Choosing Structural Screws for CLT and Mass Timber

Structural screws for cross-laminated timber must be selected as part of the connection design. Screw diameter and length matter, but so do thread type, head geometry, grain direction, fixing angle, edge distance and the load being transferred.

Start with the load path

Before selecting a screw, establish what the connection needs to do. A wall-to-floor detail, panel edge connection, steel plate connection and reinforcement detail do not place the same demands on a fixing.

Withdrawal: load acting mainly along the screw axis
Shear: load acting across the screw
Combined loading: axial and lateral actions occurring together
Head pull-through: the head-side timber resisting the screw head
Compression: forces transferred through the connection or screw arrangement

Partial thread or full thread?

Partial-thread screws are commonly used to pull two components together. The unthreaded section allows the head-side member to close against the receiving member while the thread anchors in the lower timber.

Full-thread screws can be used where load transfer and reinforcement are required along more of the screw length. Neither format is universally stronger; suitability depends on the connection model.

Selection Point	Typical Uses
Partial thread	Clamping two timber components and transferring load through the head and engaged thread.
Full thread	Specified reinforcement or load transfer where thread engagement is required through a larger section.
Flush structural head	Connections requiring head-side resistance without a projecting washer head.
Cylinder head	Deep or concealed installation where a small head is required; suitability for metal-to-timber must be checked.

CLT face, edge and end-grain conditions

CLT is layered, so the intended fixing position and direction relative to the laminations must be clear. A screw into the broad face does not behave identically to one installed into a narrow edge or end-grain condition.

Use a screw system with technical data covering the proposed direction and substrate. Schmid states that RAPID® screws are approved for relevant side- and end-grain applications and CLT side faces and narrow edges, subject to the detailed approval conditions.

Why head design matters

The screw head transfers load into the head-side component. A small countersunk head, large washer head and high-performance flush head provide different bearing areas and pull-through characteristics.

RAPID® SuperSenkFix is intended to combine a flush finish with high pull-through resistance. It can therefore suit structural timber and CLT details where a projecting head would interfere with finishes or following trades.

Installation factors to include

Correct driver bit and adequate installation tool
Specified screw angle and consistent alignment
Minimum edge and end distances
Spacing between screws and between fixing groups
Timber density, moisture and likelihood of splitting
Required thread engagement and head seating
Service environment and corrosion exposure

Do not substitute by size alone: Two screws with the same nominal diameter and length can have different characteristic values, head performance, thread geometry and approval conditions.

Questions to ask before ordering

Is a specific manufacturer and product named in the design?
Is the connection timber-to-timber or metal-to-timber?
Is the fixing into a CLT face, narrow edge or end-grain condition?
Does the design require partial or full thread?
What diameter, length, angle, quantity and spacing are specified?
Is the finish suitable for the intended environment?

Frequently asked questions

Can ordinary wood screws be used for CLT connections?

Designed structural connections require screws with suitable performance data and approval for the application. A general-purpose wood screw should not be substituted without design approval.

Do CLT screws always need pre-drilling?

No. Many approved structural screw systems are self-drilling. Pre-drilling may still be required by the design or because of timber density, edge distance or installation conditions.