DEAR TIM: Recently a deck collapsed near where I live and people got seriously injured. The report blamed the failure on the use of nails that had corroded and couldn’t hold the weight. Could this be right? Every deck I know of has been built with nails and they seem to be fine. What is the current thinking on using nails and more importantly, what would you do if you were building a new deck? Would you use nails or some other fastener, like a screw? Brad G., Lexington, KY
DEAR BRAD: Decks collapse all the time here in the USA. I can clearly remember a major deck collapse in Cincinnati, Ohio decades ago while a large group of people assembled on a deck to watch a giant fireworks display. My guess is there are quite a large number of decks that collapse that never make the news but are quite visible in data insurance companies maintain.
My thinking on this subject has changed over the years because I’ve been able to see what happens to decks over time. Years ago as a young carpenter / builder, I thought nails were fine. After all, we used nails to frame houses and houses have stood for hundreds of years being nailed together. Now I’m of the frame of mind that nails are an inferior fastener when building a deck that gets wet on a regular basis.
There’s a big difference between a house and a deck. Decks get wet and dry out. The framing lumber in houses, for the most part, stays dry for the entire useful life of the building. The wet-dry cycling that decks go through causes nails to lose their holding power.
Wood is a hygroscopic material. This means it changes size when it gets wet. When wood gets wet, it swells. When it dries out it shrinks. This back and forth movement causes internal stresses in the wood that cause cracks to develop. It’s not much different than what happens when you bend an aluminum pop top tab back and forth on a beverage can. Do it back and forth enough times and you crack the metal in two.
At first, the cracks are microscopic. After enough wet-dry cycles you may start to see tiny checking cracks. The next time it rains, this crack allows the water to get deeper into the wood. When this happens, there’s even more stress and the cracks begin to get wider and wider.
When you pound a nail into wood, you create stress around the nail and it can easily crack. You can see this happen in real time if you nail near the edge of a piece of lumber. It helps to blunt the end of a nail if you want to minimize splitting, but much of the stress that causes the crack is still there waiting to be released.
Once cracks start to open up around the shaft of a nail and water enters and causes the crack to widen, the holding power of the nail is significantly reduced. This past month I’ve been rebuilding a large deck on my own home and have come across nails in joist hangers that I’ve been able to pull out of the wood with my bare hands. That’s scary.
A side issue is the chemistry that’s at play with wood decks. Commonly available treated lumber contains copper. Nails are made from iron. When you introduce water to these two elements that are in intimate contact with one another, a chemical reaction begins. The iron sacrifices itself and starts to corrode.
This is why it’s imperative to use double hot-dipped galvanized fasteners so this corrosion does not happen. Just because a nail or fastener says galvanized on the box, don’t interpret this to mean that it’s hot-dipped galvanized. There are some galvanizing processes that apply an ultra-thin coating of zinc on the nails that can disappear in just a few years.
Then there’s the issue of end grain. If you pound a nail into the end of a piece of lumber, it has minimal holding power. If you could look at the end of a log with a microscope, it would resemble a giant bundle of tiny cocktail drinking straws. It doesn’t take much common sense to understand a nail driven into the end of a tube wouldn’t hold well at all, even a ring-shanked nail that has more surface area on the nail shaft to produce greater holding power.
Several years ago, I attended a full-day training seminar that concentrated on threaded fasteners for deck construction. I discovered that coarse-threaded screws have far greater holding power than nails. The screws hold well over time in wood that develops cracks caused by the wet-dry cycling.
You can purchase giant timber screws to connect pieces of lumber to make beams. Structural screws can be used to install joist hangers and other metal brackets that help hold two pieces of lumber together. If you use an affordable cordless impact driver, you can drive these fasteners as fast as you might hammer a nail by hand.
The screws will cost you more money when building a deck. But the small extra cost is worth it when a serious injury or death might occur if you decide to forgo their use. If you decide to use the screws, be sure they’re approved for use with the new treated lumber that’s now in the marketplace. If you use a fastener that has the wrong coating on it, it won’t take long for it to start to corrode and fail.