Guide written by:
Jeremy, Construction site supervisor, Cardiff
Screws can be classified according to several technical characteristics. These are:
Type of thread;
Head cutout and material.
To help us better understand screw distinctions, let's take type HM10-60 as an example:
H = hexagonal head;
M = pitch is "ISO metric" type (i.e. mechanical);
10 = screw is 10mm in diameter;
60 = screw is 60mm below the head.
There are many types of screws and bolts, each with a precise application (unlike screws, of course, bolts require nuts). To determine what type we want to use, we must consider a variety of factors:
The substrate material into which you are screwing;
Substrate thickness and robustness;
Size and weight of the load to be borne;
Risks of tearing and shearing - primarily mechanical applications.
In general, bear in mind that the larger the diameter, the greater the screw's resistance to tearing and shearing. The same goes for its length - with equal diameter, a longer screw will withstand greater force.
Let's go through the most commonly used types of screws.
Wood and agglomerate screws: these can be screwed without making a hole in advance. Very sharp and threaded over their entire length, they avoid splitting the wood when screwing. They have a countersunk Pozidriv or Torx cutout head and are bichromated (yellowish in appearance), stainless steel or galvanized (silver).
Woodscrews: only threaded over part of their length, with a split domed head ("flathead" type) and made of zinc-plated or stainless steel, wood screws generally come in small diameters and lengths.
Lag bolts: also partially threaded, lag bolts are very resistant to tearing and have a hexagonal head. They are made of galvanized or stainless steel.
Plasterboard screws:easily recognizable by their sharpness and black colour. Threaded over their entire length and very sharp, they let you penetrate plasterboard through to the metal framework. These screws have a countersunk head and a cruciform cutout ("Phillips" type).
Metal or mechanical screws: this is the type of screw with the most diverse range of shapes, lengths, diameters, head types and pitch types. Generally, screw characteristics are determined by the manufacturer for the particular machine or assembly they're intended for. Screw materials are very important in the context of mechanical applications because they determine resistance to shearing, tearing, adverse forces and extreme temperatures. If you have to replace any number of mechanical screws, always make sure to obtain an identical type to the original - look for an alphanumerical code on the head (as per HM1060, above).
Sheet metal screws: similar to wood / agglomerate screws but made of a different (much stronger) material and with a sheet-specific tip. It's strongly advised that you make a hole in advance, or at least a pronounced mark with tip of the screw, for best results.
Self-tapping screws: For thin metal sheet, soft metals (e.g. aluminium) and plastic. These screws offer the advantage of being able to pierce the material when turned. The threads are identical to wood / agglomerate screws, except that the tip is shaped like a drill bit. They may or may not be countersunk, having a convex or hexagonal head. Non-hexagonal heads will have Phillips, Pozidriv or hexagonal cutout.
Framework / PVC screws: these screws have the advantage of being screwed straight into the substrate (solid or hollow) without needing a rawlplug. Indeed, once you start them off they will "self-tap" into the substrate, thanks to their fine thread and highly resistant material. This makes them very suitable for screwing into plastic. Generally, they have a milled Torx cutout head.
Concrete screws: thanks to a very wide thread (and in some cases mixed), this type also don't require rawlplugs but do require a pre-drilled hole. Highlyresistant, they are mainly used to fix pipe or cable housing. Their head shape varies widely, depending on their destination. Their thread doesn't go the entire length of the screw, so as to accommodate whatever equipment you're screwing to the wall or ceiling.
Hinge screws: in case you don't follow, the "hinges" we speak of arethe pieces of ironwork fixed onto exterior doors, gates and shutters - attached to the hinge proper. Several different types of screw exist for this purpose. Why? Because the materials involved differ widely (wood, aluminium, pvc), as can the colour of the ironwork. Aesthetic factors can be taken into account when choosing head shape. You'll find milled heads, large round heads and domed heads. Hinge screw threads are wide-pitch and the tips depend on substrate material. The most common varieties are RHSQ, or "round head square corners", intended for ironwork in joinery.
