Design Decisions

The basics of timber home design

By Jeff Davis
Published in Timber Homes Illustrated
2007 Annual Buyer's Directory

I became interested in Timber framing growing up in a small Vermont town surrounded by working dairy farms all with large barns. Several of our friends were farming folk, and I spent time playing and working in the large old timber frame structures. Timber framing has been around for a few thousand years, and recently has seen a strong resurgence in the residential home market as well as the light commercial industry. Many of the old New England homes were timber frame structures that had their timbers covered up. Thankfully today the new generation of timber frame homes is proud to expose the beauty of the structure.

Forming a Frame

Basically, timber frame structures utilize a framework of larger timbers that are spaced farther apart than the smaller dimensional lumber pieces used in conventionally framed houses. In a traditional frame, timbers are joined together using carved wooden joints (with interlocking mortise and tenons) and fastened with wooden pegs. In conventional stud framing, the frame members are nailed together. When designed correctly and finished with a properly maintained roof, timber frame homes can be stronger and last longer than conventionally framed homes.

There are two ways to approach the design of your timber frame home. If you have a floor plan already in mind, the floor plan will generally dictate the kind of framing system that will be used in the home. Or, if you have a particular framing system you like, you can design the floor plan around the timber frame.

There are two basic framing systems used in timber framing, the common rafter and the bent system, with both systems having several styles to choose from. Both systems are beautiful and provide flexibility for designing a floor plan that meets your spatial requirements.

The common rafter system is the simplest timber framing system, most often using 6x8 rafters placed every 4 feet on the center. These rafters support the roof loads and extend from the ridge (the top of the frame) to eave plates at the top of the walls. In a common rafter system, the rafters rest on the eave plate. Posts along the eave wall can be moved to accommodate window and door locations without affecting the whole framing system.

By combining both a ridge beam and a principal purlin system, even wider structures can be achieved with common rafter frames. Purlins are timbers that run parallel to the ridge beam of the home and perpendicular to the rafters.

A bent system is more complicated than a common rafter system. Instead of having common rafters which run from ridge to eave, bent systems have principal rafters each connected to a principal post. Instead of being put together one by one as the frame goes up, bents are assembled on the deck of the home and raised into place by a crane.

Common purlins span between the principal rafters 4 feet on center and parallel to the ridge. The principal rafter in a bent system is generally a larger sized timber than in the common rafter system because it is carrying more roof load.

Bent spacing is limited by the size of the common purlin at no more than 14 feet apart, which is less than the common rafter system at 20 feet.

There are many bent framing styles to choose from: King post, queens post, hammer beam, scissor truss, and others. Bent systems can be modified according to its location in the house - for instance, at a loft, a modified king post truss can define the space and carry loads.

In a bent system the principal rafter is connected to the principal post, creating one unit. This means the whole bent must move to accommodate specific placement of windows and doors or room sizes defined by the timber frame.

From the Top

How does a timber frame system affect the design? The main purpose of a home is to provide shelter, and the most important component of shelter is the roof - keeping out rain and wind, yes, but also supporting snow and wind loads. A good design should start with the roof system - what kind it is and how the roof load is transferred to the foundation. These loads are concentrated, and the interior space is often defined by the timber frame system and where posts are located. The most important design considerations are how the interior spaces work with the roof framing system, and the symmetry of the spaces created. Because the timbers are visible within the home, one has to take into account the interior spaces created by the framework of timber. For this reason, I prefer to design a timber frame system first and then design the floor plan to fit the frame.

Plans with a lot of hips, valleys, and intersecting roof lines need careful consideration in designing a timber frame home. These can create complex angles which, in a conventional home might be covered by dropped ceilings, but in a timber frame will be visible from the interior space. Cathedral spaces are common in a timber frame structure, so attention also needs to be given to interior partitions separating rooms where walls continue up to intersect a sloping roof.

Endless Possibilities

Floor plans should be designed so they do not break up the architectural balance and symmetry of the timbers which define the space. Therefore, cathedral ceilings may not be appropriate for small areas such as hallways and bathrooms because the interior partition walls divide the open ceiling space above the room. One solution is to build smaller living areas without timber framing, using conventional material for the roof structure instead.

Timber framing provides many possibilities: It is easy to combine conventional, structural insulated panel (SIP) and timber frame construction to create customized spaces that fit your design concept and your budget. Timber frame porches provide a welcome design addition to a home, and timbers can be used as division between open spaces without using partition walls. Curved beams and arched braces create nice transitions between cathedral and flat ceilings. And of course, timber framing lends itself to open spaces for entertaining and informal living.

However you design them, they are sure to bring you enjoyment and comfort for generations to come.

Jeff Davis is President of Davis Frame Company in Claremont, New Hampshire.

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