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What are Coach Screws?

Mary McMahon
Updated May 16, 2024
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Coach screws are heavy duty screws designed for metal to timber connections, or to join heavy timbers. Also known as lag bolts or lag screws, they have a square or hexagonal head engineered to be used with a wrench, spanner, or pliers. These screws come in a wide range of sizes, from the very small ones packed with furniture designed to be assembled by the consumer to extremely big ones used in large scale construction projects.

Coach screws consist of an externally threaded cylindrical shaft that tapers to a point, with a head at the other end. When they are driven into timber, the threads bite into the wood, while the head stops the screw from going all the way through as it provides compression. The essential form of the screw has been in existence for over 2,000 years, although metal screws for construction and woodworking have only been in use since it became possible to mass-manufacture them in the late 19th century. It was not until the Second World War that screw sizes became standardized, so that they could be manufactured and shared between Allied forces with ease.

These types of screws are often used at critical joins in construction because of their sturdy manufacture. Screws are often superior to nails for construction projects in general, because they can be easily removed and reused. Coach screws are preferred for joints that see heavy use and other areas in which additional support through the use of metal flanges is needed.

When working with coach screws, it is important to make sure to use the correct wrench or spanner so that the head of the screw is not damaged during installation. When checking the size of a wrench or spanner, the user should measure along both flat ends, not from point to point. When using these screws on wood to wood connections, or in other applications where the screw will pass through wood first, it is advisable to use a washer with the screw so that the head won't crush the wood and reduce the load handing capacity.

HomeQuestionsAnswered is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Mary McMahon
By Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a HomeQuestionsAnswered researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Discussion Comments

By anon990520 — On Apr 26, 2015

Yes, but you forget one important bit of information: how long should they be. A metal screw achieves its designed strength when is has 1.5 time the screw diameter. An M8 screw should have a threaded hole of that has a minimum of 12mm of thread.M12 would have 18mm thread required.

How much is needed for a coach screw ?

By anon239190 — On Jan 07, 2012

Coach screws that are supplied with the kit are for metal on wood. You need reach bolts to attach the blinds to the veranda.

By anon71043 — On Mar 17, 2010

I've been trying to attach some bistro blinds to my steel veranda, but cannot get the coach screws supplied in the kit to screw in.

I've drilled the right size hole as dictated in the instructions (9mm) but they will strip out before they get through the steel and pull in. I'm at a loss as to do next. It almost seems as though i will need either larger holes drilled or a higher quality coach screw.

The metal I'm screwing into is a colorbond steel stratco item, so its not ridiculously thick.

Any tips or pointers? Greg

By frankjoseph — On Jun 24, 2008

Anon2436 - Yes, coach screws can be good for garden swings. You'll often see coach screws used in backyard furniture, garden furniture, or outdoor furniture. It's because the screw is a sturdy type that can handle heavy use -- like a garden swing. Regardless of the screw type you use, you should be careful to make sure you build a safe-functioning swing.

By anon14289 — On Jun 13, 2008

Coach screws are wood screw one end and machine screw on the other end. They have no heads.

By anon2436 — On Jul 11, 2007

are they any good for making garden swings?

Mary McMahon

Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a...

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