Appearing in: Period Homes Magazine, September 2008

 

The Keys to Successful Timber Framing
By Amy Cornelius, LEED-AP

Timber Framing is one of the oldest construction techniques known to man but remains one of the least understood in the marketplace. Evocative pictures greet us from magazines and coffee table books that encourage visions of grand rooms and entrances for homes, churches, office buildings, barns and even enhancements to our landscapes. The problem arises after the vision: how the heck do we design, build and afford the dream?

There are many factors that affect the price of a timber frame from specialty finishes to species of wood but the two main keys to a successful timber frame project are all in the design: keep it simple and use a collaborative process to develop the plans. In this article we will discuss the timber frame in general, the collaborative design process, and how to intelligently weigh options vis-a-vis your budget.

So What is Timber Framing?
The traditional definition of timber framing: ‘a frame constructed of heavy timbers using mortise and tenon joinery that is held together with wooden pegs’ is expanding as the industry and design aesthetics evolve.

Designer's desires for longer spans and more complex joinery coupled with increased engineering review rigor is driving timber framers to include more steel in their projects and to use a multitude of metal fastening systems. This steel can be hidden within the frame or it can be visible or expressed. Most often expressed steel is seen as an integral part of the overall design of the structure.

Many timber framers are also working with manufactured timbers including glued-laminated and PSL (parallel-strand lumber) material. These timbers, manufactured from thin pieces or 'slices' of kiln dried wood and then adhered with adhesive under pressure, have high tensile strength and allow designers to achieve significant spans. They can also be quite beautiful.

Timber frames are manufactured in a variety of ways. Most shops use some combination of power tools (chain mortise machines, etc) and hand tools such as chisels and mallets for joint clean up. Some specialized companies work in environments that are powered by water wheels and use only hand tools, others are slightly more automated and there are an increasing number of shops which employ CNC (Computer Numerical Control) technology. CNC technology allows computer design programs to communicate directly with large robotic milling machines which then cut the entire frame. Some companies cut entire frames using CNC technology while others cut repetitive components leaving the unique or more visible pieces to be hand-cut. Even in the fully automated frame shops, there is still a certain amount of hand work involved in cleaning up and finishing the frame, and sometimes in the more specialized compound joinery.

Regardless it is still about the wood
“Deciding to build a timber frame is a very emotional one,” says Hugh Lofting, President of Hugh Lofting Timber Framing, Inc. “Our clients are heavily invested in every decision in the design process from wood species to the curve of the braces we use. We like to tell them these are decisions that will last for generations and that makes them really sit up and take notice.” Timber frames historically are long lived structures. In fact, there is one frame in China that is 4000 years old and it is still in use.

In a timber frame the structure of the building is exposed inside the building. This is akin to inviting the outdoors inside bringing the occupants closer to nature while providing almost all of the decoration that the home or structure may ever need. The interior spaces created are unencumbered by structural walls allowing for an open floor plan that can be reconfigured to the occupants needs and have a tangible feeling that exudes warmth, strength and security. The client may almost want to pet their house.

Timber framers work with many species of wood and many work with the wood that is local to their community. Each wood has its own characteristic look and personality. Newly sawn or green Oak is particularly personable when it is introduced to the heated environment. A framer once told me that his client complained of gunshots downstairs in the night – it was the oak frame checking as it dried in the dry heat of the house. Timber framers use a variety of woods including hardwoods (beech, birch, chestnut, elm, oaks, maple, cherry and hickory) and softwoods (cedar, firs, hemlock, larch, pine, spruce and tamarack)

A Word About Sustainability
Timber framing is also one of the most sustainable building methods available supporting good forest management, low-carbon input manufacturing processes and energy efficiency. Timber frames are typically manufactured off-site and raised in a matter of days. The prefabricated nature of the frame reduces construction time and site waste. Also the choice of enclosure systems for the frame can make it very energy efficient and air tight.

A significant percentage of timber frames are enclosed with SIPS (Structural Insulated Panels) that, when installed, form a blanket of insulation around the frame that provides significant R-values, very little thermal bridging and an air tight barrier. Much like a sandwich, SIPS are constructed of two outer layers of OSB (Oriented Strand Board) that are filled with a thick layer of foam (polystyrene or polyurethane). The SIPS are applied to the outside of the timber frame leaving the timbers fully exposed. SIPS are manufactured off-site and can be pre-cut for windows and door penetrations further lessening on-site construction waste and construction time.

Simple Design and Simple Shapes
Simple, uncomplicated design does not mean boring. It does mean that keeping the design simple, like a barn, without complex roof systems, multiple dormers and various appurtenances will help minimize costs while showcasing the timbers without clutter. Why? Craftsman labor and the total board footage of the wood.

Timber framing is extremely labor intensive. Even frames that have been cut on CNC machines often require hand finishing. Every time the timber framer works with the timber, the cost increases. So, if you have a kiln dried, S4S (smooth four sides) timber that is perfectly straight and the final design requires same, the framer will work with it about three times: assessing, laying out and cutting. Each time the design adds complexity such as: antique wood that is by nature often out of square and/or filled with nails, hand hewn finishes, hand carved embellishments and compound joinery, the time spent with the timber increases. This makes a direct impact upon the project budget. Not that any of these things are bad but they should be recognized up front by the design team.

Complexity also brings with it increased volumes of wood. Timber orders are measured in board footage where a board foot equals a piece of wood that measures one inch by one foot long by one foot wide. Adding hips and valleys and dormers to the frame increases the total board footage that is purchased for the project but the loss factor in each piece increases as steep angles are cut into the timbers. In the case of the dormer, the total board footage of the roof increases and the pieces that are added to the project are much more complex adding to materials and labor expense.

