When we think of woodworking, we think of either nails or wood screws to connect the pieces. However, in many instances, through-bolts can provide greater strength and stability.
A through-bolt is a bolt that goes all the way through (whatever it is that is being connected) and is held in place with a nut at the other side. A “sandwich” connection.
There is quite a bit to think about when both choosing and using fasteners on a woodworking project.
Dealing with Shrinkage
One of the big problems with fastening any type of wood is the shrinkage that occurs to the wood. A tightly fastened screw can become loose and wobbly in no time as the wood dries out and shrinks or deforms. Retightening a screw or lag bolt works most of the time, but securing a connection with a through-bolt always works.
For example: When attaching handrail posts or important safety supports to a newly constructed deck — or any structure for that matter — the overzealous do-it-yourselfer often underestimates the degree of wood shrinkage that can occur and chooses lag bolts to make the connection. Big mistake. When shrinkage occurs, the bolt hole can strip out, often leading to a need for a large lag bolt or screw.
When shrinkage is a possibility — which is almost all the time when the wood is green — then it’s a good practice to use a through-bolt. With a bolt-head and washer on one side and a nut and washer on the other side, tightening things up after a bit of shrinkage has occurred is light work. Granted, use of a through-bolt is not always possible. Sometimes you can’t get to the other side to put a nut on, but that, in our estimation, is the only time that a through-bolt should not be strongly considered.
There is another consideration — bolt grade. Nuts and bolt vary widely in strength from very soft to extremely high tensile strength.
It used to be that one could ask for an A-grade bolt and get something that a gorilla couldn’t break. Today there are more grades and specifications than you can shake a stick at.
If you’re building a guard rail or some other important safety device, be sure to investigate this before laying down cold hard cash on lower grade connectors. All you need to do is ask the clerk for the higher grade fasteners. There is usually a section devoted to better-quality connectors in most hardware stores and home centers.
Unfortunately, most consumers aren’t aware that a difference exists and thus don’t know to ask. If you choose high tensile strength you should be prepared for a limited selection. High tensile strength nuts and bolts are expensive and not normally stocked in the variety of sizes offered with softer, less expensive, more widely used connectors.
We often have the tendency to use bolts that are too small in diameter for the task at hand. Remember, with increased diameter, there is increased holding power and vice versa for smaller diameters. For example: Most engineers will tell you that two half-inch through-bolts are necessary to develop the proper amount of strength necessary for a rail post to qualify under the building code as handrail support. Yet we often see home-built rail systems that use three-eighths inch or even quarter-inch bolts.
The smaller diameter seems to hold well and looks good at first, but if someone falls against a rail with these smaller-sized fasteners holding the post to the structure, the rail will probably go over at the same time. Not what a guard or handrail is supposed to do.
Another important consideration when fastening with a bolt (regardless of whether it is a through-bolt or lag bolt) is the use of washers. A washer adds to the bolts area of holding power. The bigger the washer the better the holding power.
If we were going to build a wood deck rail at our house today we would not only use through-bolts but every one would have a large washer at both ends. In fact, when we build with the soft woods that are predominantly used for decks, we use “fender” washers. Fender washers are have the same size hole in the middle, but are greater in diameter at the outside. A fender washer costs a little more, but holds a lot better.
The other thing we like to use is a lock washer. There are a couple of different kinds here. We think any lock washer is better than none at all.
Even better than a lock washer is a lock nut. They hold better, can be tightened with no problem and don’t twist off like some lock washers. Locknuts are the ones with the plastic in the center that grabs the bolt threads as it is tightened. All you have to remember to ask for is a “lock nut.”
5 Tips When Purchasing a Fastener
So, when purchasing a fastener:
- Use a through-bolt whenever possible.
- Use high tensile strength nut and bolts.
- Always use washers on wood projects (fender washers when possible).
- Always use either a lock washer or a lock nut. We favor the lock nut; your choice.
- Remember that part of the project construction is to go back after several months and tighten loose connections that result as the wood dries out and shrinks.
And, that’s all there is to it.