The structural timber screw as an alternative connection method

By | 2020-03-29T17:21:37+00:00 March 30th, 2020|

In timber engineering the joining of members is generally a critical factor in the design of a structure, which places emphasis on the type of connection used. Metal dowel-type fasteners and bearing-type connectors are the most common form of connector.

Image credit: Wolf fasteners & Accessories

Image credit: Wolf fasteners & Accessories

Why is the Structural Timber Wood Screw different?

  • It is a high-strength wood screw manufactured from heat-treated carbon steel in different lengths. This is an alternative to connections via bolting and traditional coach (lag) screws.
  • The 6 and 8mm washer head screw has a 14 and 22mm washer head with a serrated tread point.
  • Installs easily with a six-lobe driver bit and requires no pre-drilling.
  • Is used for connections in load bearing timber structures between members of solid timber, glued laminated timber, cross laminated timber and laminated veneer lumber, similar glued members, wood-based panels or steel. The high-strength wood screw is also used as tensile or compressive reinforcement perpendicular to the grain.
  • Corrosion treatment is equivalent to hot-dip galvanisation.

Typical Applications

  • Purlin to rafter connection
  • Connection of insolation board material onto rafter or between rafters
  • Connection of multi-ply truss members
  • Connection of timber frame panels
  • Truss to top plate connection
  • Tensile or compressive reinforcement of timber members
  • Structural insulated panel erection
  • Shed and stable erection
  • Timber fence pole connections
  • Gum pole connections for thatched lapa framework

Advantages

  • Easy connection for wood components without pre-drilling and timber splitting
  • Tested engineering design values via ETA approval
  • Exceptional aesthetic appearance
  • Misplacement easy to rectify by unscrewing and refitting
  • Time-saving contributed to labour-saving
  • Visual characteristic head marking for inspectorate

Frequently asked questions:

Q: Are all the imported structural timber screws of the same quality?
A: Certainly NO. Through an in-house corrosion test we proved that certain structural screws do not comply with the advertised specified corrosion classification.

Q: How can I be certain the structural screw will perform as advertised?
A: It is very important that users are aware that structural timber screws are imported and not all comply with international required standards.

Unfortunately, the SABS does not have a National Standard for structural timber screw product requirements, and it is the responsibility of the user to make an informed decision on the correct strength and corrosion-treated structural timber screw to use in the connection.

To make the correct decision on which screw to use, clarify with the supplier the following:

  • Does the product have an ETA (Euro Technical Assessment) approval?
  • Does the product have a DOP (Declaration of performance) approval?

Q: What is an ETA?
A: The European Technical Assessment (ETA) is a documented assessment of the performance of a construction product, in relation to its essential characteristics.
This is the definition given by the Construction Products Regulation (CPR, Regulation (EU) No 305/2011), which governs the marketing of construction products in the European Economic Area.
The ETA provides manufacturers with a voluntary way for CE marking their innovative non-standard construction product and thus bringing it to the European internal market. As an independent assessment, the ETA also contributes to creating trust in the performance of the construction product related to its essential characteristics and taking into account its intended use.

Q: Will the structural timber screw be the answer for insulation over rafter connections?
A: Certainly YES. IsoBoard has developed an over-rafter connection detail with structural timber screws which were tested by the University of Stellenbosch and approved by the ITC SA under the following conditions:

  • Board suppliers’ fixing details to be used, or alternatively an engineer’s rational design or deemed to satisfy detail.
  • The detail needs to provide adequate lateral bracing to the rafter, in accordance with the normal design assumptions.
  • Roofs to be inspected prior to fitting of covering. This is a general requirement for tiled and sheeted roofs, especially if insulated boards over rafter are used.
  • Where BC bracing is not present, the ceiling grid must be installed prior to installation of roof covering (loading).