Wood Screw Sizes and Wood Screw Types
Wood screws enjoy a long and noble history, as one of the easiest and most secure ways of adding strength, precision, and rigidity to woodworking joints. Large woodworking screws carry a huge load-bearing capability, while tiny wood screws are perfect for the important embellishments that add character and elegance to carpentry projects.
Due to the sheer number of range of sizes and styles available, finding the perfect screw for your woodworking project can be a monstrous chore, especially when every manufacturer seems to use different standards. Diameter, length, head type, and thread density are all crucial, but you also have to factor in the various types of material, from decorative brass to hardened steel. In this wood screws guide, we will show you how to navigate the multitude of choices.
Obviously, price is a major issue, especially for large projects such as decking, where a small difference in price can add up to a huge final cost. However, bear in mind that cheap does not always equal cost-effective; cheaper screws are often fabricated from inferior alloys, which can easily snap, bend, or break.
Wood Screw Types
There are four main types of woodscrew head, each designed to suit different tasks
- Flat Head/Countersink Screw: These are the most common type for woodworking, consisting of a flat head with a countersink designed to sit flush with the surface of the wood, or they can be buried and disguised. Flat head screws require a little more work, as a countersink has to be drilled after the pilot hole, which can be time-consuming for larger projects.
- Round Head Screws: These have a flat underside and rounded head, so they do not require a countersink hole and sit proud of the surface. Roundhead screws are useful for fixing thin substrates, such as metal and plastic, and many woodworkers use them as decorative features.
- Pan-head Screws: These also sit on the surface of the wood, and have a flat, wide head for gripping thin substrates. They are rarely used in woodworking, but can be useful for affixing and gripping thin hardboard.
- Oval Head Screws: Oval headed screws contain both a countersink and a rounded head, making them perfect for use with washers and decorative features, such as door furniture, where brass and chrome screws are particularly common.
Wood Screw Head Types
In the US, the two most common types of screw head are the Phillips and slotted screws, old stalwarts that are readily available, yet carry some disadvantages. In Canada, the Robertson screw is far more popular, and Europeans have adopted the Pozidriv screw as a good alternative to the Phillips. Matching the rise in power tools capable of handling high torque without damaging screw or driver, Torx heads have rapidly grown in popularity.
Slotted head screws are the most familiar type, with a simple slot designed to fit a flat screwdriver. The rise of power drivers has seen this type become much rarer, but they are still the favored option for many carpenters and cabinet-makers.
- Naturally, screws are often painted over or varnished, and this can fill the recess, preventing removal. Slotted screws are simple to clean with a knife, making them a popular option for interior door hinges.
- Slotted screws can handle a reasonable amount of torque, making them a decent option for larger, heavier screws
- Slot heads are easy to source, and the simple slot means that it is easy to use an improvised screwdriver in an emergency.
- Slotted heads do not always fit securely into the recess, which means that powered drivers can cam out, risking damage to screw and driver head alike
- These screws were never designed with power tools in mind, and the driver head can easily slip out of the slot at high speeds, potentially scratching the surface of the wood
- Slotted screws do not self-center, so it is easy to accidentally drive the screw in at an angle
- The slot only has two angles of engagement, making it difficult to apply force in restricted spaces
Phillips Head Screws
Phillips screws were originally designed for use with power tools, and are designed to cause cam-out under high torque, preventing over tightening.
- Because the head is cruciform, there are four positions of engagement, a useful trait where space is restricted
- Phillips screws are self-centering, so it is fairly safe to drive them at high speeds without the screw entering the wood at an angle
- Phillips heads are designed to promote cam-out at high speeds, which is a less desirable characteristic in the days of slipping clutches. When driving screws into hardwood, camming out leads to stripped heads and damage to the driver head.
Torx screws, incorporating a six-pointed star recess in the head of the screw, are rapidly growing in popularity. These screws, often called ‘star drive screws,’ are designed for use with power tools, and they have become the screw of choice for many professional contractors laying decking or fitting roofs.
- Torx screws grip the driver snugly and rarely cause cam out, so both screw and driver last for much longer
- They are designed for high speeds, so many self-tapping screws are of the star drive variety
- There are six potential positions of engagement, making them a great choice for tight spots
- Torx screws and drivers can be difficult to find, although the growth of the online marketplace is rapidly negating this particular disadvantage
Robertson Screws/Square Drive Screws
Square drive screws, also called Robertson screws, consist of a square recess in the head of the screw, and they are particularly popular in Canada. Due to issues with the patent and licensing restrictions, they were unavailable in most parts of the world, so they were largely superseded by Phillips heads and hex heads. Robertson screws differ slightly from square drive screws in that the bottom of the recess is tapered rather than flat.
