require different head types and I hope to shed a little light on that here.
Flat Head Wood Screws
Flat head wood screws are used when you need the screw head to either be flush with the the surface of the wood you are working with, or you're going to countersink it and bury it in the timber under a bung. As can be seen in the photo on the left, the head is of course perfectly flat, but also to note is that the underside of the head is tapered to fit a countersink. That taper will either fit perfectly in the correctly sized countersink hole on a piece of hardware or a hold that you've drilled using a countersink bit. Alternatively, if no countersink is drilled in to the timber it will press the wood once the screw is driven home and form a watertight seal in cooperation with the full bodied shank that proper cut thread wood screws have (more about cut vs. rolled threads here). The exact angle of the taper underneath the thread can be determined by studying the ANSI wood screws standards, particularly ANSI B18.6.1
Flat head wood screws are what you'll want to use for planking a boat when a bung will sit over top of the screw.
Oval Head Wood Screws
An oval head wood screw is similar to a flat head when it comes to the taper underneath the head. That taper again can be used to help seal the screw penetration which of course is a great thing in boatbuilding, to fit perfectly in a piece of hardware, or to sit nicely on a drilled out countersink.
An oval head should be used when the screw head will be exposed, and rather than the flush look of a flat head wood screw a protruding head is preferred for aesthetic purposes. It should not be used if it is going to be sunk in to a piece of timber and covered with a bung. If a bung will go over the top of the screw, a flat head is preferred, as the bung will sit flush with the top of the screw and there won't be any open cavities.
Round Head Wood Screws
Round head wood screws are missing that tapered portion of shank underneath the head as can be seen in the photo above. The head itself protrudes further than an oval head, and the underside of the head will sit flush against a flat surface.
Use a round head when you want the big, beautiful head to be seen or if you're mounting a piece of hardware that has flat, non-countersunk screw holes. Upholstery edges are often accented with round head screws, mast plates are mounted with them, and a variety of other hardware with no countersinks.
What it boils down to?
Do you require the shank to be tapered for a countersink or not? That's the number one question. If it needs to be tapered then a flat or oval head wood screw is the way to go. If no taper: round. Then will it be seen and exposed? If exposed, an oval or round head is beautiful unless you prefer a flush look. If buried in a piece of timber with a bung over top, a flat head wood screw is the way to go.