A retaining block (Dutch: borgklosje) is a small wooden block used in timber frame construction to secure, lock, or prevent movement of structural members at their connection points. It acts as a mechanical stop that holds components in their intended position without relying solely on friction or adhesive.
This traditional fastening technique has been used in European timber construction for centuries and remains relevant in restoration work and high-quality timber framing today.
Application
Retaining blocks serve several important functions in timber structures:
- Joint locking: Placed within or adjacent to mortise-and-tenon joints, they prevent the tenon from backing out of the mortise due to vibration or load cycling.
- Beam positioning: Small blocks nailed or screwed to a supporting member prevent a beam or joist from shifting sideways under load.
- Panel securing: In traditional panelled construction (such as wainscoting or door panels), retaining blocks hold floating panels in their grooves while still allowing for seasonal wood movement.
- Temporary bracing: During assembly of timber frames, retaining blocks can serve as temporary stops to keep members aligned until permanent connections are made.
- Glazing support: In traditional wooden window frames, small blocks (glazing blocks) position the glass pane within the rebate before putty is applied.
Retaining blocks are typically cut from hardwood for durability, especially when used in structural applications. Their dimensions are small — often no more than 20 to 40 mm in each direction — but their placement is precise and critical to the joint’s long-term performance. In modern engineered timber construction, the function of the retaining block has largely been replaced by metal connectors, dowels, and structural screws, though the principle of mechanical locking remains the same.
Related terms
- Mortise and tenon
- Timber frame joint
- Peg (treenail)
- Wedge
- Cleat
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