In woodworking, a knee is a natural or cut, curved piece of wood. [1] 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 [2] 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.
Grown - The term "grown knees" refers to any knee which is made from a natural crook or bend in a tree. Grown knees can be taken from several locations within a tree. The roots are a particularly useful source as the root structure of many species of trees naturally spreads out laterally just beneath the ground in order to help anchor the tree. This provides a fairly reliable source of approximately 90 degree crooks which may be impossible to find in other portions of the tree. In order to obtain this raw material for knees builders may dig up a stump in its entirety, as unlike other portions of the tree, it is impossible to judge the quality and quantity of available material in the roots as they are underground. Once the stump has been dug up the knees can be sawn or split from suitable natural crooks. However, knees sawn from a stump can quickly dull tools used to shape and finish them - as the roots grow they envelop small particles of soil and rock, which acts as an embedded abrasive and accelerates the wear of edged tools. For species of wood with appropriate splitting characteristics, such as oak, the stump can be split into wedges, with one large root on each wedge; each wedge is then carved into a rib for a small boat. In principle tree shaping could be used to grow a tree into the desired shape.
A sharp bend in a piece of wood is also called "cranked". Commonly used in shipbuilding known as ship’s knee for their advantage of reducing the encroachment into the usable space of the structure since there is no spandrel. Also knee rafter increases the usable space in an attic by creating a kneewall-like space.
A ship’s knee has two parts called the arm (shorter) and a body (longer). The outside surfaces come to a corner, (typically 90 degrees in buildings) called the heel. The inside surface retains its natural shape and the curve is called the bosom. The thickness between the heel and bosom is the throat. The ends of the arm and body are the toes.
The names of ship’s knees are based on their position: [3] [4]
Knees can be blind pegged with foxtail wedges on both ends or bolted in place.
Due to tradition, ease of workability, strength and durability characteristics, some species of wood are particularly prized for making knees. Tamarack (also known as hackmatack) stumps are among the preferred softwood species for grown knees, while white oak, live oak, and elm are preferred for hardwoods for bent knees due to their ease of steam bending.
Lumber is wood that has been processed into uniform and useful sizes, including beams and planks or boards. Lumber is mainly used for construction framing, as well as finishing. Lumber has many uses beyond home building. Lumber is sometimes referred to as timber in England, Australia and New Zealand, while in most parts of the world the term timber refers specifically to unprocessed wood fiber, such as cut logs or standing trees that have yet to be cut.
Plywood is a composite material manufactured from thin layers, or "plies", of wood veneer that are glued together with adjacent layers, having their wood grain rotated up to 90° to one another. It is an engineered wood from the family of manufactured boards, which include medium-density fibreboard (MDF), oriented strand board (OSB), and particle board.
Joinery is a part of woodworking that involves joining pieces of wood, engineered lumber, or synthetic substitutes, to produce more complex items. Some woodworking joints employ mechanical fasteners, bindings, or adhesives, while others use only wood elements.
Engineered wood, also called mass timber, composite wood, man-made wood, or manufactured board, includes a range of derivative wood products which are manufactured by binding or fixing the strands, particles, fibres, or veneers or boards of wood, together with adhesives, or other methods of fixation to form composite material. The panels vary in size but can range upwards of 64 by 8 feet and in the case of cross-laminated timber (CLT) can be of any thickness from a few inches to 16 inches (410 mm) or more. These products are engineered to precise design specifications, which are tested to meet national or international standards and provide uniformity and predictability in their structural performance. Engineered wood products are used in a variety of applications, from home construction to commercial buildings to industrial products. The products can be used for joists and beams that replace steel in many building projects. The term mass timber describes a group of building materials that can replace concrete assemblies.
Boat building is the design and construction of boats and their systems. This includes at a minimum a hull, with propulsion, mechanical, navigation, safety and other systems as a craft requires.
A beam is a structural element that primarily resists loads applied laterally to the beam's axis. Its mode of deflection is primarily by bending. The loads applied to the beam result in reaction forces at the beam's support points. The total effect of all the forces acting on the beam is to produce shear forces and bending moments within the beams, that in turn induce internal stresses, strains and deflections of the beam. Beams are characterized by their manner of support, profile, equilibrium conditions, length, and their material.
Glued laminated timber, commonly referred to as glulam, is a type of structural engineered wood product constituted by layers of dimensional lumber bonded together with durable, moisture-resistant structural adhesives so that all of the grain runs parallel to the longitudinal axis. In North America, the material providing the laminations is termed laminating stock or lamstock.
Clinker built is a method of boat building in which the edges of hull planks overlap each other. Where necessary in larger craft, shorter planks can be joined end to end, creating a longer strake or hull plank.
