Drive screws that don't corrode
Among your most-common options are stainless steel (which comes in different grades), silicon bronze, and steel with one of several available coatings. Here, we look at each. Although aluminum and brass resist corrosion, we don't include them because they're too soft for most applications. Also, aluminum will corrode quickly in contact with the chemicals in treated lumber.
Silicon bronze. This material is the standard in marine fasteners because it resists corrosion and doesn't promote rot in the wood around the fastener. It costs about twice as much as 304-grade stainless steel, so reserve screws made of this material for boat-building.
Stainless steel. Screws made of this material may cost twice as much as coated steel fasteners. But stainless steel is well worth it if you don't want to see any rust or stain marks on your project. Whenever you put time or expensive lumber into a clear-finished project, it doesn't make sense to use anything but stainless steel.
Most widely available stainless steel fasteners are either 304 or 305 grade. Both are appropriate for general use, although 305-grade fasteners have slightly greater corrosion resistance. For maximum corrosion resistance in extreme environments, say one where salt spray is common, spend the extra money for 316-grade stainless.
The Golden Rule for outdoor fasteners
Use screws, not nails, to join your outdoor projects. The additional gripping strength provided by their threads better resists the inevitable and considerable wood movement caused by outdoor exposure.
Coated steel fasteners
Because of the many available coatings and different processes for applying them, this category can confuse many DIYers. Most local hardware outlets still sell the galvanized fastener as their basic exterior-grade fastener. Generally speaking, the thicker the galvanized coating (consisting mostly of zinc) the more durable it will be. You'll find the thickest zinc coatings on hot-dipped nails, but you may have a hard time finding hot-dipped galvanized screws. That's because the thick, globby coating tends to clog the threads and driving recess of the screw, making it hard to drive.
Commonly available galvanized screws are either mechanically galvanized or electro-galvanized. At the lumberyard, you can quickly tell these two types apart because mechanically galvanized screws have a dull gray surface and electro-galvanized screws have a shiny silver-color surface. We do not recommend electro-galvanized screws (also called clear-zinc coated) for exterior applications. They will corrode quickly in contact with the elements.
Mechanically galvanized screws are generally suitable for decks and other outdoor projects made of pressure-treated lumber. Nevertheless, you should not use them with PT lumber in contact with the soil, in high-moisture areas, or in areas with salt content in the air. Also, keep in mind that mechanically applied zinc contains some iron that is susceptible to attack from the tannic acids that occur in redwood and cedar. The acids combine with the iron to form a dark stain around the fastener head.
As you can see in the photo shown, manufacturers have improved the basic electro-galvanized fastener by adding a polymer coating (often pigmented) on top of the galvanized coating.
Fasteners for pressure-treated wood
If you purchase one of the new-generation pressure-treated woods (came to the market in 2004 or later), be sure to ask your retailer about recommended screws and fasneners. For example, your dealer may stock proprietary anti-corrrosion technologies matched for alkaline copper quat (ACQ) or copper azole (CA-B).
These multiple-coated screws have several advantages. First, some of the coated screws come in colors that help the fastener blend with natural and pressure-treated wood tones. Also, the additional coatings increase corrosion resistance. For example, the outer coatings resist tannic acids, making them suitable for use with cedar and redwood decking. These coatings also help prevent iron stain (if the coating remains intact). This type of stain should not be confused with extractive bleed. The latter can occur with any fastener.
Nevertheless, any coating has the potential to wear through and expose the underlying steel to corrosion. This wear typically happens when you drive the fastener. Some low-cost coated screws lose part of the coating covering their threads after one drive through dimensional pressure-treated lumber.
All of the fasteners we tried showed some wear in the coating covering the driving recess. If you are less than careful and leave the head of the screw slightly above the surface, foot traffic will abrade the coating, too.
Our advice: Buy a few sample screws and test-drive them in the material you intend to use. Also, ask the manufacturer about how well the screw holds up in a salt-spray test; disregard any fastener not test-rated for a minimum of 1,000 hours.
Yellow-zinc coated steel
Some fasteners with this electro-plated coating are labeled as being corrosion resistant, but they are not appropriate for exterior applications.
When driving screws in hard materials, such as PT lumber or hardwoods, opt for a screw in the largest gauge available. For example, many outlets carry 3" deck screws in #8 gauge, but you'll experience fewer snapped screws if you spend a few extra bucks for #10 screws.
If you plan to drive deck screws near the ends of boards without predrilling, look for a fastener with an auger, serrated, or fluted point, such as the ones shown in top photo. In our tests, these screws were less likely to split the wood. Serrated and fluted screws proved noticeably easier to drive when using long screws in hard materials.
Square-drive and star-drive (also called Torx) recesses grip driver bits better than Phillips head or combination Phillips/square recesses do. (See bottom photo.) In our tests we found it difficult to strip out a square- or star-drive recess or do serious damage to their coatings. Phillips recesses strip out occasionally, and combination recesses, although handy, strip out excessively.
For a free downloadable screw chart, go to: woodmagazine.com/screwchart