Q&A: Air Nail Holding Power

A.Scott Schiff responds: The coatings on power-driven nails are plastic polymers known as "thermoplastics." Their main purpose is lubrication: When the nail is driven, they melt from the heat of friction, making the nail slippery so it’s easier to drive. This allows the nail-gun to be built smaller and lighter.

When the coating cools, the plastic rehardens and creates a bond between the nail and the wood, so that the nail is harder to pull out.

We’ve confirmed the added withdrawal resistance of coated gun-driven nails in laboratory testing here at Clemson, using machines to measure the pull-out resistance of individual nails. We’ve also tested assemblies by nailing a whole panel to framing and applying a suction pressure to the panel. In both kinds of tests, coated gun nails hold better than hand-driven nails of the same size.

If it seems easier to carpenters in the field to pull out a gun-driven nail than a hand nail, maybe that’s because of a difference in the way the different types of nails let go. With a coated gun nail, once the bond between the coating and the wood breaks, it’s all over — after that, you just have a loose nail in a slippery hole. When a hand-driven common nail is pulled partway out, the part that is still in the wood is holding as tightly as it was before, so you need to keep pulling hard to get it completely out. But in terms of the strength of a building, I think the fact that it takes more force initially to get the coated gun nail loose than it does to pull out the hand-driven nail is more important than the suddenness of the way the coated nail finally gives up.

Keep in mind here, we’re comparing nails of the same size. These days, not all the "eight-penny nails" on the market are the same length and thickness. You can buy gun-driven nails that are as long and as thick as the hand-driven common nails they’re meant to replace (in areas covered by seismic codes, you may have to). But you could also just go by the withdrawal strength of the nails, rather than by their size. Most manufacturers have received National Evaluation Reports (NERs) from the Council of American Building Officials (CABO) that specify how their gun-driven nails or staples can be used in place of the common nails the code is based on. If you need the data that’s in the NER, ask the manufacturer for a copy, or contact the International Staple, Nail and Tool Association (ISANTA) at 312/644-0828.

If you want extra holding power, go with a ring-shank or screw-shank nail. Tests show that modified-shank nails hold much better than smooth-shank nails — and if they’re coated too, that’s just icing on the cake.

In practice, major failures of nailed assemblies, such as the building collapses that happened during Hurricane Andrew, can usually be traced to the way the fasteners were used, not the type of fastener. Make sure you hit the framing (sometimes it’s hard to tell). Put the nails close enough together — it’s better to use too many than too few. And set the pressure on your gun to drive the nails flush with the surface, not below it. Overdriving a nail or staple makes for a weaker connection.

Scott Schiff directs building research in the Department of Civil Engineering at Clemson University, Clemson, S.C.