Collaborative Design
The Collaborative Design Process is an iterative one that refines the design as the scope of the project evolves and it is very important to having successful timber frame project. Collaborative Design is a communication intensive process that has measurable benefits that may for example include: identifying synergies between building size, fenestration and mechanical equipment needs or synergies between plantings, building orientation and water use.

It is important to put together the collaborative design team early in the process, preferably at the Conceptual Design Stage, and charge them with developing complete, solid plans before starting any construction. The team should at least include: the client, the architect/designer, the builder, the timber framer, the HVAC contractor or consultant, the sustainable and/or civil engineer (if any) and the landscape architect. The plans they develop should include: architectural, structural and framing, mechanical, stormwater, electrical and plumbing.

Many times clients avoid MEP drawings to save costs, with the belief that the general contractor and subs will be able to develop the best mechanical program. When stick framing a structure this approach often results in cost overruns. Given the prefabricated nature of timber framing and the limited interior walls this approach quickly develops into problems with routing and installing utilities. These problems frustrate the construction team and often result in additional costs for materials and labor. Avoiding the MEPs may also have long term cost affects on operating the building because design synergies were not considered at the onset.

Bringing this team in early has other measurable and immeasureable benefits of laying a foundation of good communication at the outset of the project that flows throughout the entire construction process. It allows for tighter bidding, reduces cost overruns from change orders and reduces total construction time. It also sends a message to the construction team that this is a project that has been well thought out. This often causes the construction team to take greater care in their own work resulting in a better project all around.

Why Include The Timber Framer?
Timber framing is a specialty business and a Timber Framer should be included in the design process for a number of reasons. Most importantly, the lack of interior walls and the prefabricated nature of the frame present unique challenges for routing mechanical systems and there is little wiggle room for changing the frame once it is fabricated. That is not to say that you are 'stuck' with the framer for the duration of the project. The timber framer can be brought to the project as a paid consultant to address design issues and to develop a set of CAD drawings to be used in the bidding process. In this role, the timber framer will identify potential factors that affect cost and buildabilty including timber sizing, wood species and design complexities and identify design problems such as stair opening, door and frame conflicts. By identifying pitfalls and opportunities early, the client will save money and time on the frame that can be rolled into the project as a whole.

"Our goal as an industry is to become more interactive with architects earlier in the process", says Pam Hinton, Executive Director of the Timber Frame Business Council, "our membership of experienced timber framers can help architects and their clients save money and make the projects more successful. Working as a team, the timber framer can do what it takes to express the architect's vision in wood - a good collaboration will express the intent of the design, and maximize the potential of the materials used."

What Affects the Cost of a Timber Frame?
There is no doubt that timber framing is more expensive than stick framing but as we have discussed above there are many ways to control costs: collaborative design, simple design and simple shapes. The collaborative design process will help to achieve the vision with a timber frame design that optimizes structural strength, total board foot volume and manufacturing processes. During the design process other cost drivers will be determined and weighed against the overall project goals.

One of these drivers would be the choice of wood species. Choosing oak over mahogany would dramatically reduce materials costs but may not achieve the desired look. During the design process the team should review the characteristics of various species and tour several timber frames before selecting the species.

Another cost driver is the quality of the wood: green (freshly felled) versus kiln dried versus reclaimed woods. Each of these can markedly affect the price of the frame with green wood typically least expensive and reclaimed wood being the most expensive. A rule of thumb is that the drier the wood, the more stable hence less checking and tighter joints over time. These issues rarely have a structural impact on the frame and are usually aesthetic choices. Requiring certified woods (FSC or Forest Stewardship Council Certified) sometimes affects price and availability of certain species and should be reviewed during the design process.

The finish on the timbers is yet another cost consideration. As discussed above, each time the framer works with the timber, it affects the cost of the frame. A rough sawn frame is the simplest. Requiring S4S requires additional milling. Sanding and oiling, hand hewn finishes, adzing, sand blasting all provide distinct looks but all require multiple handling of the timbers.

Planning for the raising is an oft misunderstood cost driver in timber framing. Raising a timber frame takes an awful lot of space and this is something that is often lacking around construction sites. The best case scenario for the timber framer is to have an area equal to the dimensions of an entire bent (an end wall) available on the ground to assemble the frame and ready it for raising. In this best case there are no other walls existing on the site just a clear, level, squared deck. This is the easiest and least costly situation. On the opposite end raising the frame after three of the four walls are standing or trying to raise trusses in a room where the roof has already been placed makes for a more difficult and costly installation.

Access and Site requirements also drive costs. A steep sloping lot or an inaccessible driveway may preclude the use of a crane to raise the frame and add many labor hours for installation.

A Word About Hybrids
One way to control costs and get the best bang for your buck is to design a hybrid frame that utilizes timber framing in the high visibility areas and use stick frame or SIPS construction in the remainder of the spaces. Often timber frame great rooms and front porches are clad with and then married to SIPS construction. As discussed above, SIPS provide structural support. Additionally, because of their design, they can be used as structural building members so that whole buildings can be constructed of SIPS.

Another hybrid technique is to include trusses in the design that are supported by SIPS or stick framed walls. It should be recognized though that trusses can be structural or decorative but the cost for each will be about the same so they might as well earn their keep.

In Conclusion
Timber framing is not one only one of the oldest construction techniques known to man. It is also among the most beautiful. That coupled with its sustainable nature, long life and design flexibility should make it the choice method for anyone contemplating a new building project.

*You can also view the article here.

Amy Cornelius, LEED AP works with Hugh Lofting Timber Framing, Inc a LEED for Homes registered contractor.