Understanding Wood Screw Thread Sizes: A Comprehensive Guide

Wood screws are used to secure two solid pieces of wood together without the need for cut joinery. When using wood screws, it is important to consult a wood screw size chart to ensure that you are using the right size for your project. The size of a wood screw is determined by two major factors: shaft length and thickness/diameter. The length and diameter, measured in inches, belong to a gauge group from 0 to 24. The diameter of a screw is also known as the root diameter (RD) and measures the smallest diameter of the screw thread. The shank diameter measures the threadless part of the bolt between the thread and the head.

Screw length requirements

While there are no strict rules regarding screw length requirements, there are some general guidelines to follow. The screw length will sometimes be dictated by the construction details, but as a rule of thumb, the screw should go through the thinner piece and thread into the thicker piece.

It is recommended to have about 2/3 of the screw threaded into the "secondary" piece. However, as the thickness of the lumber increases, this ratio can be adjusted so that only 1/2 of the screw is threaded into the thicker piece. This ratio is also reflected in the design of 1-inch and longer screws, which are manufactured with 2/3 of the shank threaded.

The screw length should be sufficient to hold the two boards securely together. If the screws are too short, they may not hold the boards adequately, and if they are too long, they will pierce through to the other side. The screw length should be such that it enters at least half the thickness of the bottom material; for example, 3/4" into a 2 x 4. Ideally, the screw should penetrate the wood at a distance of about half to two-thirds of its threaded length.

When considering screw length, it is also important to take into account the forces that will be applied to the boards, such as lateral or withdrawal loads. For lateral loads, where there is a sliding motion between two boards attached flat, a thicker screw may be preferable. In the case of withdrawal loads, where the force tries to pull the boards apart, ensure that ample screw threads are inserted into the receiving board, typically aiming for 1 to 1 1/2 inches of thread.

Screw head types

The most common drive types for wood screws are Phillips and slotted head styles, but there are many other head types available. These include hex head, square head, Torx/star head, and Phillips square head screws.

Flat Head

Flat-head screws have a flat top and a conically-shaped underside. This allows the screw to sit flush with the finished workpiece, creating a smooth surface. Flat-head screws are the most commonly used type of wood screw.

Oval Head

Oval-head screws have the same conically-shaped underside as flat-head screws but have a decorative top that protrudes slightly above the surface.

Round Head

Round-head screws are flat on the underside of the head and have a domed top that sits above the surface of the workpiece.

Pan Head

Pan-head screws have a rounded top and a flat bottom. These types of screws either have their heads protruding out of the surface of the material or sit inside a counterbore.

Screw thread types

There are two main styles of screw threads: straight threads and tapered threads. Straight threads, also called parallel threads, have a profile that remains the same diameter throughout the part. Tapered threads, on the other hand, become smaller in diameter as the part moves down.

Some of the main types of straight threads include BSPP, UN/UNF, and metric parallel. Metric threads, denoted by the letter "M", are used as general-purpose screw threads worldwide and were the first international standards. British Standard Whitworth (BSW) threads, an imperial unit-based standard, were the world's first national pipe tapered thread standard. They are used for special purposes such as line adjustment and can be found on bolts.

The Unified Thread Standard (UTS) is the main standard for nuts and bolts and is used in Canada, the United States, and Great Britain. It consists of three classes, each with different applications: the first is for assemblies requiring liberal tolerance, the second is for general applications, and the third is for assemblies where accuracy and closeness of fit are essential.

"V"-shaped threads are widely used and can be cut with a lathe machine, tap, or die. Square threads are highly efficient and are used to transmit power in high-load applications, but they are not easy to fabricate. Acme threads are stronger than square threads and are commonly used for brass valves, bench vices, and screw-cutting lathes.

Wood screws, unlike metal-working screws, are only partially threaded. They have coarser threads with smooth shanks below the head. The partially threaded design prevents the screws from turning when the thread runs out and protects the wood from splitting or cracking if the screws are driven in too far.

Screw thickness/diameter

The thickness or diameter of a wood screw is one of the most important factors to consider when selecting the right screw for your project. The thickness of a wood screw is measured in inches and falls into a gauge group from 0 to 24.

To determine the thickness of a wood screw, you can manually measure the width of its thread. If you don't have a measuring tool, you can estimate the thickness by referring to a wood screw size chart. These charts categorise wood screws into different groups based on their nearest fractional size. For example, if your screw measures 5/64 of an inch, it falls under the #1 screw category.

The thickness of the screw is crucial in ensuring a secure and firm hold between two pieces of wood. If the screw is too thin, it may not properly hold the boards together, and there is a risk of the screw pulling out. Conversely, if the screw is too thick, it can split or damage the wood.

Additionally, the shank diameter and root diameter are also important measurements to consider. The shank diameter refers to the measurement of the bare shank, which is the threadless part of the bolt between the thread and the head. The root diameter, also known as the minor diameter, measures the smallest diameter of the screw thread. These measurements ensure that the screw fits perfectly into the designated space and provide guidance on the appropriate size of the drill bit required for the pilot hole.

By consulting wood screw size charts and considering the thickness and diameter measurements, you can make an informed decision about the right wood screw for your specific application.

Screw length and thickness

The length and thickness of a wood screw are critical factors in ensuring a strong joint between two pieces of wood. The right screw size will provide sufficient holding power, while an incorrect size can lead to accidents or damage to the wood.

The length and thickness of the screw will depend on the specific application and the thickness of the wood being joined. Generally, the screw should be long and thick enough to hold the two pieces of wood securely together.

The screw length is determined by the thickness of the wood and the depth to which the screw needs to penetrate. As a rule of thumb, the screw should penetrate the wood to about half to two-thirds of its threaded length. This ensures the screw has a strong hold without risking splitting or cracking the wood. The screw should pass through the thinner piece of wood and thread into the thicker "secondary" piece. For thicker lumber, the screw length can be reduced so that only half of the screw is threaded into the secondary piece.

The thickness or diameter of the screw is also important. The screw should be thick enough to hold the boards together securely without being too thick, which could cause the wood to split. The diameter of the screw is measured in inches and falls into a gauge group from 0 to 24. The minor diameter, or root diameter, measures the smallest diameter of the screw thread, while the shank diameter measures the threadless portion of the screw between the thread and the head.

Frequently asked questions