Understanding Deck Lag and Bolt Strengths and Uses
Most typical wood frame building construction is put together with a handful of different types of hardware and fasteners. Generally a wide variety of elements are needed to put an entirety of a construction assembly together but the most common types of fasteners are relatively small nails and screws.
These typical nails and screws, measured by quantity, generally constitute the vast majority of fasteners used in common stick framing. However, when building exterior backyard decks, there are a few places where in the deck construction, larger bolts and lags are necessary to hold the main structural elements together. Today we’re going to take a closer look at some of those large bolts and fasteners that are typically used. Three different types of lags and bolts are shown in the photograph below. Even though they might all look very similar to each other, they have different characteristics and are actually called different names.
The use of large bolts and fasteners goes back hundreds of years and overtime, depending on the region and the use of the particular types of fasteners, different names and or monikers have been used. We’ll talk about some of these differences in the etymology of these names as well.
There are a variety of names used for some of the same things today and in general the field of the construction industry related to hardware and fasteners can be a bit confusing. We will explain some of these details today as a bit of a primer on understanding the basics of structural securement and fastening with lags and bolts. The term “lag* goes back to ship building from centuries ago.
The particular fastener shown in the picture below is referred to is a carriage bolt. Like most large bolts or lags used to connect wood elements, in most cases the wood should be pre-bored to allow this bolt to pass through into the wood and through to the other side. Not all lags are intended to pass from one side of a girder or post to the other side, but carriage bolts need a nut to secure the bolt at the opposite side of the assembly of boards. In the picture below, you’ll notice a few details about this type of fastener.
The head of the fastener is round which means that it cannot be torsioned or tightened with a wrench or ratchet. Instead, the square neck that sits right below the bolt head essentially locks the bolt into the wood and prevents it from spinning when the nut and washer are tightened down into place. In this particular photo, you might notice that the part of the shank shown in the image is smooth, it doesn’t happen to have threads. The lower part of the shank has threads, but this part is less smooth.
The smooth part of the shank is actually stronger from a shear and tensile resistance perspective. A threaded area can break a little bit easier than a smooth area that is slightly thicker. Threads are essentially cut from a smooth shank as the bolts are manufactured and the threads inadvertently create an area of scoring along the shank which doesn’t necessarily debilitate the fastener but does make it slightly weaker than the part without threads. Many decades ago, carriage bolts were also referred to as wagon bolts because, like “carriages” these bolts were commonly used in the construction of wagon building.
This next particular carriage bolt, in the photo bellew, is very similar to the one shown above, however in this particular case the threads go all the way to the neck of the bolt. The higher area of threading can allow a much tighter application so that this type of bolt can be used for a shallower depth fastening. Interestingly, threads of a bolt or screw, in some regions of the United States such as the Appalachian mountain range were historically also referred to as worms.
It might sound strange at first, but when you think about a certain types of mechanization and tooling, the name for the action of rotating around the shaft is still referred to by the same name in cases of a worm drive circular saw, for example.
A collection of these bolts is shown in the bin below.
The next type of bolt that we’re looking at is referred to as a hex screw. It doesn’t work exactly the same way as a bolt because it’s not intended to pass through to the other side of the wood framing members. Instead, the sharper point is intended to stay inside of the wood framing.
Hex screws like this can self tap, in other words they don’t necessarily have to have a hole drilled into the wood for them to enter, but these particular bolts are relatively large and most bolts over about 3/16 of an inch, depending on the type of wood, should be pre-drilled to prevent the wood from splitting.
Not only the type of wood, but also where in the board the screw is placed, can affect whether or not the board will split. Other factors such as moisture content of the board can also affect whether or not the board will split from a screw installation. In most cases with screws greater than 3/16 of an inch, it’s better to pre-drill.
The next picture shows typical bolts that are different from carriage bolts because they do not have a square neck and they’re also different from lag screws because unlike lag screws, these bolts will not self tap. They must be inserted into a pre-drilled assembly of boards and must be secured by tightening both the nut and the bolt head at the same time in opposite directions.
We recommend building and homeowners here in Washington DC use a contractor, like us, who understands and cares about quality, deck and patio longevity, snd simply doing things right. Always, feel free to reach out to us here at Dupont Decks and Patios, you can give us a can or just drop us a line. We are happy to help with almost all steps of the deck and patio building and design process. Let us know about your ideas and talk to us if you have questions about possibilities. You can call us at (202) 774-9128. You can find us online at https://dupontdeckspatiosdc.com and you can email us there as well at https://dupontdeckspatiosdc.com
