There is often confusion as to what defines a coach bolt, and how it is different to a coach screw. This is especially common because even professionals even refer to coach screws as coach bolts. Not only this, but there are people out there writing about the subject online that treat coach and carriage bolts as separate entities. So we're here to clear up the confusion, then you can go ahead and call them whatever you like!
What are Coach Bolts & Are They Different to Coach Screws?
Coach bolts are also known as cup square carriage bolts. They have been used since the 19th century are regularly used in woodworking to fasten metal to wood, or to connect timber joists or decking.
They're also common in modern buildings because their design offers some advantages over other types of bolts. Their bigger dome heads make reaching tight spots along edges easy too since there won't need any additional force required beyond just turning clockwise. As long as you can get tighten the fastener, they are easy to install.
They are double-component fixings that feature an enlarged dome head so that you can pull through a hole and fasten with a nut. This makes them great at ‘clamping’ timber together.
One benefit of bolts is that they can always be tightened. This makes them a preferred choice for projects that need to stand the test of time. Our most popular size for most medium duty outdoor projects are stainless steel M10 coach bolts.
Coach bolts derived from coach nails that were very popular in 17th century England. They got their name because they were strong enough to hold together coaches and other heavy items - thus enabling transportation over rough terrain without fear of damage.
In the early 20th century coach bolts were employed for a number of purposes including being used to build aircraft, hold components together in cars and bicycles. Traditionally, M12 coach bolts in these applications were commonplace, but their use has decreased slightly due to various other fasteners being available. They are still widely used today because coach bolts offer a strong fixing that is easy to remove and relatively cheap to produce. However coach bolts require a hole to be drilled into the material before installation which means it doesn't work well on soft materials or where objects need to be slotted together quickly.
What is a Coach Screw?
Coach screws are another heavy duty fixing that is often confused with a coach bolt.
Sometimes referred to as lag screws, or more confusingly, lag bolts, they are mostly used to connect timber to timber, but can also fix timber to metal, concrete or masonry in some instances.
The coach screw is a single-component unlike the carriage bolt, meaning it does not to be fastened with a nut.
They often have a hexagonal head unlike its bolt brother that commonly has a dome-shaped head.
The hex head, combined with the pointed tip, makes this fixing better at tapping its own holes.
It’s important to note that coach screws aren’t meant to be completely self-tapping and require you to create a pilot hole before putting them in place to prevent the wood from splitting. This should be about 2mm smaller than the diameter of the screw.
When using coach screws some people like to use washers to prevent the head from embedding itself into the wood during installation. This also makes it much easier to remove the screw later.
Coach screws are manufactured to conform to DIN 571 standard.
In larger sizes, coach screws have a solid shank before their threading. This extra steel reinforces the screw and makes it stronger against resistance when timber may move over time.
In smaller lengths of approximately 50mm and less, shanks are less common, with the thread length of a coach screw always being at least 60% of its total length.
What are Coach Bolts Used For?
They are often used for joining timber to timber or wood to metal in tough conditions, generally outside. Larger coach bolts up to 12mm diameter are used for heavier duty projects.
Coach bolts come in multiple material compositions according to their purpose.
The most common carriage bolt compositions are:
- Bright Zinc Plated (BZP)
- A2 stainless steel
- A4 stainless steel (marine grade)
While these are all manufactured for heavy duty, they have slightly differing qualities that alter their uses.
Zinc plated steel has a higher tensile strength than stainless steel so you might expect this variety to be more sought-after. However, stainless steel versions of this bolt have unique properties to eliminate much disparity in demand. The largest size coach bolt available is M16, but these are only available in BZP due to this size bolt only being needed for the most extreme cases where maximum load-bearing qualities are required.
Namely, A2 stainless steel can be used where wood gets damp due to its chemical properties that prevents it from rusting quickly. In construction works such as garden decking and other timber projects where there is not extreme stress being put on the bolts, their anti-rusting properties could make them an ideal choice.
A4 stainless steel coach bolts are the least common, but their marine grade anti-rusting properties make them suitable for use on sea boats. Generally, A2 stainless provides plenty of corrosion resistance for most professional uses.
Another benefit of having stainless steel carriage bolts is that they have a greater degree of play in them, allowing them to bend slightly to adapt to conditions that might change slightly over time, such as structures sinking into the ground varying degrees. Th.is is useful for garden projects where the ground can get wet over time and cause heavy structures to move slightly
The stronger bright zinc plated variety might sound attractive and seen as a 'safe bet' in professional projects, but the lack of play in them has the potential to cause them to suddenly break. This is another reason why stainless steel are a popular choice for outdoor projects.
What are Coach Screws Used For?
Coach screws or lag screws are used in many DIY jobs where the need for heavy resistance is less. Coach bolts need to be used with a nut, so it can be awkward to tighten them. Therefore, screws can be a preferred choice.
In terms of material composition available, coach screws are the same as carriage bolts. Fusion Fixings has BZP and A2 stainless available in an array of sizes.
Are Coach Bolts Stronger Than Screws?
Coach bolts provide greater holding strength than screws. Because washers help make bolts even stronger as they help to spread the load over a greater surface area.
With bolts being a double component fixing, meaning they’re used with a nut, the bolt essentially has two ‘heads’. Both ends of the bolt provide a higher level of resistance than just the thread itself.
What is the Difference Between Coach Bolts and Coach Screws?
The main difference is the fact that the screw is a single component fixing. This means it uses its own thread to hold together pieces of timber without the need for a separate fastening. This contrasts with the bolt, which requires a nut to connect material together.
The bolt is generally better at withstanding resistance due to the surface area of the nut being used is bigger than the hole that the bolt is in, whereas the screw is held in with its thread alone.
Coach screws make a great choice where there is less demand on the hold because their self-tapping thread can easily be drilled into a pilot hole in wood for a more than satisfactory hold in most instances. This is why they are found more in a DIY workshop than on construction sites.