Laminated veneer lumber (LVL) is an engineered wood product that uses multiple layers of thin wood assembled with adhesives. It is typically used for headers, beams, rimboard, and edge-forming material. LVL offers several advantages over typical milled lumber: Made in a factory under controlled specifications, it is stronger, straighter, and more uniform. Due to its composite nature, it is much less likely than conventional lumber to warp, twist, bow, or shrink. LVL is a type of structural composite lumber, comparable to glued laminated timber (glulam) but with a higher allowable stress.
A rafter is one of a series of sloped structural members such as steel beams that extend from the ridge or hip to the wall plate, downslope perimeter or eave, and that are designed to support the roof shingles, roof deck, roof covering and its associated loads. A pair of rafters is called a couple. In home construction, rafters are normally made of wood. Exposed rafters are a feature of some traditional roof styles.
Wood shingles are thin, tapered pieces of wood primarily used to cover roofs and walls of buildings to protect them from the weather. Historically shingles, also known as shakes, were split from straight grained, knot free bolts of wood. Today shingles are mostly made by being cut which distinguishes them from shakes, which are made by being split out of a bolt.
A Windsor chair is a chair built with a solid wooden seat into which the chair-back and legs are round-tenoned, or pushed into drilled holes, in contrast to other styles of chairs whose back legs and back uprights are continuous. The seats of Windsor chairs are often carved into a shallow dish or saddle shape for comfort. Traditionally, the legs, stretchers, and uprights were usually turned on a pole lathe. Spindles may also be carved, using drawknives and spokeshaves. The back and sometimes the arm pieces are formed from steam bent pieces of wood. Traditional Windsors are typically painted, primarily to hide the different types of wood used in construction, based on their characteristics.
In woodworking, veneer refers to thin slices of wood and sometimes bark that typically are glued onto core panels to produce flat panels such as doors, tops and panels for cabinets, parquet floors and parts of furniture. They are also used in marquetry. Plywood consists of three or more layers of veneer. Normally, each is glued with its grain at right angles to adjacent layers for strength. Veneer beading is a thin layer of decorative edging placed around objects, such as jewelry boxes. Veneer is also used to replace decorative papers in wood veneer HPL.
Quarter sawing or quartersawing is a woodworking process that produces quarter-sawn or quarter-cut boards in the rip cutting of logs into lumber. The resulting lumber can also be called radially-sawn or simply quartered. There is widespread confusion between the terms rift sawn and quarter sawn with the terms defined both with opposite meanings and as synonyms.
This glossary of woodworking lists a number of specialized terms and concepts used in woodworking, carpentry, and related disciplines.
Cross-laminated timber (CLT) is a subcategory of engineered wood with panel product made from gluing together at least three layers of solid-sawn lumber. Each layer of boards is usually oriented perpendicular to adjacent layers and glued on the wide faces of each board, usually in a symmetric way so that the outer layers have the same orientation. An odd number of layers is most common, but there are configurations with even numbers as well. Regular timber is an anisotropic material, meaning that the physical properties change depending on the direction at which the force is applied. By gluing layers of wood at right angles, the panel is able to achieve better structural rigidity in both directions. It is similar to plywood but with distinctively thicker laminations.
Steam bending is a woodworking technique where wood is exposed to steam to make it pliable. Heat and moisture from steam can soften wood fibres enough so they can be bent and stretched, and when cooled down they will hold their new shape.
Engineered bamboo is a set of composite products produced from bamboo. It is designed to be a replacement for wood or engineered wood, but is used only when high load bearing strength is not required because building standards for this type of use have not been agreed by regulatory bodies. Engineered bamboo comes in several different forms, including bamboo scrimber and laminated bamboo, which has three times the structural capacity as normal timber and is defined and regulated by the ASTM International Standards.
A steam box is a long, sealed container used to steam wooden planks for the purpose of making them pliable. Once steamed and then fastened or clamped into the desired position and left to dry, the wood will hold the new shape. Steam boxes allow for much more efficient use of wood. Instead of cutting the desired shape away from a large and more expensive piece of wood and leaving much scrap to be discarded, steam boxes allow for a smaller piece to be bent in the general shape and leaving much less scrap. Steam boxes also allow the wood to bend beyond its dry breaking point, which is useful in making extreme curves with the wood. In many cases, the bent piece is stronger than an identical piece cut from larger stock. Steam bending wood allows the wood grain to follow the bend, leaving it strong where a piece cut from larger stock would snap across crosscut grains or laminated joints.
A Gothic-arched roof barn or Gothic-arch barn or Gothic barn or rainbow arch is a barn whose profile is in the ogival shape of a Gothic arch. These became economically feasible when arch members could be formed by a lamination process. The distinctive roofline features a center peak as in a gable roof, but with symmetrical curved rafters instead of straight ones. The roof could extend to the ground making the roof and walls a complete arch, or be built as an arched roof on top of traditionally framed walls.
