What Is Timber Framing?

View of Bent

From a design for a working barn, this bent created agricultural space on the lower floor and a residence above. This 4 bent, 3 bay building also featured a principal rafter and common purlin roof. This bent consists of jowled posts, cruck blades, braces, interrupted ties, and collar tie. Using many curved and tapered timbers, this frame is an ideal candidate for scribe rule.

Aisle Wall Elevation

This barn design includes a central aisle with a gable roof and shed wings on each side, (see 3D image above). Consisting of 4 walls with connecting girts, short spans, and a simple repetitive form, this utilitarian building is an ideal candidate for the square rule.

King Post Truss

The king post truss is the oldest and most common roof truss. In its simplest form, it consists of a lower chord, king post, and upper chords. Longer spans require intermediate members called struts which help stiffen the rafters.

Note that the king post is continuous from the lower chord to the top of the rafters. Often shown with struts joining the lower chord, a correct structural design joins the struts to the lower end of the king post, at least 8 inches above the bottom chord. King post trusses are generally used for spans of 32 feet or less.

Queen Post Truss

A queen post truss consists of 2 queen posts, a straining beam, braces, and upper and lower chords. The queen posts are joined to the straining beam at the top and the lower chord at the bottom. The top of the queen posts are braced to the ends of the lower chord. Queen post trusses are capable of spanning longer distances than king post trusses, up to 45 feet easily. In addition to spanning greater distances than a king post truss, a queen post truss can be built using shorter members than the king post truss. Steel queen rods may be used in place of queen posts for more utilitarian designs.

Scissor Truss

The scissor truss consists of two lower chords, two upper chords, and a king post. The lower chords are joined to the bottoms of the upper chords and rise to the approximate midspan of the opposite upper chord. Often found in churches in Europe and the eastern US, the rising lower chords permit interior vaulting. In general, the scissor truss is inefficient structurally and is suitable only for short spans.

Hammer Beam

Hammer beams in their original form were used to create open vaulted spaces in medieval churches. Made from relatively short members, the ancient truss form used heavy masonry walls to resist outward thrust. Modern hammer beams often require a horizontal metal tie rod to replace the heavy masonry walls. The example shown here resolves the outward thrust with a continuous hammer beam, heavy tapered posts, and outside braces allowing the use of traditional wooden joinery and eliminating the need for steel tie rods. In this case, the principal rafters are joined with a half lap and a clasping joint at the ridge purlin.

Construction of Typical Timber Frame Joinery

Exploded here are many of the common components in timber frame structures. Note how mortise, tenon, housing, and pegs work together to form a connection.

Bay: The space between bents or the area between structural cross frames.

Beam: A main horizontal member in a building’s frame.

Bent: A cross section of a frame generally including principal rafters and the shape of the roof.

Brace: A smaller, diagonal timber used to prevent distortion of the frame. Many of our timber frames have naturally curved braces.

Cantilever: Any rigid structural member projecting beyond its last vertical support. Floor joists and rafters are the most common cantilevered timber frame members.

Chamfer: A decorative bevel cut on the corner of a timber.

Checks: Separation of wood fibers common in all timbers. Checks do not affect the structural capacity of timbers and do indicate the true direction of fibers in the timber.

Chord: In a truss, the major uppermost timber (upper chord) and the lower major timber (lower chord). In a roof truss, the rafters serve as the top chord and the tie beam as the lower chord.

Collar Tie: A horizontal connector between a pair of rafters used to reduce sagging and increase the possible span of the rafters.

Common Purlins: Horizontal members parallel to the ridge that generally span between principal rafters.

Common Rafters: Closely and regularly spaced inclined timbers that form and support the roof surface.

Cruck: Technically, a timber frame bent supported by two large, curving members. Loosely used to include many large, curved timbers. Crucks are a traditional English style of timber framing.

Dimensional Lumber: Planed, standardized lumber that is sold at any lumberyard, such as a 2x4.

Eave: The lower end of a roof surface that projects beyond the wall.

Gable: A double-sloping roof that forms an “A” shape.

Gambrel Roof: A double-pitched roof with the lower slope steeper than the upper slope, common in barn architecture.

Girt: Secondary horizontal member that connects posts in either walls or bents.

Green Wood: Freshly harvested wood that has not been dried or seasoned. Green wood is common in timber framing.

Half Lap: A simple joint in which two timbers are lapped or let into each other so that they pass each other in the same plane.

Hammer Beam: A roof bracket projecting from the top of the wall that supports roof framing above. Hammer Beams were first used in medieval cathedrals where the thrust from the roof was resolved into thick masonry walls. Modern hammer beams often require the use of a horizontal metal tie rod.

Hand Riven Pegs: Pegs split from straight-grained hardwood, then drawknifed and shaved to size.

Hardwood: Any wood from a deciduous tree. e.g. oak, cherry, ash, etc.

Heartwood: The inner, older part of the three that is typically more durable and traditionally turned to the exterior of buildings.

Housing: A shallow mortise or cavity for receiving intersecting timbers. Housings may be used for structural purposes or to account for irregularity in timber sizes. Housings are common in square rule layout systems, less so in scribe rule.

