A cruck is a curved timber post in a roof frame, in particular the post forming the lower part of the frame. The Dutch term is krommer or kromstijl, and the timber was usually oak.
The frame around it
The curved post does not stand alone. Two members normally go with it:
- The brace — strengthens the corner joint between the curved post and the tie beam above it.
- The sole or wall piece — makes the connection to the outer wall or the wall plate.
Together they form a triangular, stiff corner. That is exactly the point of the curve: the curved post gives the transition from post to beam a flowing load path, which makes the corner stiffer than a post set square to the beam.
Grown, not sawn
Curved timbers were deliberately grown: young trees were bent or pruned into the wanted shape. That had two advantages over sawing a curve out of thick boards:
- Less material — sawing out a curve wastes timber, because the board has to be at least as wide as the curve is deep.
- More strength — in grown curved timber the fibre follows the bend. In sawn curved timber the fibre is cut through, so the curve is weakened at precisely the most heavily stressed point.
The principle still holds, and it is why grown curved timber remains sought after in boatbuilding and boat restoration.
Reuse and history
When buildings were pulled down, curved timbers were regularly reused — an early form of reclaimed building materials. Around 1600 demand, especially from shipbuilding, grew so strong that curved timbers were sawn lengthways to get two out of one. Pine was tried, but oak stayed the preferred material.
Curved timbers appear outside roofs too: in timber-framed houses, in stair construction and in shipbuilding.
Related terms
- Roof truss
- Brace
- Sole plate
- Wall plate
- Tie beam
- Reclaimed building materials
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