- Square drive screws are designed to handle high torques without camming out or stripping
- The snug fit and self-centering properties mean that Robertson screws can be tightened one-handed
- Robertson screws can be tightened with socket wrenches, a great benefit in restricted spaces.
- The screwdriver can be engaged in four positions, handy when access is difficult.
Combination Head Screws
Combination screws, a hybrid of two other types, are another useful addition to the toolbox, ensuring that you always have the right screwdriver to hand. One popular type is a combination between the Phillips screw and the slotted head, although a hybrid between the Phillips head and the Robertson screw is also common
The disadvantage of these hybrid types is that they don’t grip the driver as snugly, and cannot handle high torque without camming out. As a result, they tend to be reserved for smaller screws, where the forces involved are much less.
Wood Screw Gauges
The diameter of wood screws is usually given as the gauge, a measurement of the thread diameter, with larger gauges meaning thicker screws. An eight-gauge screw is four times as thick as a two-gauge, and sizes larger than 12 are often given as fractions of an inch or millimeters.
Wood Screw Lengths
The next important measurement is the length of the screw, given in inches or millimeters, and you should seek a screw that drives as far into the wood as possible, for maximum grip and load bearing. Harder woods grip the screw much more securely, so you can get away with slightly shorter screws.
Wood Screw Materials
Wood screws are available in a range of types, from durable, hardened steel to elegant brass. Each alloy carries a unique set of pros and cons, helping you to find the perfect type for your project.
Mild Steel Wood Screws
- For general, interior use, mild steel screws are great value, combining strength and availability.
- Mild steel screws are prone to rusting, so they are unsuitable for use in wet areas, such as bathrooms or external decking.
- Mild steel tends to be a little too soft for self-tapping screws, especially if you intend to use hardwood.
- For general interior use, mild steel screws are perfect, cost effective and fairly strong.
- Mild steel screws are not corrosion resistant, so they are unsuitable for exterior use or in areas, such as bathrooms, with humid atmospheres.
Hardened Steel Wood Screws
- Hardened steel screws are a little more expensive, but they are much stronger than their mild steel counterparts, so they are less likely to bend, break, or strip.
- The high carbon content makes them even more prone to rusting, so you should avoid using them outside or in damp bathrooms or kitchens.
- The tough, rugged nature of these screws means that they are perfect for self-tapping screws, able to bore their way into the hardest of woods without complaint.
Brass Wood Screws
- Brass wood screws carry a timeless appeal, making them the perfect choice where aesthetics are a concern.
- Brass is a very soft alloy, with poor load-bearing capacity, so brass screws tend to be restricted to smaller sizes.
Stainless Steel Wood Screws
- Stainless steel screws are more expensive than plain steel, but their resistance to rusting makes them invaluable for outdoor constructions.
- For large projects such as decking, the cost may be prohibitive, and most contractors substitute galvanized or coated screws.
Galvanized Wood Screws
- Galvanized screws are cost-effective option for outdoor work, where rusting is a major concern.
- Hot-dipped screws are cheap and perfectly adequate for most tasks, although the rough coating can deform the head recess and make them difficult to drive into hardwoods.
- Mechanically coated woodscrews are another option, with a finer quality of finish, but they are not as robust or durable and tend to corrode much more quickly.
Anybody with woodworking experience is fully aware of the extra work involved in drilling pilot holes, especially if you had to add a countersink for flat head screws. Taking advantage of harder alloys, manufacturers now offer self-tapping screws, which happily bore their own way into the wood, and they are a welcome addition to the woodworker’s box of tricks.
- Self-tappers are popular with roofers, shuttering carpenters, and drywall installers, due to the significant time savings.
- Self-tapping woodscrews incorporate a steep thread that extends for almost the entire length of the screw, as opposed to the two-thirds found with traditional woodscrews.
- To aid driving into wood, the shank of the screw is thinner than the thread, allowing the screw to bite firmly and provide strength.
- Self-tapping screws should be avoided where strength is the main priority, and they are too rough for fine carpentry.
Buy Wood Screws Online
In the good old days, it was easy to visit your local ironmonger and buy woodscrews by the scoop. In the age of ripping off consumers, many high-street retailers sell woodscrews in small packets, at a very high price. Fortunately, online retailers have filled the breach and are happy to send screws by the box load, reducing the cost of larger projects. With this wood screws guide, you know that you will have found the best wood screws for the job.
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