Knee (construction)

In woodworking, a knee is a natural or cut, curved piece of wood. Knees, sometimes called ships knees, are a common form of bracing in boat building and occasionally in timber framing. A knee rafter in carpentry is a bent rafter used to gain head room in an attic.

Wood is a highly anisotropic material (its strength varies considerably with the direction of applied force, i.e. parallel, radial, or tangential to the grain). Because wood is strongest when loaded in tension or compression along the grain, the best knees are those in which the wood grain follows the bend. For a knee with relatively little bend, it may be possible to cut the knee out of a single straight-grained board and still achieve sufficient strength. However, with increasing bend this method becomes problematic since more and more of the knee is aligned across the grain and is therefore considerably weaker. A knee laid out this way might easily snap in two under hand pressure alone, even if it is generously sized. In boat joinery constantly subject to shock and fatigue loading this method is unsuitable.

To avoid this issue knees requiring sharper curves are made using methods which ensure that the wood grain and direction of load are closely aligned. This can be achieved by steam bending, laminating, or selecting a natural crook with matching grain - a "grown knee". Grown knees are generally considered as the "best" method among boat builders and have a strong traditions associated with their use, but they may not achieve the same strength as a good laminated knee.

Bent - Bent knees are formed by plasticizing the wood to make it flexible via boiling, steaming, or microwaving (for small components). While still hot, the wood can be bent into a shape suitable for the location - either on a form or by forcing and securing it directly into the final service location. Steam-bending is a time-honored method for shaping boat frames, but it does weaken the wood slightly, it can leave residual stresses which may cause breakage or spring-back over time, and it is limited in the degree of bend which it can achieve, particularly for thick members. Also, not all species of wood steam bend well.

Laminated - Laminated knees are formed by coating thin, flexible strips of wood with adhesive, layering them to achieve the required thickness, then forcing the desired bend into the layup and securing it until the adhesive sets. Laminated knees are very strong and can be made in shapes which would be difficult to achieve using other methods, but they require time for the adhesive to cure, they are messier to construct, and they must use a jig or fixture to secure them until the adhesive cures.

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