Specialized decking screws: these are a type of wood screws specific to screwing down decking. They are made of stainless steel due to their exterior application. The thread has braking fins just under the screwhead, facilitating screwing yet also locking the screw once in place. The head is countersunk and the Torx cutout allows the screw to be tightened to a greater degree. Some specialized screws also have a double thread for improved decking stability.
Connecting screws: frequently used in carpentry applications, such as on doors or furniture, connecting screws are made up of two distinct parts. One is a screw, the other a hollow threaded rod that accommodates the screw. They can be found on door handles, locks, etc. Their length depends on the thickness of the wood involved. Often made of brass for aesthetic reasons, they are also found in nickel-plated steel or painted iron. Their head is often round and domed and has a flathead cutout.
Multi-purpose screws: as the name suggests, these screws have a variety of uses! Often aimed at assembling wooden structures, they have a wide thread pitch and a flat or domed head. They are not threaded over the entire length. Their cutout may be flat, Phillips or Pozidriv.
You won't really need to choose the material of your screws, in the sense that they are purchased for a particular use and the manufacturers have therefore chosen an appropriate steel or alloy and treatment to give them the best properties. Thus wood / agglomerate types are mainly phosphated, wood screws zinc-plated, decking screws stainless steel etc. You can rule out the cheapest screws as they're likely to be brittle and poorly treated.
Bichromated: fairly resistant to corrosion (provided screws are high quality). The cheapest tend to lose their surface treatment, and hence corrode, and also break when overtightened. They are yellowish in colour, and when chipped, they lose their protective treatment.
Zinc-plated: same deal as above - it's all in the quality of the treatment and the pressures the screw is exposed to once in use. Corrosion resistance average, colour silver.
Phosphated: black in colour. These screws are fairly strong and have good corrosion resistance.
Chrome-plated: same appearance as zinc-plated and similar robustness.
In terms of base material, screws can be made of:
Brass: for interior use, mostly in cabinetry. Brass has a poor corrosion resistance and suffers from verdigris. These screws are untreated and therefore unsuitable for many uses;
Stainless steel: for outdoor use. Stainless steel is durable and by definition stainless (i.e. doesn't corrode). There are however two different quality standards, A2 (cheaper and more common) and A4(alloyed with molybdenum, providing protection against acids for increased resistance).
Variable proportion alloys: these materials benefit from surface treatments (by hydrolysis) specific to the use of the screw. Caution: screws with the same intended use can be made of a more or less dense alloy.
Think carefully about the alloy quality of a screw if it isn't made of stainless steel. Quality is crucial, since breaking a screw while tightening wastes time and can damage your substrate if you end up having to drill to pull it out.
Screw treatment is also important - since when go to tighten it, a screw can flake (low-end phosphate wood screws are a good example), speeding the process of corrosion.
Screws and bolts for specific uses and replacement mechanical screws must be identical in size, strength and pitch (refer to alphanumeric codes on the screwhead).
If you want to tighten a screw with force, be very careful not to damage the head and where possible choose Torx, Pozidriv, Alen or hexagonal cutout screws.
And one last thing - try not to lose your screws!
To find out more about screws and related topics, follow our editors' advice and check out their other guides:
How to choose your electric screwdriver?How to choose your plasterboard screwdriver?How to choose your garage door?How to choose your gate operator?How to choose your shutter driver?How to choose your outdoor ironwork?How to choose your carpentry and cabinetmaking tools?How to choose your lock?How to choose your clamping tools?
Guide written by:
Jeremy, Construction site supervisor, Cardiff
Electrician by trade, I first worked in industrial estates where I installed, wired and fixed a large number of electrical installations. After this, I managed a team of electricians for this type of work. 10 years or so ago, I turned to building and construction. From the modest family home, to gyms and theatres, I have been able to coordinate, audit and organise all sorts of construction sites. For 4 years now, I am restoring and building an extension to a bungalow in the heart of the Welsh countryside. My experience in manual work and my knowledge means I am proud to be of service. Terraces, interior design, roofing, plumbing, electrics, anything goes! My wife, daughter and I, built almost everything we have from scratch! So to answer all of your questions and advise you on choosing your tools? Easy!