Joint: Any connection of two or more timbers, in traditional work accomplished without metal fastenings.

Joists: Horizontal timbers use to form and support the floor above and the ceiling below.

Jowled Post: A post wider at the top than the bottom. The wide top provides more wood for joinery and is common in traditional English timber frames.

King Post: A centered, vertical post extending from the bottom chord to the top chords of a truss.

Layout: The drawing of a joint on a timber before it is cut.

Mortise: A groove or slot in a timber which receives the corresponding tenon.

Mortise and Tenon Joint: The most common joint in timber framing, most joints are a variation of this simple form. A tenon is inserted into a mortise and then pinned in place with one or more pegs.

Nominal Size: Undressed or original sawn size of a timber. Most roughsawn beams retain their original size; most planed beams are .” smaller than their nominal size. For example a 6x8 planed timber will usually measure 5” x 7”.

Peg: The common fastening element of timber frame, usually made of hardwood and either .” or 1” in diameter. Pegs may be riven or turned.

Plates: Horizontal timbers at the top of the wall frames that generally support common rafters and tie the frame together.

Plumb: Exactly vertical.

Post: Any vertical timber.

Post and Beam: Generally similar to timber frame, but usually involving simpler configurations of posts and horizontal timbers connected with metal hardware. It may also refer loosely to timber frame.

Prick Post: A minor post, generally one story tall.

Principal Post: Major frame members that are generally included in both wall and bent assemblies and are the full height of the frame.

Principal Rafters: Large rafters that form the top roof surface of the bents. Principal rafters are generally used to support other roof timbers such as principal purlins with common rafters or common purlins alone. Principal rafters are similar to top chords in trusses.

Purlin Plates: Similar to principal purlins in that they generally support common rafters, but are not joined into principal rafters.

Queen Post: Paired, vertical posts in a truss that extends from the bottom chord to the top chords. Queen post trusses can span further distances than king post trusses.

Raising: Erecting the bents, walls, or other components of a timberframe. Raising is commonly accomplished with the assistance of a crane, and ends with a celebration.

Ridge: A horizontal timber at the peak of the roof that supports the top ends of the common rafters.

Roof Pitch: The angle of the roof, commonly expressed in inches of rise per foot of run (horizontal distance) i.e. a 6 in 12 roof rises 6 inches for every 12 inches of horizontal distance.

Roof Truss: An assembly of timbers working together to span long distances without interior supports. Scissor, king post, and queen post are common types of timber trusses.

Scarf: A joint used to join two short timbers into one longer timber. Scarfs are commonly used in plates in new work, and in posts and other members in restoration work.

Scribing (Scribe Rule): The original method of laying out timbers for joinery requires laying timbers on top of each other and transferring information vertically to mating timbers. While ancient in origin, scribing remains the most powerful and versatile method of layout and easily allows for the use of curved, shaped, sculpted, and organic pieces. Most of our frames are laid out using scribe rule.

Sheathing: Boards or plywood used to cover a floor or roof surface.

Shed Roof: Similar to a gable, but having only one slope. Commonly used on the side of two-story buildings.

Shim: A thin, often tapered piece of wood, metal, or plastic used to fill gaps between any two materials, often between concrete and timber.

Shrinkage: The reduction in the size of timber as it dries.

Sill Timbers: Horizontal timbers that rest on the foundation or floor system. Sill timbers are uncommon in modern timber framing.

SIPS Panel: A sandwich with two outer layers of plywood or OSB, (oriented strand board), and an inner layer of foam. SIPS are structural and also provide excellent insulation. Most timber frame homes are enclosed with SIPS.

Soffit: The underside of any overhang.

Softwood: Any wood from a coniferous tree. i.e. Pine, Doug fir, Spruce.

Span: The unsupported distance a structural timber covers.

Spiral grain: Wood fibers that do not lie parallel to the edge of timbers, but cross it at an angle. Spiral grain causes twisting and makes timber significantly weaker.

Square Rule: A system of layout, developed in America, where a perfect, smaller timber is imagined within a larger, imperfect timber and is the basis for all joinery. Many timbers such as rafters, joists, girts, and braces are interchangeable in a square rule frame.

Summer Beam: A major beam that spans between two other beams and supports floor joists.

Tenon: The projecting end of a timber that is inserted into a mortise.

Tension: A force causing the tendency of extension, most commonly occurring in roof structures. Good timber design converts tension to compression by forming trusses or similar configurations. Tension is difficult to resolve in wood joinery, while compression is generally very simple to resolve.

Through Tenon: A tenon that passes through the timber it joins.

Timber: A large rectangular piece of wood, typically 5x5 or greater in cross-section.

Timber Frame: A frame of large timbers joined and pegged together, generally supporting smaller timbers to which the roof, walls, and floors are fastened.

Vernacular Architecture: Local building styles that are built using local labor and materials; these styles are directly influenced by regional culture, conditions, and climate.

Wall: Generally occurring at right angles to bents, walls form the eave sides of the building, rather than the gable ends. In some common rafter frames, there are no bents and only walls.

Wane: A rounded edge on a timber where the original round surface of the log remains after sawing into a timber.

Wind Brace: Like a wall or bent brace, but occurring in the plane of the roof.