BASICS
Timber framing is a traditional building practice that creates the skeleton framework of a structure by connecting and fastening the ends of both large and small wooden members with wooden joinery. Joinery consists of carving or fitting two pieces of timber together with the connection being secured and fastened with a wooden pin or peg. A primary method of joinery is mortise-and-tenon, which is the hole or recess cut of one wooden member receiving the corresponding projection of the other wooden member. Other forms of wooden joinery include lap joint dovetail, tying joint, and scarf joint.
Along with joinery, timber framing’s distinctive style often involves using heavy timbers instead of slender, pre-cut, dimensional, and pre-sized lumber that is readily available from lumber yards (e.g., 2x4s or 2x6s). Instead of building with pre-cut dimensional lumber, traditional timber framers often create and work directly with timber harvested from locations having a suitable and sustainable species of trees. These locations may include trees from a builder’s property or trees from a building site. Builders can harvest the trees rough-sawn, hand hewn, or adzed on site or process the trees at a mill as green, air-dried, or kiln-dried timber. Recycled timber and reclaimed wood are also other methods of procurement, but those forms of timber must be suitable for joinery.
Although timber framing has distinctive attributes, the building method does have some intermingling with post and beam construction. Post and beam structures have upright posts supporting horizontal beams and those wooden members may include round logs or squared timbers that are held together by metal hardware that is either hidden or exposed. Although timber framing can be a specialized version of timber post and beam, its connections generally remain distinctive with wood-on-wood joinery, not metal hardware.
Distinctions of traditional timber framing can also become blurred as owner-builders, architects, engineers, and code enforcement officials dictate the use of certain technologies that involve engineered and metal connections for some joints. In addition, designs may require hybrid structures where builders construct timber framing and stick building in different parts of a structure.
GALLERY
HISTORY
Timber framing has a long-standing architectural history as archeologists have found evidence of the building method in sites throughout the world including the Middle East, Europe, and Asia. Archaeological evidence at Neolithic sites suggests that rafter systems existed for houses in Europe 5,000 years ago. Ancient stone temples, like Stonehenge, and Greek temples also show an homage to wood design. In India, archaeologists have dated teak timbers shaped and connected with joinery and bamboo pegs to 200 BC. In Japan, early builders discovered that wooden joints and fibrous timbers could help withstand the forces of typhoons and earthquakes. Japan also holds historical evidence of timber framing by being home to some of the world’s oldest surviving wooden structures, located at a Buddhist temple complex named Horyuji. Historians and archaeologists believe builders reconstructed the structures at Horyuji after a fire in 670 AD. In Europe, as virgin forests dwindled and became scarce by 1200 AD, the scarcity forced changes to building methods such as shorter timbers, cruck frames, and the use of masonry. But as Europeans discovered and settled the Americas by the early 1600s, they also discovered virgin forests which allowed them to use long timbers and build an assortment of structures.
In New England, early Americans also developed a unique cultural significance to timber framing by using community-driven labor. As farmers or community members would need a new barn or a town hall, they would gather the wood in the winter months, prepare the wood in the spring, and then raise the structure together under the direction of a master timber framer. Raising-day still holds cultural significance in the modern era by allowing participants an opportunity to celebrate the ingenuity and craftsmanship that goes into making a timber-framed structure, while also witnessing the unveiling of assemblies as they piece them together.
Even with timber framing’s rich cultural significance, the building method eventually became extinct, primarily caused by factors of the industrial revolution and as timber resources dwindled along the eastern seaboard. With dwindling tree resources, numerous sawmills responded by readily cutting smaller dimensional lumber from smaller trees. By the early 1900s, the industrial revolution ushered in inexpensive stick-built homes constructed by using two-by-fours and two-by-sixes that are fastened with nails and other metals. Builders favored the new method, as they could build structures faster with lighter materials and less labor. The preference continues today as builders still use pre-set lumber or stud framing for many new homes.
Although timber framing did eventually die out as a popular building method, it has reemerged in an era of revival by those dedicated to preserving historical and artisan practices. The revival began in the 1970s by a group of skilled artisans and technicians in the United States and has spread to other parts of the world. Today, along with traditional techniques, timber framers can also use modern technological processes, such as industrial mill processing and improvements for insulation and design.
MATERIAL PROPERTIES
Choosing and Evaluating Timber:
When considering wood for structural timber framing, different species of wood will have unique design characteristics and different capabilities, including how well a piece of timber will season or dry. Common species of hardwoods include American Chestnut, Ash, Beech, White Birch, Yellow Birch, Butternut, Black Cherry, Hickory, Black Locust, Red Maple, Sugar Maple, Red Oak, White Oak, and Walnut. The range of softwoods include Balsam Fir, Northern White Cedar, Red Cedar, Eastern Hemlock, Eastern White Pine, Red Pine, Norway Pine, Eastern Spruce, and Tamarack (Eastern Larch).
Although there are several tree or wood varieties suitable for a structure, there are also several factors and considerations in determining what will be the best choice for a project. Those factors include sustainability, cost, weight, workability, strength, suitability, and if the exterior will have exposed timber. As a step in narrowing the factors, experts recommend owner-builders research if there are suitable trees or timber available locally and select a species that is harvested in a sustainable and responsible manner.
Experts also recommend that owner-builders hire a structural engineer or certified grading expert to verify timber choices, as well as the architectural plans for the project. Local building codes and/or the structural engineer may also require professional grading of the timber, especially if an owner-builder fells their own trees. Professional grading of timber not only involves determining the grade classification – Select Structural No. 1 and Select Structural No. 2 – but also referencing a vast array of concepts and standards from the American Society of Testing Materials (ASTM) and the allowable stress values shown in the National Design Specification for Wood Construction (NDS). Professional graders may also reference guidelines from their own grading organization or an organization, such as the Northeastern Lumber Manufacturers Association in Maine.
