5.0 x 100mm Wood Screws: Size, Uses & How Many

A 5.0 x 100mm wood screw is a general-purpose structural timber screw: 5.0mm shank diameter, 100mm long. The "5 x 100" is simply diameter by length. Use it to fix through timber up to about 50mm thick into a second piece behind, so it passes through the front timber and still bites deep into the base. As a rule, you want at least as much screw in the base timber as the thickness of the piece you are fixing through. A trade tub of 1000 works out around 5p a screw.

What does "5.0 x 100mm" actually mean?

Two numbers, and they are always the same order: diameter first, length second. So 5.0 x 100mm is a screw with a 5.0mm shank (roughly a number 10 gauge) that is 100mm long from under the head to the tip. Nothing else is hidden in the size, everything else (head type, thread, point) is about how it drives, not how big it is.

The 5.0 x 100mm Performance Plus screws we stock are a double-countersunk pozi head, serrated thread and a Type 17 slash point, in a zinc and yellow passivated finish. In plain terms: the head pulls flush and is less likely to snap, the serrated thread cuts in with less splitting, and the point lets you drive straight into most softwood without pre-drilling.

 

What is a 5.0 x 100mm screw used for?

This is the length you reach for when a shorter screw would not give enough bite in the timber behind. Typical jobs:

• Studwork and timber framing - fixing 47mm CLS studs to plates, noggins between studs, sole plates down to timber.
• Decking and fencing substructure - joists to posts, rails to posts, bearer fixings where you need depth rather than a surface screw.
• Battening thick material - counter-battens, thick sheet or board fixed back to timber.
• General structural carpentry - anywhere you are joining two decent-sized timbers and a 60 or 75mm screw would only just reach the second piece.

For lighter or thinner work, or if you want one screw that also does masonry with a plug, look at the multipurpose screws range instead. For a fuller run of timber screw sizes, the performance wood screws collection carries the shorter and longer options alongside this one.

How deep should the screw go? The two-thirds rule

The screw has to do two things: pass all the way through the piece you are fixing, then bite firmly into the timber behind. The trade rule of thumb is that the screw should give at least as much thread in the base timber as the thickness of the piece you are fixing through, and ideally around two-thirds of the screw's length ends up in the base.

Work it through for a 100mm screw. Fix through a 47mm CLS stud and the screw pushes 53mm into the timber behind, more than the 47mm it passed through, so that is a solid fixing. Here is what a single 100mm screw gives you through common timbers:

 

So 100mm is the right screw up to about a 50mm front timber. Once the piece you are fixing through is 63mm or more, a 100mm screw does not leave enough bite and you want a longer one.

5.0mm or 6.0mm gauge - which do I need?

The gauge is the shank diameter, and it sets how much load the screw carries and how likely it is to snap under strain.

• 5.0mm is the everyday structural gauge. It suits studwork, decking sub-frames, fencing and general carpentry, and drives easily in softwood.
• 6.0mm is the heavier gauge for landscaping timbers, sleepers, ledger boards and anywhere the fixing takes real shear or you are into dense timber. It needs more torque and a pilot hole more often.

For most timber-to-timber work at 100mm long, 5.0mm is the sensible choice. Only step up to 6.0mm when the timber is heavy or the load is high.

Do you need to pilot-drill?

With a Type 17 point and a serrated thread, these screws drive into most softwood without pre-drilling. You still want a pilot hole when:

• You are fixing close to the end or edge of a timber, where driving straight in would split it.
• You are going into hardwood or old, dry, dense timber that resists the point.
• You want a dead-flush, neat head in visible work, a quick countersink stops the head sitting proud.

In fresh softwood framing, out in the open away from ends, you can usually drive straight and move quickly, which is the point of the Type 17 tip.

How many screws in a box, and what do they cost?

These come in a re-sealable trade tub of 1000 at £49.99, which is 5p a screw, and the tub includes a free pozi driver bit. For anyone using this size regularly, the tub is far better value than buying small boxes, and the re-sealable tub keeps the rest dry and findable on site. Price is correct at the time of writing; check the product page for live pricing. Delivery is free on UK orders.

Common mistakes to avoid

• Screw too short for the job. A 100mm screw through a 63mm timber only bites 37mm, that is a weak fixing. Match the length to the timber (see the table).
• Reading the size backwards. 5 x 100 is 5mm diameter, 100mm long, never the other way round.
• Driving into an end grain edge with no pilot. The serrated thread helps, but too close to the end will still split timber. Pilot near ends and edges.
• Over-driving the head. The double-countersunk head pulls flush, keep driving past that and you strip the timber or snap the head. Stop when it sits home.
• Wrong finish for outdoors. Zinc and yellow passivated is fine for general and covered external work. For decking or exposed structural work in wet conditions, use an external-grade or stainless screw instead.

The short version

5.0 x 100mm means 5mm diameter, 100mm long, a general structural timber screw. It is the right length for fixing through timber up to about 50mm thick, leaving good bite in the base. Go 6.0mm for heavy or high-load work, longer than 100mm once the front timber passes 63mm, and pilot near ends or in hardwood. Buy the tub of 1000 if you use them often, or browse the full performance wood screws range for other sizes.

By Steve, Builders Hardware Direct. Steve runs the BHD trade counter and has spent years supplying fixings to builders, joiners and fencing contractors across South Wales. Not sure on a size? Call the counter on 01269 267030 or order online for free UK delivery.