Fasteners for a lasting, firm hold
Fasteners for a lasting and firm hold
Your basic common screw isn't quite so common anymore. The ever-changing climate of tools, bits, and materials has led to an explosion of specialized designs. The greater use of dense hardwoods, the development of composite materials, and changes in preservative treatments also have exerted their influence.
To begin with, the density of composite materials creates problems of splitting, mushrooming (when material is pushed up and out around the screwhead), and screws "spinning out" (when threads lose their bite) before the heads are fully countersunk.
While better for the environment, the changeover from the old CCA wood treatment to the new ACQ treatment has proven to be far more corrosive on fasteners. Many experts recommend only stainless-steel or hot-dipped zinc fasteners for use with ACQ; however, many other screw types are billed as suitable for ACQ. Currently, the fastener industry is self-governed and sets its own standards for what constitutes an ACQ-rated screw for treated stock.
Choosing suitable fasteners
Finally, as a means of maintaining outdoor projects, we rely on all kinds of chemical solutions. We use bleaches and cleaners to kill mildew and revive surfaces. On the downside, we sometimes add salt to dissolve ice on a deck and improve traction. But while these additives may well keep outdoor wood looking good or make it safer to walk on, they abuse and destroy the protective coatings on fasteners. So, after pondering the type of project you're building, carefully read the following pages on screw features and the chart located at the end of the story, to choose suitable fasteners for the job. Then, check out "Screw-Driving Tips" below and the photo, to see how you can best put your selected fasteners to work. If using screws, consider going with an impact driver.
Lubricate: Extra lubrication reduces torque and helps save protective coatings. Beeswax works, and the cheapest and easiest source for the lubricant is a toilet wax ring. Just be sure to clean off the excess wax with mineral spirits before finishing.
Predrill and plug: If you don't have hundreds of screws to drive, it's worth your time to predrill. For a furniture-quality look, counterbore to hide screwheads deep in the wood. Then fill the counterbored recesses with plugs made from leftover scrap. Doing this improves appearance, and protects the fastener as well.
Anatomy of a screw
Close examination of screwheads reveals a whole world of engineering that governs their design. The more you know, the easier time you'll have choosing the right one.
Material: Exterior fasteners are made from various grades of case-hardened or stainless steel. While stainless-steel products are left bare, all others have layers of electroplated zinc coatings and sometimes a polymer coating for color or lubrication. Or they're dipped in molten zinc to prevent corrosion. Note that yellow zinc and black screws (not shown in this article) are often not suited for the rigors of exterior use.
Head size: With large-headed screws, your fastener's visibility increases and countersinking becomes harder and time-consuming. Further, the chances of splitting the wood increase. Many types of trim-head exterior screws, such as GRKs, come with head sizes similar to a same-size finish nail, making them less noticeable.
Screw driving basics
Countersink cutters: Many exterior screws have countersink cutters on the underside of the head that aid in sinking them. The number and prominence of the ridges tell how effective they are in that task. One screw, Titan's Splitstop (shown below, has aggressive cutters to both countersink the head and minimize splitting.
Thread pitch and count
Thread pitch and count: As a general rule, the steeper the thread pitch and the lower the thread count, the faster you can drive a screw. However, such screw designs demand more torque, placing more stress on tools and users, and increasing the chance of snapping screws and stripping heads. One screw, has a secondary set of threads inside the main threads that reduces the torque demand on the drill and the need for predrilling, particularly with softer woods. (below)
Designed for driver ease
Driver shape: Exterior screws come with square (Robertson), combination, star (Torx), and Phillips drives. Those that tend not to slip: star and square drives. However, it's not only the shape of the driver bit that plays a role in driver ease, but also the depth of the recess. The deeper the pocket, the better the bit's bite.
Shank size: Shank size affects a screw's sheer strength and pull-out power. A wider shank means the screw threads can be more aggressive. In general, use #10 screws for heavy-duty projects, such as deck building, and #7 or #8 for lighter-duty tasks, such as outdoor furniture construction.
Shank slot: Look for screws with a shank slot like those shown. This feature improves a screw's ability to auger into a hole, improving self-tapping capability without having to predrill.
Know your threads
Thread shape: Threads are changing rapidly. A composite screw's shank usually has opposing threads designed to keep the lower threads from spinning out in less-dense natural material (i.e., a wood floor joist). Spax, a composite screw, even has serrated threads (shown) to improve cutting ability and reduce torque.
Salt spray and splitless
Salt-spray specialists: If you live in a coastal environment, make sure your fasteners can withstand salt corrosiveness. Key West lumber dealers recommend hot-dipped galvanized and stainless-steel screws.
Splitless features: Predrilling and counterboring are still your best bets to prevent splitting your material, particularly along the ends and edges of stock. However, these methods take time. Trimhead screws help greatly in this area, but other features also have been developed. In addition to the aggressive countersink cutters mentioned earlier, Titan's screws have a vertical knurl above the threads (shown below) that helps hog out a hole to make room for the shank. If you choose nails instead, consider the ring-shank type. The blunt tip on the ring-shank nail reduces splitting.