A mending plate is a metal plate fitted over or between two pieces of timber. It is used to stop splitting, to tie two members together, or both at once. In Dutch it is called a kramplaat.
Three uses
- Against splitting. A plate on the end grain of a log, post or beam holds the fibres together so the inevitable drying split does not run on. It is used the same way along the edges of scaffold boards.
- As a tie. Across the joint between two end grain faces or two edges, to keep the pieces in line.
- As a toothed connector between two members — the bulldog. A round or square toothed disc pressed between two timbers and pulled up with a bolt through the middle. The teeth bite into both members and carry the shear; the bolt holds the assembly together. Single-sided versions are used timber-to-steel, double-sided ones timber-to-timber, and the double-sided type gives a stiff connection.
An important distinction: structural or not
A mending plate is not automatically a structural connection. A plate screwed over a split cannot simply be counted on to carry load. For structural timber connections, modern practice uses the nail plate (pressed in at the factory on trussed rafters) or a bolted connection with special connectors such as the bulldog, designed to the rules for timber structures.
Practical points
- Corrosion. Outside or in a damp room, a galvanised or stainless grade is essential. Rusting steel in timber causes black staining and attacks the fibre; in oak and other tannin-rich species it happens especially fast.
- Edge distances. A fixing too close to the end or the edge splits the timber — precisely what you were trying to prevent. Keep to the manufacturer’s edge and spacing distances.
- Bulldogs need force. They are pressed in, preferably with a threaded rod and nuts or a press, not by tightening the final bolt with a spanner.
- Timber keeps moving. A rigid plate across a large area can create new stresses as the timber shrinks. A short plate at the critical point beats one long plate down the whole beam.
Related terms
- Nail plate
- Timber connection
- Dowel
- End grain
- Shrinkage crack
- Hardwood
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