When it comes to handling wood defects – knots, spiral grain, cross grain, crotches, reaction wood, shakes, wane, stain, decay, insect damage, shrinkage, splitting or checking – experienced and skilled timber technicians may employ strategies to work around the defects or may simply know what areas of the timber they should avoid for structural loads or what defects they can use in low-stress locations. An example of what timber framers may avoid are the hidden seams below tree forks or crotches or knots for key joints.
Thermal Characteristics:
A typical average R-value for softwood is 1.25 per inch of thickness and for an 8-inch log, it will give an R value of about 10. Timber also has some mass benefits that will improve its overall energy performance. But, since additional coverings and layers are usually added around timber structures, supplemental insulation layers are also added to improve the building’s insulating abilities.
Fire Resistance:
Although wood is a combustible material, builders have long recognized that timber structures can maintain structural integrity while exposed to fire. Experts attribute the fire resistance of timber to the charring effect of wood. As large timbers burn, they develop an insulating char layer on the surface, which protects the inner portions of the timber from the heat of the fire. This attribute then enables the timber to carry a load for a longer period.
Durability and Strength:
Timber-framed structures are capable of being durable, long-standing, and sturdy because of the combined strength of the timber and the joinery. Technicians, architects, and engineers design and build timber-framed homes to last a very long time or as legacy homes. A well-built timber frame structure will also not require its wall mass to hold it up; therefore, builders can use expanses of glass in a passive solar wall without weakening the frame.
The complexities of structural design will likely make it necessary for owner-builders to employ the expertise of design and construction professionals. Their training allows them to determine the loads for a structure, what types of loads and stresses each timber member must resist, and how well the joinery will transfer the stresses. Owner-builders will also need to find structural engineers that understand the capacities and capabilities of wood-to-wood joinery, including the design criteria for wooden pegs.
ADDITIONAL NOTES
Owner-Builder Friendliness:
For aspiring timber framers, the methods of selecting and grading timber, handling defects, getting timbers cut to size, scribing, and joinery will require an acquired skill level and experience in order to achieve proficiency. To get the experience, experts recommend owner-builders attend a workshop, take part in a project, or start framing small structures. Along with experience, owner-builders will also need to acquire specialized tools in order to implement the traditional methods of timber framing.
Combining with other Natural Materials:
Timber-framed structures are a suitable adjunct to natural building methods such as straw, hempcrete, and cob as the natural wall systems can work in combination with the skeleton of timber framed structure or assembly.
Standards and Codes:
Timber framing is widely accepted in most jurisdictions because of its established history although most local building codes will require design and approvals from architects, structural engineers, certified grading experts, and other accepted professionals.
Beauty and Structural Transparency:
Exposed timbers provide beauty, charm, and esthetic appeal. The exposed structure can also provide a peripheral or subconscious feeling of security and shelter, as the inhabitant can view first-hand how the roof of a structure is being held up. Timber framing also has a rich tradition of craftsmanship with the use of historical methods and specific hand-held tools.
FAQ
Yes, there are many design options, including decorative timber trusses and adding an entire timber frame addition to a house.
Wood used inside timber-frame buildings, protected from weather, is usually not ‘treated’, so decay is theoretically possible under (accidental) persistently moist conditions, like a hidden plumbing leak. Attention must be given to build the timber-frame correctly and treat timber members exposed to the exterior environments.
Yes, most likely, as timber will shrink and swell as the moisture content of each member changes. Although humidity and temperature fluctuations may cause cracks and twisting, if an owner-builder designs and builds the home well, the defects will not affect the structural integrity of the structure. Experts advise that owner-builders should embrace the changes as natural aspects of the wood.
The term “mass timber” in the context of the International Building Code refers to a type of construction or product that is inherently fire resistant and suitable for larger commercial structures, where the standard would not permit light frame construction. Common mass timber products include glulam timbers (glued-laminated timber), cross-laminated timber panels, nail-laminated timber panels, or dowel-laminated timber panels. According to Woodworks – Wood Products Council, there is a growing trend by builders and designers to use mass timber products because of their aesthetic appeal as well their strength, stability, and lighter carbon footprint as compared to steel, concrete, and many masonry applications.
Additional Reading
Reference:
Learn to Timber Frame: Craftsmanship, Simplicity, Timeless Beauty by Will Beemer, Storey Publishing, LLC, 2016.
A Timber Framer’s Workshop: Joinery & Design Essentials for Building Traditional Timber Frames by Steve Chappell, Fox Maple Press, 2011.
Discovering Timber-Framed Buildings by Richard Harris, Shire Publications, 2008.
The Timber-Frame Home: Design, Construction, Finishing by Tedd Benson, Taunton Press, 1997.
Build a Classic Timber Frame House: Planning & Design/Traditional Materials/Affordable Methods by Jack Sobon, Storey Publishing, LLC, 1994.
Timber Frame Construction: All About Post-and-Beam Building by Jack Sobon & Roger Schroeder, Storey Publishing, LLC, 1984.
Organizations:
Canelo Project – Elgin, AZ – founders Bill & Athena Steen
https://caneloproject.com/
Yestermorrow Design/Build School – Waitsfield, VT
https://yestermorrow.org/
Timber Framers Guild – Alstead, NH
https://www.tfguild.org/
