Construction screws

Full thread or partial thread?

This question decides the capacity of a connection more than the diameter does, and it is confused more often than any other. It is not about which thread is stronger, but about which job it has to do.

A partial thread has a smooth shank under the head. That shank passes through the member being fixed without gripping it, so the screw pulls the two parts together. There is one condition, and it is often missed: the smooth part must pass through the whole member being fixed. If the board is 40 mm and the smooth section is 30 mm, the thread starts biting in the upper member and no clamping force develops at all.

A full thread grips both members at once. It pulls nothing together; it transfers force along the entire shank, and the parts stay where they were. The guide Full or partial thread - when to use which settles the rest.

• KonstruX - thread over the full length, cylinder head, Ø 5.2-8.0 mm, lengths 80-330 mm, TX 25-TX 40 drive, blue zinc plated, pack of 100 (Ø 8.0 mm - 50).

When do you need an ETA?

An ETA is a European Technical Assessment - a document stating characteristic load capacities, minimum spacings, edge distances and conditions of use. Without it (or without a declaration of performance under a harmonised standard) the structural engineer has nothing to calculate with.

For a load-bearing connection subject to inspection, a structure covered by building consent, or strengthening and repair work that carries a calculated load, the designer needs documented properties of the fastener - from a European Technical Assessment (ETA) or from a declaration of performance based on a harmonised standard. Eurotec timber construction screws are covered by ETA-11/0024; before inspection, check the trade name and size in the current issue. When the document really matters and when it does not is explained in the guide ETA-11/0024 - when you need the assessment at handover.

Above-rafter insulation - how to choose the length?

Screw length is the sum of the layers, not the thickness of the insulation: counter batten plus insulation plus sarking plus the required embedment depth in the rafter. The last term is the one most often left out, because the first three can be reached with a tape measure and the fourth has to be read off the assessment. Then there is the angle: a screw driven at a slant travels further than the layer thickness, so a length calculated perpendicular comes out too short.

• Topduo with washer head - Ø 8.0 mm, lengths 165-472 mm, TX 40 drive, pack of 50.
• Topduo with cylinder head - Ø 8.0 mm, lengths 165-435 mm, double thread, TX 40 drive, pack of 50.

The calculation step by step is in the guide Choosing screw length for above-rafter insulation.

Beech plywood and LVL - why a separate screw?

Laminated veneer lumber in beech is several times stronger than spruce and exactly that much harder to screw into. Resistance rises so far that an ordinary structural screw driven without pre-drilling can reach its breaking torque before it gets to full depth - the head shears off and the shank stays in the material. The worse version of this failure is silent: the TX recess rounds out halfway in, the screw looks seated, and the connection does not have the assumed capacity. That is why KonstruX goes into pre-drilled holes in beech LVL - ETA-11/0024 gives the drill diameters and design values for this. Details in the guide Screws for beech plywood and laminated veneer lumber.

What to order alongside?

Drives in this category run from TX 25 to TX 50, so the standard bit from a cordless driver is often not enough. The colour-coded 25 mm TX bit set covers TX 10 to TX 40, and the long 50 mm version reaches TX 50 and gets where the short one cannot. For the decking surface itself, see decking screws; for cladding on a substructure, see facade fastening.