What is a zwalp?
A zwalp is a Dutch term for a heavy, rectangular sawn timber beam from which the sapwood has been removed, usually of oak. In English it is comparable to a baulk. The term mainly comes from historic hydraulic engineering, for example in locks. The word also has a military and a water-related meaning.
Meanings
- Sawn baulk: a heavy timber beam with a rectangular cross-section, using only the durable heartwood. In a lock floor a zwalp was used to resist the upward water pressure beneath the floor.
- Military: a frame of planks and beams on which cannon barrels not in use were stored.
- Water: a wave or a sudden surge of water.
Why remove the sapwood?
Sapwood is the outer, youngest part of the trunk. It is softer and much less durable than heartwood and is more readily attacked by fungi and insects. For beams that are permanently wet or alternately wet and dry, as in locks, removing the sapwood is essential for a long service life.
Use and restoration
- Historic locks and waterworks: in restoration, zwalpen are often reproduced in the same species and dimensions.
- Choice of timber: oak heartwood has good natural durability. Tropical hardwoods are also used for underwater structures, provided they come from responsibly managed forests.
- Structural assessment: decay in or replacement of load-bearing beams in a hydraulic structure calls for a structural engineer or hydraulic specialist.
Related terms
- Sapwood
- Canal lock
- Durability class
- Exterior durability
Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.