What is a self-drilling dowel?

A self-drilling dowel (Dutch: schroefdeuvel) is a steel dowel with a screw head and a special drill point. This makes it self-drilling: in a single operation it drills through the timber and through one or more steel plates slotted into the timber. According to manufacturers’ data, this is limited to a few plates of limited thickness (for example three plates of 5 mm). The self-drilling dowel is intended specifically for steel-to-timber connections.

With a traditional connection using steel dowels, the timber and steel plate must first be pre-drilled so that the holes line up exactly. With a self-drilling dowel this is not necessary: the steel plate can be slid into a saw slot in the timber, after which the dowels are screwed straight through it.

Application

Self-drilling dowels are used for:

• connections in post-and-beam structures, trusses and lattice girders with slotted-in steel plates;
• connecting timber beams and columns to steel base plates and nodes;
• exposed timber construction where clean detailing without external toothed-plate connectors or steel angles is wanted.

Advantages:

• the steel plate sits inside the timber: a neater result and better protected against fire than an external plate;
• no pre-drilling of timber and steel, which saves time and avoids fit-up errors;
• available in long lengths, for large timber sections;
• the head can be left exposed or countersunk.

Points to watch:

• Design: the number of dowels, their spacing and edge distances follow from a structural calculation and the manufacturer’s approval; stick to them strictly, or the timber may split.
• Plate thickness and steel grade: check with the manufacturer which plate thickness and steel grade the dowel can handle.
• Proper tools: use a powerful screwdriver at the correct speed; too high a speed will overheat the drill point.
• Outdoors: choose suitable corrosion protection or a stainless steel version for external use.

Related Terms

• Split ring connector
• Post-and-beam construction
• Wood joints
• Timber construction

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